The Reason Why A Careful Collection of Many Hundreds of Reasons for Things Which, Though Generally Believed, Are Imperfectly Understood by Philp, Robert Kemp

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THE

REASON WHY:

A CAREFUL

COLLECTION OF MANY HUNDREDS OF REASONS FOR THINGS WHICH, THOUGH GENERALLY BELIEVED, ARE IMPERFECTLY UNDERSTOOD.

A BOOK OF CONDENSED SCIENTIFIC KNOWLEDGE FOR THE MILLION.

By THE AUTHOR OF "INQUIRE WITHIN."

This collection of useful information on "Common Things" is put in the interesting form of "Why and Because," and comprehends a familiar explanation of many subjects which occupy a large space in the philosophy of Nature, relating to air, animals, atmosphere, caloric, chemistry, ventilation, materia medica, meteorology, acoustics, electricity, light, zoölogy, etc.

NEW YORK:

DICK & FITZGERALD, PUBLISHERS,

No. 18 ANN STREET.

PREFACE.

We are all children of one Father, whose Works it should be our delight to study. As the intelligent child, standing by his parent's knee, asks explanations alike of the most simple phenomena, and of the most profound problems; so should man, turning to his Creator, continually ask for knowledge. Not because the profession of letters has, in these days, become a fashion, and that the man of general proficiency can best work out his success in worldly pursuits; but because knowledge is a treasure which gladdens the heart, dignifies the mind, and ennobles the soul.

The occupation of the mind, by the pursuit of knowledge, is of itself a good, since it diverts from evil, and by elevating and refining the mind, and strengthening the judgment, it fortifies us for the hour of temptation, and surrounds us with barriers which the powers of sin cannot successfully assail.

It is not contended that the mere acquisition of knowledge will either ensure a good moral nature, or convey religious truth. But both religion and morals will find in the diffusion of knowledge a ground work upon which their loftier temples may discover an acceptable foundation.

The man who comprehends the order of Nature, and the immutability of Divine law, must of necessity bring himself in some degree into accordance with that order, and under submission to the law: hence the _tendency_ of knowledge will always be found to harmonise the fragment with the mass, and to subvert the evil to the good.

The troubles of the world have arisen from the want of knowledge, not from the possession of it. And in proportion as man becomes an intelligent and reflective being, he will be a better creature in all the relations of life. If these benefits, vast and incalculable as they are, be the real tendency and result of knowledge, why is ignorance allowed to remain, and why is the world still distracted by error?

It is because the moral and intellectual qualities of man are, like all creations and gifts of God, the subjects of development, whose law is progression.

We can aid human improvement, but we cannot unduly hasten it. Whenever man has sprung too rapidly to a conclusion, he has alighted upon error, and has had to retrace his steps.

The greatest philosophers have been those who have clung to the demonstrative sciences, and have held that a simple truth well ascertained, is greater than the grandest theory founded upon questionable premises. Newton made more scientific revelations to mankind than any other philosopher; and his discoveries have borne the searching test of time, because he snatched at nothing, leaped over no chasm to establish a favourite dogma; but, by the slowest steps, and by regarding the merest trifles, as well as the highest phenomena, he learnt to read Nature correctly. He discovered that her atoms were letters, her blades of grass were words, her phenomena were sentences, and her complete volume a grand poem, teaching on every page the wisdom and the power of an Almighty Creator.

When he observed an apple fall to the ground, he asked the "Reason Why;" and in answer to that enquiry, there came one of the grandest discoveries that has ever been recorded upon the book of science. With that discovery a flood of light burst upon the human mind, illustrating in a far higher degree than had ever previously been conceived, the vastness of Almighty Power.

Why should not each of us enquire the "Reason Why" regarding everything that we observe? Why should we mentally _grope_ about, when we may _see_ our way? When addressed in a foreign tongue, we hear a number of articulated sounds, to which we can attach no meaning; they convey nothing to the mind, make no impression upon the in-dwelling soul. When those sounds are interpreted to us, in a language that we can understand, they impart impressions of joy, hope, surprise, or sorrow, because the words convey to us _a meaning_. In like manner, if we fail to understand Nature, its beauties, its teachings are lost. Everything speaks to us, but we do not understand the voices. They come murmuring from the brook, trilling from the bird, or pealing from the thunder; but though they reach the ear of the body, they do not impress the listening spirit.

Every flower, every ray of light, every drop of dew, each flake of snow, the curling smoke, the lowering cloud, the bright sun, the pale moon, the twinkling stars, speak to us in eloquent language of the great Hand that made them. But millions lose the grand lesson which Nature teaches, because they can attach no meaning to what they see or hear.

"THE REASON WHY" is offered as an interpreter of many of Nature's utterances. Great care has been taken that these interpretations may be consistent with the latest knowledge, obtained from the highest sources. If the author finds that his work if accepted for the good of those who seek not only to know, but to _understand_, he will make it his constant care to read the Book of Nature, and to add to the pages of this volume whatever interpretations the progress of enquiry and discovery may demand and supply.

INDEX, AND INDEX LESSONS.

[Pointing hand symbol] _The numbers refer to the Questions. The Index Lessons do not correspond with the Chapters, but are designed to bring together in their alphabetical connection, all the Questions and Answers upon each particular subject included in the work._

LESSON I.

Acids, vegetable, whence are they obtained, 1256

Actinism, what is it, 552

Actinism, what effect has it upon vegetation, 559

Actinism, at what season of the year is it most abundant, 560

ærial spectra, what is the cause of, 527

Affinity, what is the attraction of, 777

Air, why do we breathe it, 4

Air, what is the composition of it, 12

Air, what is its state after it has been breathed, 13

Air, is that sent from the lungs light or heavy, 14

Air, is it a good or bad conductor of heat, 150

Air, why is a still summer said to be sultry, 171

Air, why does it feel cooler when in motion, than when still, 175

LESSON II.

Air, when is it hot enough to destroy life, 179

Air, why is it often excessively hot in chalk districts, 219

Air, is it heated directly by the sun's rays, 221

Air, why does it ascend the chimney, 240

Air, why does it fly through the doors and windows towards the fire-place, 241

Air, what does the motion of it in our rooms illustrate, 242

Air, why does it become charged with watery vapour, 346

Air, when is it said to be saturated with vapour, 430

Air, what proportion of water is air capable of holding in the form of vapour, 431

Air, what is its weight relative to that of water, 647

LESSON III.

Air, which is heavier, dry or vaporised, 697

Air-pistol (or pop-gun), why does pressure applied to the handle propel the cork, 854

Air-pistol, why must the handle be drawn out before the cork is placed in, 855

Air, why does fresh impart a healthy appearance, 915

Albumen, what is it, 899

Alkalies, what are they, 36

Amber, or electrum, what is it, 564

Animals, how is their greater warmth in winter provided for, 168

Animal forms, why are there so many, 1029

Animal furs, why do they become thicker in the winter, 1035

Animals, why have carnivorous, long pointed teeth, 1038

LESSON IV.

Animals with long necks, why have they large throats, 1049

Animals, why cannot flesh-eaters live upon vegetables, 1071

Animals, why can ruminating recover the food from their paunches, 1089

Animals, why can ruminating keep the chewed from the unchewed food in their stomachs, 1090

Animals, why do the smaller animals breed more abundantly than the larger ones, 1094

Animals, domestic, why may change of weather be expected when they are restless, 1107

Animals, plants, and minerals, what are the differences between them, 1140

Animals that graze, why do they crop the tender blades, but leave the tall grass, 1287

LESSON V.

Animals, distribution of, 1326

Arms and hands, why do we see blue marks upon them, 896

Arms and legs, why does it require the influence of the will to set them in motion, 917

Arms and legs, why are they made subject to the will, 919

Ascending, what is the cause of bodies, 775

Atmosphere, how is it heated, 232

Atmospheric humidity, why does it sometimes form clouds, at others form dews, fogs, mists, &c., 409

Atmospheric pressure, how high will it raise water, 652

Atmosphere, what is the, 639

Atmosphere, to what height does it extend, 639

Atmosphere, what is the amount of its pressure, 641

Atmosphere, what is the proportion of watery vapour in the, 642

Atmosphere, why are its upper regions intensely cold, 444

Atmosphere, what is the greatest height ever reached in the, 648

LESSON VI.

Atmospheric pressure, what is the total amount on the earth's surface, 644

Atmosphere, what is the amount of its pressure upon the human body, 645

Atmosphere, why do we not feel its pressure, 646

Atmosphere, to what extent may its pressure vary, 702

Attractive agent, what is an, 76

Attraction, what is it, 772

Attraction, how many kinds are there, 773

Aurora Borealis, what is the cause of, 590

Bananas or plantains, where are they cultivated, 1220

Bark Peruvian, where is it produced, 1228

Barometer, what is a, 691

Barometer, why does it indicate the pressure of the atmosphere, 692

Barometer, why is it also called a "weather-glass", 693

Barometers, why are they constructed with circular dials, 694

Barometer, why does the hand change its position when the mercury rises or falls, 695

Barometer, why does tapping its face cause the hand to move, 696

Barometer, why does its fall denote the approach of rain, 699

LESSON VII.

Barometer, why does its rise denote the approach of fine weather, 700

Barometer, how does it enable us to calculate the height of the mountains, 701

Barometer, when does it stand highest, 704

Barometer, when does it stand lowest, 705

Barometer, what effect has heat upon the, 707

Barometer, what effect has cold upon the, 708

Balloons, why do they ascend in air, 830

Balloons, why do air balloons become inflated, 831

Balloons, why do they sometimes burst when they reach a high altitude, 832

Bat and ball, what principles of natural philosophy are illustrated in the play, 867

Bats, why have they hooked claws in their wings, 1079

Bats, why do they fly by night, 1080

LESSON VIII.

Bats, why do they sleep during winter, 1081

Beds, why should they be raised two feet from the ground, 15

Bed-room windows, why are they sometimes covered with ice crystals, 344

Beer, why will it not run out of a cask until a hole is made at the top, 660

Beer, why does it get flat, 805

Bees, why have they stings, 1101

Bees, why may we expect fine weather when bees wander far from their hives, 1114

Beetles, why are they called "coleoptera", 1319

Beetles, why have they hard horny wing-cases, 1320

Beetles, why have many of them hard horns, 1321

Bile, why does it separate nutritious from innutritious matter, 884

Birds, why have water-fowls feathers of a close and smooth texture, 1033

Birds, why are they covered with feathers, 1030

Birds, why does black down grow under their feathers on the approach of winter, 1036

Birds, why have they hard beaks, 1040

Birds, why are their beaks generally long and sharp, 1041

Birds, why are their bones hollow, 1050

Birds, why do they lay eggs, 1051

LESSON IX.

Birds, why have those with long legs short tails, 1052

Birds, why have aquatic web-feet, 1059

Birds, why have those that swim and dive short legs, 1060

Birds, why have some deep rough notches on the under surfaces of their feet, 1061

Birds, why have they gizzards, 1072

Birds of prey, why have they no gizzards, 1084

Birds, why may wet and thunder be expected when they cease to sing, 1121

Birds of passage, why, if they arrive early, may severe weather be expected, 1121

Birds, geological distribution of, 1326

Birds, tameness of in unfrequented countries, 1327

Birds, why are birds of song not also remarkable as birds of plumage, 1328

Birds, what are the velocities of their flights, 1329

Birds, what is the cause of their migrations, 1330

Black, why should parts of kettles and saucepans be allowed to remain, 204

Blood, in what proportions are the gases found in it, 39

LESSON X.

Blood, what is venous, 41

Blood, what is arterial, 42

Blood, what is the constitution of the, 899

Blood, what quantity does the human body contain, 920

Blood, how frequently does the whole quantity pass through the system, 922

Blowing upon tea, why does it cool it, 174

Bow, why does it propel the arrow, 847

Bow and arrow, what line does the arrow describe, 848

Bow and arrow, what forces tend to arrest the arrow, 849

Bow and arrow, why are there feathers at the ends of arrows, 850

Bones, how many are there in the human body, 923

Bones, of what substances are they composed, 924

Bones, what are the uses of the, 925

Bones, why are those of the back hollowed out, 927

Bones, why are those of the skull arched, 930

Bones, why are those of the skull divided by small sutures, 931

Bones, why are they hollow, 934

LESSON XII.

Bones, why are those of the arms and legs formed into long shafts, 935

Bones, why are those of the feet and hands numerous and small, 936

Botanical geography, 1208

Brain, why is it placed within the skull, 926

Bread-fruit trees, where are they natives of, 1223

Breathing, is it a kind of combustion, 17

Breaths, have people ever been poisoned by their own, 24

Breezes, why are summer said to be cool, 170

Breezes, what is the cause of sea and land, 235

Bubbles, why do they ascend in the air, 236

Bubbles, why do they fall, after having ascended, 237

Bubbles, why do they display rainbow colours, 499

Bubbles, why are they round, 825

Bubbles, why are they elongated when being blown, 826

Bubbles, why do they close, and become perfect spheres when shaken from the pipe, 827

Bubbles, why do they change their colours in the sunshine, 828

Bubbles, why do they burst, 829

LESSON XI.

Burning or supporting combustion, what is the difference, 45

Burning-glasses, why do they appear to set fire to substances, 80

Butterflies, why do they lay their eggs upon cabbage-leaves, 1099

Butterflies' eggs, why do they lie dormant in the winter, 1288

Butterflies, why do they fly by day, 1296

Calms, why do they prevail at the equator, 671

Caloric, what is it, 72

Caloric, what is the source of it, 73

Caloric, what are the effects of it, 74

Caloric, why is it called a repulsive agent, 75

Caloric, how may it be excited to develope heat, 79

Caloric, is there any in ice, snow, water, marble, &c., 89

Caloric, how do we measure the quantity of in any substance, 329

Caloric, how does it travel, 332

Caloric, how do we know that it is caloric which fuses metals, 334

Calves and lambs, why have they no horns, 1069

Camel, why has its stomach a number of distinct bags, 1065

Candles, why do tallow require snuffing, 264

LESSON XIII.

Candles, why do composite and wax not require snuffing, 265

Candle, what becomes of it after it is burnt, 269

Capillary blood-vessels, why are they found in every part of the system, 919

Capillary attraction, what is it, 780

Carbonic acid gas, how is it formed, 9

Carbonic acid gas, what becomes of it, 10

Carbonic acid gas, is it heavier or lighter than air, 11

Carbonic acid gas, what are the chief sources of, 16

Carbonic acid gas, what is its effect upon the human system, 21

Carbonic acid gas, what becomes of that formed by combustion, 59

Carbonic acid gas, what proportion is dangerous to life, 60

Carbonic acid, what is it, 798

Carbonic acid, where does it chiefly exist, 799

Carbonic acid, what are its pure states, 800

Carbon, what is it, 18

Carbon, will it produce flame when burnt in oxygen, 106

Carbon and hydrogen, what differences characterise the combustion of, 268

LESSON XIV.

Carbon, what is the purest form of it, 277

Card, why do the images on each side blend while a card is revolving, 846

Caterpillars, why do they appear in the spring, 1289

Caterpillars, why do they eat voraciously, 1290

Caterpillars, why do they pass into the state of the chrysalis, 1291

Caterpillars, why do they become torpid in the chrysalis, 1292

Caterpillars, why do they attach themselves to the leaves of plants when in the chrysalis, 1294

Cats, &c., why do they see in the dark, 981

Cats' eyes, why are the pupils of nearly closed by day, 982

Cats, &c., why have they whiskers, 1096

Cattle, why if they run round in meadows may thunder be expected, 1122

Ceiling, how did Mr. Sands walk on the, 664

Champagne, why does it effervesce, 806

Champagne, why do bubbles rise from it in two or three columns, 807

Charcoal, why is it dangerous to burn it in rooms, 20

LESSON XV.

Charcoal fires, why do they not give flames, 107

Charcoal, what is it, 275

Charcoal, why does it act as a disinfectant, 809

Chicory, what is it, 1196

Chimney, why does it cease smoking after the fire has been lighted a little while, 680

Chimney, why does a long one create a better draught than a short one, 681

Chimneys, why do some smoke when windows and doors are closed, 683

Chimneys that stand under elevated objects, why do they smoke, 685

Chimneys, why do sooty smoke, 687

Chimneys, why do they smoke in damp and gusty weather, 688

Chimney, why does it smoke when first lighted, 679

Chocolate, what is it, 1194

Circulation, why are the venous blood and chyle sent to the lungs, 887

Circulation, what is the course of the arterial blood, 888

Circulation, why does the blood impart vitality, 891

LESSON XVI.

Circulation, how do we know the blood is alive, 892

Circulation, why does the blood circulate, 893

Circulation, how is the body renewed by the blood, 894

Circulation, how does the blood return to the lungs after it has reached the extremities, 895

Circulation, why are the veins more perceptible than the arteries, 897

Circulation, why when we prick the flesh does it bleed, 893

Circulation, what occurs during the, 899

Circulation, what becomes of the matter collected by the blood, 901

Circulation, how is the blood propelled through the arteries, 916

Circulation, why are the capillary vessels capable of receiving the quantity of blood sent through larger vessels, 917

Cinnamon, where is it produced, 1239

Cleanliness, why does it promote health, 1015

Clothes on fire, why should persons throw themselves down, 252

Clothing, why do some articles feel cold, and others warm, 120

LESSON XVII.

Clothing, are conductors or non-conductors the warmer, 121

Clothing, why are white and light-coloured articles cool, 218

Clothing, why are dark-coloured dresses worn in winter, and light in summer, 230

Clouds, what are they, 373

Clouds, why do we not see them ascend, 375

Clouds, why are they invisible when they rise, but become visible when they have ascended, 376

Clouds, why do they not descend to the earth, 377

Clouds, at what altitudes do they fly, 378

Clouds, how many descriptions are there, 379

Clouds, what produces their various shapes, 380

Clouds, what are their dimensions, 381

Clouds, how are they affected by winds, 382

Clouds, what do Cirrus foretell, 389

Clouds, what do Cumulus foretell, 390

Clouds, what do Stratus foretell, 391

Clouds, what do Nimbus foretell, 392

Clouds, what do Cirro-cumulus foretell, 393

Clouds, what do Cirro-stratus foretell, 394

Clouds, what do Cumulo-stratus foretell, 395

LESSON XVIII.

Clouds, why are cloudy days colder than sunny days, 396

Clouds, why are cloudy nights warmer than clear nights, 397

Cloudy days and nights, why are they not always wet, 432

Clouds, why are they white, 531

Clouds, why are they sometimes yellow, 533

Clouds, what develops the electricity in the, 581

Clouds, why do they sometimes move towards each other from opposite directions, 778

Clouds, why do they gather around mountain tops, 781

Cloves, where are they produced, 1231

Coal, what is it, 271

Coal, why do we know that it is of vegetable origin, 273

Coal, what are the chemical components of, 274

Coals, why do they produce yellow flame, 279

Cockles, why have they stiff muscular tongues, 1087

Cocoa, what is it, 1195

Cocoa, what tree produces it, 1221

Coffee-pot, why has it a wooden handle, 125

LESSON XIX.

Coffee, what is it, 1193

Coffee, where is it cultivated, 1224

Cohesion, what is the attraction of, 776

Coke-fires, why do they not give flames, 107

Coke, what is it, 278

Cold, when is a body said to be, 78

Cold, what is the cause of the sensation, 133

Cold, does it radiate, 207

Colour, why is a substance white, 466

Colour, why is a substance black, 467

Colour, why is a rose red, 468

Colour, why is a lily white, 469

Colour, why is a primrose yellow, 470

Colour, why are there so many varieties of colour and tint in nature, 471

Colours, remarks upon, 501

Combustion, what takes place during, 48

Combustion, how many kinds are there, 93

Conductors of heat, what substances are good, 110

Conductors of heat, why do bad conductors readily ignite, 295

Combustion, what is spontaneous, 311

Combustibles used for lighting, which vitiates the air most, 62

Conduction of heat, what is the, 113

Convection, what is the chief effect of the law of, 243

LESSON XX.

Cork, what is it, 1271

Cork-tree, why does it shed its own bark, 1272

Corns, why when they ache may rain be expected, 1115

Corn, why does the young ear come up enfolded in two leaves, 1167

Cotton, what is it, 1199

Cough, why do we, 1023

Crickets, why do they make a chirping noise, 1123

Cross-bills, why do their mandibles overlap each other, 1136

Cup in a pie, why does it become filled with juice, 631

Cup in a pie, does it prevent the juice from boiling over, 662

Dates, what are they produced by, 1221

Death-watch, why does it make a ticking noise, 1301

Dew, what is it, 345

Dew, what causes the decline of temperature which produces it, 347

Dew, why is there little or none, when the nights are cloudy, 348

Dew, why does it form most abundantly on cloudless nights, 349

Dew, why is there little under the branches of thick foliage, 351

LESSON XXI.

Dew, why is there none formed on windy nights, 352

Dew, why are valleys and low places chiefly subject to it, 353

Dew, what bodies receive little of it, 355

Dew, what bodies receive most of it, 354

Dew, at what period of the night is it chiefly formed, 356

Dew, why is it chiefly formed at that period, 357

Dew, in what parts of the world is the greatest quantity formed, 358

Dew, in what parts of the world is the least quantity formed, 359

Dew, why is it seldom formed at sea, 360

Dew, why is it, when heavy, regarded as a precursor of rain, 361

Dew, what is honey-dew, 364

Dew, why does it lie on the exposed sides of screens of plants, 401

Dew, why does it rest upon the upper surfaces of leaves, 402

Dews, why are cultivated lands more subject to them than those that are uncultivated, 403

LESSON XXII.

Dew, why does the gravel-walk receive less dew than the grass, 404

Dew, why does little of it form at the base of hedges, walls, and trees, 406

Dews, why do morning dews and mists usually come together, 407

Dew, what effect have winds upon its formation, 408

Dew, why does it form in round drops upon leaves, 794

Diamond, what causes the brilliant colours of the, 501

Digestion, why does food flow more freely to the stomach during, 879

Digestion, why does excess in eating impair, 880

Digestion, what changes occur to food in the stomach, 881

Digestion, what causes bilious attacks, 882

Digestion, how is the nutritious matter taken from that which is innutritious, 885

Digestion, what becomes of the nutrition when it enters into the circulation, 886

Dish-covers, why should they be plain in form, and have bright surfaces, 203

Dogs, geographical distribution of, 1333

Dream, why do we, 1020

Drowsiness, why is it felt in crowded rooms, 22

LESSON XXIII.

Ducks and geese, why do they dash water over their backs on the approach of rain, 1105

Ducks and geese, why have they square-pointed bills, 1044

Dyes, vegetable, what are they, 1259

Ear, why is it spread out externally, 984

Ears, why do hairs grow across them, 986

Ear-wig, why is it so called, 986

Ears, why is wax secreted in the, 987

Ears, why do singing noises occur in the, 988

Ears, why do people become deaf, 989

Ears, why do people accustomed to loud noises feel no inconvenience from them, 990

Ears of animals of prey, why do they bend forward, 1063

Ears of animals of flight, why do they bend backward, 1064

Earth, why is it warmer than air during sunshine, 398

Earth, why is it colder than air after sunset, 399

LESSON XXIV.

Earth, how can man weigh the, 784

Earth, what is the weight of the, 788

Earth-worms, why have they no feet, 1085

Echoes, why do some occur immediately after the sounds, 742

Echoes, why do some occur at a considerable interval after the sound, 743

Echoes, why do some change the tone and quantity of sounds, 744

Echoes, why are there sometimes several to one sound, 745

Echoes, are they caused only by distant objects, 746

Echoes, what are they, 741

Electricity, what is it, 561

Electricity, why is it so called, 562

Electric fluid, why is it so called, 566

Electrics, what substances are, 567

Electricity, what is positive, 568

Electricity, what is negative, 569

Electricity, what is vitreous, 570

Electricity, what is resinous, 570

Electrical phenomena, what produces them, 571

Electricity, how does its equilibrium become disturbed, 572

Electricity, how does it seek to regain its equilibrium, 573

Electricity, what substances are conductors, 574

Electricity, what substances are non-conductors, 575

Electricity, what substances are insulators, 576

LESSON XXV.

Electricity, what is the effect when a current of electricity meets with an insulator, 577

Electric sparks, what is the cause of, 578

Electricity, why does lightning attend it, 584

Electricity, why does it affect the shape of clouds, 613

Electric storms, why are they more frequent in hot than in cold weather, 624

Electric storms, why do they frequently occur after dry weather, 625

Electric storms, why do they purify the air, 629

Electricity, what is Voltaic, 636

Electrical attraction, what is it, 778

Elementary body, what is meant by, 19

Element, what is the most abundant in nature, 756

Elephant, why has it a short unbending neck, 1076

Elephant, why has it a trunk, 1077

Elephants' hind legs, why do they bend forward, 1078

Etna, what are the botanical regions of Mount, 1240

Endogenous stems, what are they, 1280

LESSON XXVI.

Endogenous stems, why do they abound in tropical climates, 1281

Endogenous stems, why have they no bark, 1282

Endogenous stems, why do they grow to a great height, 1283

Exogenous stems, what are they, 1279

Exercise, why does it promote health, 1016

Exercise, why does it make us feel warm, 839

Explosions of gas, what is the best method of preventing, 65

Eye-balls, why are they white, 911

Eye-balls, why do they sometimes become blood-shot, 912

Eyes, why are they placed in the sockets of the skull, 929

Eyes, why are we able to move them, 965

Eye, why does the pupil look black, 968

Eye, why is the pupil larger sometimes than at others, 969

Eyes, why have we two, 970

Eyes, why having two, do we see singly, 971

Eyes, why are they provided with eyelids, 972

Eyelids, why are they fringed with eyelashes, 973

Eyes, whence are their humours derived, 976

LESSON XXVII.

Eyes, why do we feel inconvenienced by sudden light, 978

Eyes, why if we look upon a bright object and turn away, are we unable to see, 979

Eyes, why are we able to see better after a little while, 980

Eyes, why do cats, bats, owls, &c., see in the dark, 981

Eels, why are their eyes covered with a transparent horny coat, 1130

Falling, what is the cause of bodies, 774

Fanning the face, why does it make us feel cooler, 172

Fata morgana, what is the cause of, 527

Fatigue, why do we feel, 1017

Feather, why does it fall to the earth more gradually than a shilling, 791

Features, why do we preserve the same through life, 903

Feel, why do we, 1004

Feeling, why is it impaired when the hands are cold, 1006

Feeling, why do the fingers prick and sting when they again become warm, 1007

LESSON XXVIII.

Feeling, why do persons whose legs or arms have been amputated, for some time continue to feel the part that has been removed, 1008

Fibrin, what is it, 899

Fingers, why can we raise them, 943

Fingers, why can we draw them back after they have been raised, 944

Fire, why does it burn more brightly when blown by a bellows, 27

Fire, what is it, 82

Fire, what are its properties, 83

Fire, what elements take part in its maintenance, 84

Fire, how does its combustion begin, 85

Fire-screens, how do they contribute to keep rooms cool, 217

Fire, why is it sometimes put out by blowing it when it is low, 262

Fire, why does poking it cause it to burn more brightly, 287

Fires, why do "blowers" improve the draft of air, 288

Fire, why does it sometimes appear red, and without flame, 283

Fire, what effect has it upon air, 284

Fires, why do charcoal and coke burn without flame, 292

Fires, why are charcoal and coke difficult to light, 293

Fires, a new plan of kindling, 293

LESSON XXIX.

Fires, why in lighting them do we first lay in paper, wood, and coal, 298

Fire, why does a poker laid across the top revive it when dull, 302

Fire-places, why should they be near the ground, 303

Fire, why does the blacksmith sprinkle water upon the coals of his forge, 308

Fires, what is the best method of conveying air to, 684

Fish, why does putrifying look luminous, 95

Fishes, why have they fins, 1053

Fishes, why are their fins proportionately so much smaller than the wings of birds, 1054

Fishes, why have they scales, 1055

Fishes, why do they float in streams, with their heads towards the current, 1056

Fishes, why have they air-bladders, 1057

Fishes, why do not their eyes contract, 1129

Fishes, why have they no eyelids, 1132

Fishes, why have they the power of giving their eye-balls a sudden motion, 1133

Fishes, why are their tails so much larger than their fins, 1137

Flame, what is it, 96

LESSON XXX.

Flame, what temperature is required to produce it, 101

Flame of a candle, why does the lower part of the flame appear blue, 246

Flame of a candle, why does the middle of the flame look dark, 217

Flame of a candle, why does the upper part produce a bright yellow light, 248

Flame of a candle, why is there a fringe of pale light around the flame, 249

Flame of a candle, why does it terminate in a point, 250

Flame of a candle, why does it lengthen when anything is held over it, 251

Flame or spark, why does pressing it put it out, 253

Flame of a candle, why does holding a candle upside down put it out, 257

Flame of a candle, why is it more difficult to blow out the flame of a candle with a cotton wick than one with a rush wick, 258

Flame of a candle, why does blowing sharply on it put it out, 259

Flame of a candle, why will a gentle puff of air sometimes relight it, 260

LESSON XXXI.

Flame of a candle, why will not a similar puff rekindle a rushlight, 261

Flame of a candle, why will a piece of paper twisted to form an extinguisher put it out, 263

Flame of a candle, why does it become dim when the wick is loaded with carbon, 267

Flames of a fire, why do some appear much whiter than others, 280

Flames of a fire, why do some of them appear blue, 282

Flesh, why does it heal when we cut it, 902

Flesh-eaters, why do they satisfy themselves with a rapid meal, 1092

Flesh, why do the marks of deep cuts remain, 905

Flesh, why does that under the nails look red, 907

Flies, how can they walk on the ceiling, 663

Flies, why have they fine hairs growing on the extremities of their legs, 1102

Flowers, what is the chief cause of the differences of their temperatures, 227

Flowers, why may wet weather be expected when their perfume is strong, 1103

LESSON XXXII.

Flowers, why, if certain close, may rain be expected, 1116

Flying-top, why does it rise on the air, 843

Flying-top, why does it return to the earth, when its rotations are expended, 844

Focus, what is a, 81

Fogs, what are they, 365

Fogs, why are certain coasts very liable to them, 366

Fogs, what are dry, 367

Fogs, why do they frequently rise in the morning and fall in the evening, 371

Fogs, why do they sometimes rest upon a locality for days together, and then disappear, 372

Food, why do we eat it, 869

Food, why do we eat animal and vegetable, 172

Food, why do we masticate it, 871

Food, how does it descend into the stomach, 875

Food, why do we not feel it being transmitted through the throat, 876

Food, why do we feel uneasy after eating to excess, 877

Food, why do we feel drowsy after eating heartily, 878

Food, why do some portions nourish us, while other portions are useless, 883

LESSON XXXIII.

Fossil, vegetable, what is it, 272

Friction, why does it produce heat, 321

Friction, why does rubbing two surfaces together attract latent heat, 322

Frost, why is the air warmer during, 163

Frost, what is white, 412

Frost, what is black, 413

Frost, why are black frosts said to last, 414

Fruit, why do gooseberries, plums, &c., taste acid, 1184

Fruit, why do ripe taste sweet, and unripe sour, 1185

Fruits, why are succulent most abundant in tropical climates, 1207

Fruits and vegetables, why do they ripen in succession, 1284

Furs, why are they worn in winter, 166

Galvanism, what is it, 635

Gas, what kind lights our streets, 56

Gas, why does it expand in thin air, 833

Geology, what has been its influence upon botanical geography, 1249

Giraffes, why have they small heads, 1322

Giraffes, why have they long necks, 1323

Giraffes, why have they long and flexible tongues, 1324

LESSON XXXIV.

Giraffes, why are their nostrils narrow and small, and studded with hairs, 1325

Glass upon a lamp, why does it increase the brilliancy of the light, 266

Glass, why is it transparent, 506

Glass, does transparent reflect any light, 507

Glow-worms, why have they brushes attached to their tails, 1127

Glow-worms, why do they emit light, 1128

Gnats, why are their larvæ and pupæ found in water, 1314

Gnats, why may fine weather be expected if they fly in large numbers, 1110

Grasses, why are they so widely diffused throughout nature, 1166

Grasshoppers, why are they comparatively active in their pupa stages, 1293

Gravitation, what is the attraction of, 774

Guano, why is it a good manure, 1262

Gum resins, what are they, 1255

Gums, vegetable, what are they, 1254

Gunpowder, why does it explode, 808

Gutta-percha, whence is it obtained, 1254

LESSON XXXV.

Hail, what is it, 448

Hail, why is it supposed that the electrical state of the clouds affects the formation of hail, 449

Hail-storms, why do they usually occur by day, 450

Hairs, why do they grow across the passages of the nostrils, 993

Halo, what is a, 494

Halo, what is the cause of a, 495

Haloes, why are they sometimes large, and at other times small, 496

Haloes, why do they foretell wet weather, 497

Hands, why when we hold them against the candle do we perceive a crimson colour, 906

Hay-stacks, why do they sometimes take fire spontaneously, 316

Head, why is it set upon the neck, 928

Hearing, why do people engaged in battle frequently lose their, 991

Hearing, death of a dog through noise, 991

Heart, why does it beat without any effort of the will, 948

Heart, why is it placed in the chest of the body, 932

Heart and lungs, why are they enclosed by ribs, 933

Heart, why are its motions made independent of the will, 950

LESSON XXXVI.

Heat, what is it, 71

Heat, what are the properties of, 86

Heat, what is animal, 87

Heat, what is latent, 88

Heat, what is a conductor of, 108

Heat, what is a non-conductor of, 109

Heat, how is it transmitted from one body to another, 119

Heat, what is the radiation of, 114

Heat, what is the reflection of, 115

Heat, what is the absorption of,116

Heat, what is the convection of, 117

Heat, what is the cause of the sensation, 134

Heat, what becomes of that which the hearth-stone receives, 136

Heat, how long does a substance feel hot or cold to the touch, 139

Hearth-rug, and hearth-stone, their relative states of heat, 142

Heat, which are the better conductors, fluids or solids, 146

Heat, why are dense substances the best conductors, 147

Heat, why are fluids bad conductors, 148

Heat, why are woollen fabrics bad conductors, 149

Heat, is air a good or bad conductor, 150

LESSON XXXVII.

Heat, is water a good or a bad conductor, 151

Heat, how do we know that air is not a good conductor, 155

Heat, how do we know that water is not a good conductor, 156

Heat, why are bottles of hot water wrapped in flannel, 158

Heat, why are hot rolls wrapped in flannel, 159

Heat, what is the highest degree that man can bear, 176

Heat, why may man endure a high degree without injury, 178

Heat, why may we believe that the internal heat of the earth does not arise from terrestrious combustion, 224

Heat, what is the radiation of, 181

Heat, what becomes of that which is radiated, 186

Heat, when does a body radiate it, 184

Heat, does the movement of the air affect the radiation of, 188

Heat, why will not the motion of air disturb the rays of, 190

Heat, what bodies are good radiators of, 192

Heat, what bodies are bad radiators of, 193

Heat, why does water keep hot longer in a bright metal jug, 195

LESSON XXXVIII.

Heat, may it be reflected, 210

Heat, are light or dark-coloured bodies the best reflectors, 211

Heat what bodies are the best reflectors of, 212

Heat, why is that which is reflected less intense than the primary, 214

Heat, can it be reflected in any great intensity, 215

Heat, are good reflectors also good absorbers, 216

Heat of the sun's rays, how is it diffused, 219

Heat, how do we know that it is absorbed by the earth, 222

Heat, do plants absorb it, 225

Heat, how is it diffused through the atmosphere, 232

Heat, how is it diffused through the ocean, 233

Heat, why is it developed during chemical changes, 312

Heat, what are the sources of, 323

Heat, what is the accepted theory of, 328

Heat, what is the point at which it becomes luminous, 545

Heat, what is the relative intensity of primary and reflected, 548

Heat to cold, why does a sudden change bring on illness, 1013

LESSON XXXIX.

Heat of the earth, Humboldt's opinion upon the internal, 1335

Heat of the earth, Hunt's opinion upon the internal, 1336

Hear, why do we, 983

Hiccough, why do we, 1027

Hoar-frost, what is it, 362

Hoar-frost, why does it foretell rain, 363

Hoar-frosts, why are they so frequent, and black-frost so unfrequent, 412

Hogs, why have Indian large tusks turning back towards their eyes, 1068

Hog, why is the under-jaw of the, shorter than the upper-jaw, 1039

Hoop, why does it roll without falling to the ground, 861

Hoop, why does it when falling make several side revolutions, 862

Horses, why have they smaller stomachs proportionately than other animals, 1097

Horse, why has it no gall-bladder, 1098

Horses, why is the secretion of their eyes thick and glutinous, 1134

Horses and cattle, why do they stretch out their necks, and snuff the air on the approach of rain, 1106

Horse-chestnut, why is it unfit for food, 1203

Hot, when is a body said to be, 77

LESSON XL.

Hot water, why does it feel hotter in a metal jug than in an earthenware one, 126

Hot metal, why does it feel hotter than hot wool, 130

Hot metal and hot wool, which would become cold first, 131

Hot and cold bodies, why, when placed near each other, do their temperatures approximate, 331

Humming-tops, why do we see the figures painted on them before they spin, but not while they are spinning, 813

Humming-top, why does it make a noise, 810

Hydrogen, what is it, 49

Hydrogen, will it support animal life, 50

Hydrogen, will it support combustion, 51

Hydrogen, why will it explode, 52

Hydrogen, where does it chiefly exist, 54

Hydrogen, how is it obtained from coal, 57

Hydrogen-gas, is an escape of it dangerous to life, 63

Hydrogen-gas, what proportion mixed with air will explode, 64

Hydrogen-gas, does it rise or fall, 66

LESSON XLI.

Hydrogen-gas, what proportion is dangerous if inhaled, 67

Hydrogen-gas, what proportion may be recognised by its smell, 68

Hydrogen-gas, may the use of it be considered dangerous, 70

Hydrogen-gas, what sources of it are there in our dwellings, 69

Hydrogen, why will it burst into flame when coals become heated, 100

Hydrogen, why does hi-carburetted burn with a whiter flame than common coal-gas, 281

Ice, why are blocks of it wrapped in flannel in summer time, 154

Ice, why does it melt, 325

Ice, why does it occupy more space than water, 752

Ignis fatuus, what is it, 318

Incidence, what is the line of, 519

Incidence, what is the angle of, 521

India-rubber, where is it obtained, 1254

Infants, why have they no teeth, 1070

Insects, why have they a large number of eyes, 1083

Insects, why have they long projections from their heads, 1100

Insects, why do they attach their eggs to leaves, 1295

Insects, why do they multiply so numerously, 1300

LESSON XLII.

Insects, what is a larva, 1302

Insects, what is a pupa, 1303

Insects, what is a chrysalis, 1304

Insects, what is a nymph, 1305

Insects, why do they abound in decaying substances and in putrid waters, 1313

Insects, why do we see them in tanks of rain-water, 1314

Iron articles, why are they intensely cold in winter, 132

Iron articles, why do they usually feel cool, even when near a fire, 132

Ironing-box, why does the iron sometimes become too large for the box to receive it, 335

Ironing-box, why does the iron enter it, when partially cooled, 336

Iron, why does it rust when wetted, 768

Jew's-harps, why do they give musical sounds, 851

Jew's-harps, why will they not produce loud sounds unless they are applied to the mouth, 852

Jew's-harps, why does the alteration of the form of the mouth alter the sounds, 853

Kangaroos and opossums, why have they pouches in which they carry their young, 1139

LESSON XLIII.

Kettle-holders, of what use are they, 128

Kettles and saucepans, why should the lids and fronts of be kept bright, 206

Kettles, why do they become encrusted with stony deposits, 762

Kite, why does it rise in the air, 840

Kite-string, why does it feel hot when running through the hand, 841

Kite, why does running with it cause it to ascend, 812

Knowledge, why should we seek it, 1

Knowledge, why does the possession of it give us power, 2

Knowledge, what are the benefits of, 3

Latent heat, will the abstraction of it reduce the bulk of bodies, 337

Latent heat, how do we know that it exists in air, 339

Laugh, why do we, 1026

Ligaments, what are they, 937

Ligaments, why are they wrapped around the joints, 938

Light, what is it, 451

Leaves, why are they green, 1181

Leaves, why do some turn yellow, 1186

Leaves, why do they fall off in the autumn, 1187

Leaves, how to make skeleton, 1159

LESSON XLIV.

Leaves, why do they turn brown in autumn, 1183

Legs crossed, why do we see the elevated leg move at regular intervals, 918

Light, at what velocity does it travel, 453

Light, how long does it take to travel from the sun to the earth, 454

Light what are the minor sources of, 457

Light, what is a ray of, 458

Light, what is a medium of, 459

Light, what is a beam of, 460

Light, what is a pencil of, 461

Light, what is the radiant point, 462

Light, what is the focus of, 463

Light, what is the constitution of a ray of, 464

Light, why are its rays white, 465

Light, what are the estimated vibrations of, 465

Light, what is the refraction of, 472

Light, is it refracted when it falls upon a surface in a straight line, 473

Light, is the direction in which its rays are best dependent on the densities of the media, 474

Light, why does a spoon look bent when placed in a basin of water, 477

Light, why do we see the rays of the sun burst through the clouds in different directions, 478

LESSON XLV.

Light, why is the apparent depth of water always deceptive, 479

Light, why are some substances opaque to it, 511

Light, how do we know that bodies reflect it in every direction, 517

Light, meditation on, 517

Light and heat, what are the differences between, 540

Light and heat, are they combined in the sun's rays, 541

Light and heat, how do we know that they are separate elements, 542

Light and heat, in what respects are they similar, 543

Light and heat, in what respects are they dissimilar, 544

Light, what is polarized, 549

Light, what is the velocity of artificial, 546

Light, at what rate does the light of the stars travel, 547

Light, why does it tarnish silver, 553

Light, why does it affect colours, 554

Light, why can pictures be taken by the sun's rays, 555

Light, when does it scorch plants, 558

Lightning, what is the cause of, 580

Lightning, why does it sometimes become zigzag, 586

Lightning, why is it sometimes forked, 587

LESSON XLVI.

Lightning, why is it sometimes like a lurid sheet, 588

Lightning, when is the flash straight, 587

Lightning, when does the flash appear blue, 591

Lightning, why is it sometimes red, at others blue, yellow, or white, 592

Lightning, does it ever pass from the earth to the clouds, 593

Lightning, does it pass from the clouds to the earth, 594

Lightning, what is the force of, 595

Lightning, why is it dangerous to stand near a tree during a storm, 596

Lightning, why is it dangerous to sit near a fire during a storm, 598

Lightning, why is it dangerous to be near water during a storm, 599

Lightning, are iron houses dangerous, 600

Lightning, why does it seize upon bell wires, &c., 601

Lightning, are umbrellas with steel frames dangerous, 603

Lightning, are iron bedsteads dangerous, 604

Lightning, what is the safest situation during a storm, 606

Lightning-conductors, do they attract electricity, 608

LESSON XLVII.

Lightning-conductors, which is the best metal for, 610

Lightning-conductors, why have they sometimes been found ineffective, 609

Lightning-conductors, why should large buildings have several, 611

Lightning-conductors, why should they have several branches penetrating the earth, 612

Lightning, through what distance will its light penetrate, 622

Lightning, how may we calculate the distance of a storm, 623

Lightning, why is the flash generally succeeded by heavy rain, 626

Lightning, why is a flash generally followed by a gust of wind, 627

Lime-kilns, why do persons viewed through the hot air of, appear distorted and tremulous, 483

Limpets, why is it difficult to strike them from rocks, 665

Lips, why are they red, 913

Liver, what effect has it upon the circulation, 901

Looking-glass, why, if you hold one at an angle towards the sun, will light be thrown in an opposite direction, 208

Loss, is there any such thing in the operations of nature, 270

LESSON XLVIII.

Lungs, why are they placed in the chest of the body, 932

Magnetism, what is it, 633

Magnetic bodies, what are they, 634

Magpie, why is a single one said to foretell bad weather, 1118

Mahogany, what is it, 1190

Mahogany and other woods, why are there curious markings in, 1273

Man, why is he born without a covering, 1034

Man, why has he no external appendage to his mouth, 1037

Marbles, why do they revolve when propelled along the ground, 818

Marbles, why do striped appear to have more stripes when they are rolling than when at rest, 819

Marbles, why do they rebound when dropped upon the pavement, 820

Marbles, why do they roll furthest upon smooth surfaces, 821

Marbles, how many forces contribute to stop their rolling, 882

Marbles, why do the stripes upon them disappear when they are spinning rapidly, 823

Marbles, why are the rings upon them most perceptible at the "poles," while they are spinning, 824

LESSON XLIX. Matches, do they ignite spontaneously, 313

Meat, why is it sooner cooked when a tin screen is placed before the fire, 213

Metal, why does it run like a fluid when melted, 333

Mirrors, why do we see our features therein, 209

Mirrors, why do we see our faces in them, 512

Mirrors, what has the glass of to do with the reflection, 514

Mirrors, why do compound mirrors reflect many images of one object, 515

Mirrors, why do reflections appear as far behind a mirror as the object reflected stands before it, 518

Mirage, what is the cause of the, 527

Mists, what are they, 368

Mists and fogs, what is the difference between them, 369

Mists end fogs, why do they disappear at sunrise, 370

Mistletoe, why have its berries a thick viscid juice, 1177

Mistletoe, how are its seeds conveyed to the bark of trees, 1178

Moles, why have they hard flat feet, 1074

Mole's fur, why is it glossy and smooth, 1075

LESSON L.

Moles, why are they permanent, 904

Moles, why may rain be expected when they throw up their hills, 1117

Monsoons, what are they, 672

Monsoons, what is the cause of, 673

Mornings, why are they usually clear when frosty, 410

Mother-of-pearl, what causes the rich tints displayed by, 500

Moths, why do they fly by night, 1297

Moths, why are their bodies generally covered with thick down, 1228

Moths, why do they fly against the candle-flame, 1299

Muscles, how many are there in the human body, 941

Muscle, what is the constitution of a, 942

Muscles, what degree of strength do they possess, 945

Muscles, what is the stimulus which sets them in action, 946

Mussels, why have they tendinous cords proceeding from their shells, 1086

Natural phenomena, a world of miniature, 690

Nails, why have we got them at the fingers' ends, 908

Nails, why is there a circular line of whitish colour at the root of the, 909

LESSON LI.

Nails, why do white spots occur upon the, 916

Needle, why will it float if laid carefully upon water, 795

Nerves, what are they, 951

Nervous system, of what does it consist, 955

Nerve, what is the constitution of a, 956

Nervous fluid, what is the, 957

Nerves, how many kinds are there, 958

Nerves of motion, what are they, 959

Nerves of sensation, what are they, 960

Nerves of special sense, what are, 961

Nerves of sympathy, what are the, 962

Nights, why are they usually cold when clear, 411

Nitrogen, what is it, 33

Nitrogen, where is it found, 34

Nitrogen, could animals live in it, 37

Nitrogen, is it taken into the blood from the air, 40

Nitrogen, will it burn, 44

Nitrogen, what becomes of that which is inhaled, 46

Nitrogen, where does it find a fresh supply of oxygen, 47

Nitrous oxide, why does it excite the system, 38

Non-conductors of heat, what substances are, 111

Nostrils, why are they directed downwards, 994

LESSON LII.

Nose, why is it placed over the mouth, 995

Nutmegs, where are they produced, 1232

Ocean, how is it heated, 233

Oils and fats, what are vegetable, 1204

Oils and fats, animal, why are they found most abundantly in cold climates, 1205

Oils and fats, vegetable, why are they found most abundantly in hot countries, 1206

Opium, what is it, 1258

Ostriches, why have they small wings, 1031

Ostriches, why are their feathers soft and downy, 1032

Otters, seals, &c., why have they web-feet, 1062

Owls, why does their screeching denote change of weather, 1111

Owls, why does their moping foretell death, 1112

Oxen, sheep, deer, &c., why do they ruminate, 1088

Oxen, and other quadrupeds, why have they a tough elastic ligament in their necks, 1138

Oxygen, why is it necessary to life, 5

Oxygen, what is its union with carbon called, 7

LESSON LIII.

Oxygen, what is it, 25

Oxygen of the air, why does it not take fire, 28

Oxygen, why do we know that it will not burn of itself, 29

Oxygen, why do we know that it is necessary to our existence, 30

Oxygen, where is it found, 31

Oxygen, why is it mixed with nitrogen in the air, 32

Oxygen of water, why does it not support fire, 307

Oxygen, in what way does man use it, 757

Ozone, what is it, 630

Ozone, why do we know that electricity produces it, 631

Ozone, what are the properties of, 632

Paleness, what is the cause of, 914

Palms, what are their characteristics, 1221

Paper held over a candle-flame, why does it become scorched, 244

Paper held below a candle-flame, why does it scarcely become warm, 245

Paper, why does it more readily ignite than wood, 294

Paper, why, if it is laid flat upon the fire, will it "char" before it ignites, 304

LESSON LIV.

Paper on a fire, why will it ignite when you send a puff of air to it, 305

Parachutes, why do they fall gradually to the ground, 834

Parrots, &c., why have they crooked bills, 1047

Parrots, why can they move their upper as well as their lower beak, 1048

Particles of matter, why do they draw near each other, 776

Particles of matter, why will they attach themselves to sealing-wax excited by friction, 779

Pea and pin, why do they rotate upon a jet of air blown through a tobacco-pipe, 845

Peg-top, why does it make less noise than a humming-top, 811

Peg-top, why does it sometimes hum, and at other times not, 812

Pelican, why has it a large pouch under its bill, 1093

Pepper, where is it produced, 1229

Perspiration, why does it cool the body, 173

Perspire, why do we, 1009

Perspiration, how is it formed, 1010

Perspiration, what is insensible, 1011

Perspiration, what is sensible, 1012

LESSON LV.

Phosphorous, why does it look luminous, 94

Phosphorous, does it ignite spontaneously when placed upon a hot surface, 314

Phosphorous, why does it ignite when sprinkled with powdered charcoal, 315

Photographic pictures, how does light produce them, 556

Photographic rooms, why are they glazed with blue glass, 557

Pith-tumblers, why do they always pitch on one end, 868

Planets, how can man weigh them, 785

Planets, how can man measure their distances, 786

Plants, do they absorb heat, 225

Plants, how do we know that they absorb heat, 226

Plants, why do screens prevent frost from killing them, 400

Plants, why, if we cut across their stems, do we see tubes arranged in parallel lines, 1159

Plants, why do some droop and turn to the earth after sunset, 1167

Plants, why are the seeds of many enclosed in rich juice or pulp, 1172

Plants, why have some tough curly tendrils, 1173

LESSON LVI.

Plants, why have peas tendrils, and beans none, 1174

Plants, why have grasses, &c., joints or knots in their stalks, 1176

Plants, what is the circulation of sap in, 1179

Plants, why does their sap ascend and descend, 1180

Plants, why do they suffer from the smoke of cities, 1188

Plants, why is it understood that some of them feel, 1141

Plants, why is it understood that some of them move, 1142

Plants, of what elementary substances are they composed, 1143

Plants, whence do they derive their elementary substances, 1144

Plants, how do they obtain carbon, 1145

Plants, how do they obtain oxygen, 1146

Plants, their decomposition of gases by day and by night, 1147

Plants, how do they obtain hydrogen, 1147

Plants, how do they obtain nitrogen, 1148

Plants, how do they apply their elements to the formation of their structures, 1149

Plants, how is their nutritive sap applied to their growth and nourishment, 1150

LESSON LVII.

Plants, why do they grow, 1154

Plants, why if we break the stem of a hyacinth do we see a glutinous fluid exude, 1155

Plants, why if we split the petal of a tulip do we see cells containing different colouring matters, 1156

Plants, why if we break a pea-shell across do we find a transparent membrane, 1157

Plants, why if we cut through a cabbage-stump do we find a tough coating enveloping a cellular substance, 1158

Plants, why are their seeds formed within the corollas of flowers, 1168

Plants, why does the flower of the poppy turn down during the early formation of seed, 1169

Plants, why have those of the pea-tribe a folding blossom, 1170

Plants, why are leaf-buds enclosed in scales which fall off as the leaves open, 1171

Plants, what is meant by "species" of, 1209

Plants, what is meant by "genus" of, 1210

Plants, progressive discovery of new species, 1211

Plants, what are the three great classes of, 1212

LESSON LVIII.

Plants, what are the characteristics of exogenous, 1213

Plants, what are the characteristics of cryptogamus, 1216

Plants, what are the effects of diverse climates upon, 1242

Plants, what are the effects of altitude upon mountains, 1243

Plants, what agencies influence their geographical distribution, 1250

Plants, what are the chief facts connected with the distribution of by man, 1251

Plants, why are herbaceous less solid than woody, 1265

Plants, why are the stalks of herbaceous generally cylindrical, 1266

Plants, what are the stomata of, 1267

Plants, why are their stomata generally on the underside of their leaves, 1268

Plants, why have they pith in their centres, 1269

Porter, why does bottled produce large volumes of froth, 801

Pulse, why do we feel it beat, 900

Quadrupeds, geographical distribution of, 1331

Quicksilver, why does it reflect light from mirrors, 513

LESSON LIX.

Radiation, why does scratching a bright metal surface increase its powers of, 199

Radiation of heat, what benefits arise from it, 415

Radiation of light, what is it, 503

Radiation, do all bodies radiate light, 504

Rain, what is it, 416

Rain, does it ever occur without clouds, 417

Rain-drops, why are they sometimes large, and at other times small, 418

Rains, at what seasons are they most prevalent, 419

Rainy days, in what months of the year are they most frequent, 420

Rainy days, why are there more from September to March, 421

Rain, in what part of the world does the greatest quantity fall, 422

Rain, in what part of the world do the heaviest rains fall, 423

Rain, in what part of the world does the least rain fall, 424

Rainy days, how many are there in a year, 425

Rain, in what part of England does the greatest quantity fall, 426

Rains, why do the heaviest occur in hot countries, 427

LESSON LX.

Rain, why does the greatest quantity fall at the equator, 428

Rain, why are some parts of the earth without it, 429

Rain, why does it purify the air, 433

Rainy localities, why are mountainous countries more rainy than flat ones, 434

Rain, why does more fall by night than by day, 435

Rain, why do bunches of dried sea-weed indicate its coming, 436

Rain, why do weather-toys foretell its coming, 437

Rain, the Capuchin toy; mysterious walk of a wooden horse, 438

Rain, why does ladies' hair drop out of curl as rain approaches, 438

Rain, why is it said to be coming, when the mountains are "putting on their night-caps", 439

Rainbow, what causes a, 486

Rainbow, why does it exhibit colours, 487

Rainbows, why are there sometimes two, 488

Rainbows, why are the columns of the secondary bow, arranged in the reverse order of those of the primary, 489

LESSON LXI.

Rainbow colours, why do glass lustres and chandeliers exhibit them, 498

Rainbows, why are the colours of the secondary bow fainter than those of the primary, 491

Rainbow, what is a lunar, 492

Rainbow, why is a lunar bow fainter than the solar, 493

Rainbow, why is one in the morning the shepherd's warning, 538

Rainbow, why is one at night the shepherd's delight, 539

Refraction of light, what is it, 472

Refraction and reflection of light, what is the difference, 502

Reflections, why are they reversed, 490

Reflection, why do black bodies reflect any light, 505

Reflection, why does a window-pane appear to reflect better by night than by day, 516

Reflection, what is the line of, 520

Reflection, what is the angle of, 522

Repulsion, what is it, 792

Respiration, why does the chest expand when we breathe, 889

Respiration, how does blood communicate with the air in the lungs, 890

Rest, why does it invigorate us, 1018

Rice, where is it cultivated, 1219

Rosewood, what is it, 1191

LESSON LXII.

Saliva, why does saliva enter the mouth while we are eating, 872

Sea, why is it salt, 764

Sea, what is the estimated amount of salt in the, 765

Sea, what is the depth of the, 766

Sea-gulls, why are they numerous in fine weather, 1119

Sea-gulls, why do they fly over the land on the approach of stormy weather, 1120

Sea-crow, why is its lower bill longer than the upper, 1135

Sealing-wax, why does rubbing it attract to it small particles of matter, 565

See, why do we, 963

Seeds, why are they generally enveloped in hard cases, 1152

Seeds, why do they throw out roots before they form leaves, 1153

Seeds, why does the leaf-germ come up to the light and the root-germ penetrate the earth, 1285

Seeds, why are they indigestible, 1286

See-saw, why may a little boy balance a larger boy, 863

See-saw, why does the little boy sink to the earth, when the larger boy kicks the ground, 861

LESSON LXIII.

See-saw, why may the little boy keep the larger one up, when once he is up, 864

Shadows, what is the cause of, 508

Shadows, why is there some light where shadows fall, 510

Shadows, why do they lengthen as the sun goes down, 526

Shuttle-cock, why does it travel slowly through the air, 836

Shuttle-cock, why do we hear a noise when we strike it with the battledore, 837

Sight, why are two persons able to see each other, 551

Sight, why can we see so many upon the small retina, 966

Sight, why are we able to see at long or short distances, 974

Sigh, why do we, 1025

Silica, what is it, 1260

Silk, what is it, 1200

Skin, why does a chill of the produce inflammatory action in the lungs, 1014

Sky, what is it, 530

Sky, why is it red at sunset, 532

Sleet, what is it, 447

Sleep, what is it, 1019

Smell, why do we, 992

Smoke, what is it, 102

Smoke, why is there so little when the fire is red, 105

Smoke, what is it, 289

LESSON LXIV.

Smoke, why do fresh coals increase the quantity of, 291

Smoke, why does it issue in folds and curls, 632

Smoke, why does it ascend in mild and fine weather, 689

Snails, why can they move in an inverted position, 666

Snails, where do they obtain their shells, 1306

Snails, why do their shells grow, 1307

Snails, why are their shells spiral, 1308

Snails, why have they four tentacula attached to their heads, 1309

Snails, why are they able to move without feet, 1310

Snails, why do we see none in the winter time, 1311

Snails, why can they live in sealed shells, 1312

Sneeze, why do we, 1024

Snipes and woodcocks, why have they long, tapering bills, 1042

Snore, why do we, 1028

Snow, why does it keep the earth warm, 160

Snow, why is it a good non-conductor of heat, 162

Snow, what is it, 440

Snow, why is it white, 441

LESSON LXV.

Snow, why is it warm, though white garments are cool, 442

Snow, why is it always on the tops of high mountains, 443

Snow-line, what is meant by the, 445

Snow, what is red, 446

Soda-water, why does it effervesce, 802

Soils, why are clayey unfavourable to vegetation, 1160

Soils, why are sandy unfavourable to vegetation, 1162

Soils, why are chalk unfavourable to vegetation, 1162

Soils, why are mixed favourable to vegetation, 1163

Soils, why do farmers manure their land, 1165

Soot, why should it be prevented from accumulating at the bottom and sides of saucepans, 205

Soot, what is it, 290

Sound, what is it, 716

Sounds, what causes the air to produce, 717

Sounds, how do we know that they are produced by vibrations, 718

Sounds, how do we know that without air there would be none, 719

Sounds, how are the vibrations of sonorous bodies imparted to the air, 720

LESSON LXVI.

Sounds, how rapidly do the vibrations of travel, 721

Sounds, do all sounds travel at the same rate, 722

Sounds, why are bells and glasses stopped from ringing by touching them with the finger, 723

Sounds, why does a cracked bell give discordant, 724

Sounds, why do we see the flash of a gun before we hear the report, 725

Sound, why does the marching of long ranks of soldiers appear to be irregular, 726

Sounds, what are the numbers of vibrations that produce various, 727

Sounds, why does the length of a wire or string influence the sounds it produces, 728

Sound, why does the tension of a wire or string affect its vibrations, 729

Sound, why are some notes low and solemn, and others high and quick, 730

Sound, why can our voices be heard at a greater distance when we speak through tubes, 731

Sound, is air a good conductor, 732

Sounds, why can we hear them at a greater distance on water than on land, 733

LESSON LXVII.

Sound, why do sea-shells give a murmuring noise when held to the ear, 734

Sound, why can people in the arctic regions converse when more than a mile apart, 735

Sounds, why do savages lay their heads upon the earth to catch sounds, 736

Sounds, why can church clocks be heard striking more plainly at some times than at others, 737

Sound, why may the scratching of a pin at one end of a long pole be heard at the opposite extremity, 738

Sound, why is the hearing of deaf persons assisted by ear-trumpets, 739

Sounding-boards, why are they used to improve the hearing of congregations, 740

Sounds, why, when we are walking under arches or tunnels, do our voices appear louder, 747

Spark, what causes it when a horse's shoe strikes against a stone, 340

Specific gravity, what is it, 789

Spiders, why have they the power of spinning webs, 1082

Spiders, why may fine weather be expected when they build their webs, 1108

LESSON LXVIII.

Spiders, why may wet weather be expected when they hide, 1109

Spiders, why may wet weather be expected when they break off their webs and remove them, 1113

Spider, why, if the webs of the gossamer fly about in autumn, may east winds be expected, 1124

Spiders, gossamer, why can they float through the air, 1125

Spinal cord, what is the, 952

Spinal cord, why is it placed in the back-bone, 953

Spinal-cord, how do the branches pass out from it, 954

Spontaneous combustion, what substances are liable to it, 317

Spontaneous combustion, has it ever occurred in living bodies, 319

Spontaneous combustion, why does it occur in the case of the drunkard, 320

Spoon-bill, why has it a long expanded bill, lined with sharp, muscular points, 1045

Spoon-bill, why has it long legs, 1046

Squint, why do some people, 967

Starch, what is it, 1202

Star-lit nights, why are they usually colder than cloudy nights, 350

Stars, why do they twinkle, 484

LESSON LXIX.

Stars, why does their twinkling foretell bad weather, 485

Steam, why does it issue from the spout of a kettle, 750

Storms, what is the cause of, 676

Storms, why do the most violent occur in and near the tropics, 677

Straw, why is it frequently used for manure, 1264

"Sucker," why does it raise a stone, 860

Sugar, what is it, 1197

Sugar-cane, where is it cultivated, 1226

Sun, what is its distance from the earth, 452

Sun, what is the, 455

Sun, from what does its luminosity arise, 456

Sun and moon, why do they appear smaller when near the meridian, than when near the horizon, 525

Sun, why do we see it before sunrise, and after sunset, 482

Sun, what is the magnitude of the, 787

Suppers, why do they cause dreams, 1021

Swallows, why may wet weather be expected when they fly low, 1104

Syringe, why does pressing in the handle force out a jet of water, 856

Syringe, why will not the water run out, unless the handle is pressed in, 857

LESSON LXX.

Syringe, why will the water leak out, but not run, 858

Syringe, why cannot the handle be pressed in, if a finger is kept on the orifice, 859

Tannin, what is it, 1257

Taste, why do we, 996

Taste, why are some substances sweet, others sour, &c., 997

Taste, why is it most powerful after substances have been a little while in the mouth, 998

Taste, why if we put out the tongue and touch it with a nub of sugar, shall we perceive no taste, 999

Taste, why when we draw in the tongue do we then perceive the taste of the sugar, 1000

Taste, through what nerves are we made sensible of the contact of sugar with the tip of the tongue, 1001

Taste, why do connoisseurs of wines hold wine in their mouths a few seconds when judging of it, 1002

Taste, why do they also pass the fumes of the wine through their nostrils, 1003

Tea-pot, why does a bright metal one produce better tea than a black earthenware, 200

LESSON LXXI.

Tea-pot, if the earthenware one were set by the fire, why would it then make the best tea, 201

Tea, what is it, 1192

Tea, where is it cultivated, 1225

Tears, what is the cause of, 977

Temperature, why do some articles feel colder than others, 138

Temperature, why does it feel warmer after a frost has set in, 163

Tendons, what are they, 939

Tendons, why are they used to attach the muscles to the bones, 940

Teneriffe, what are the botanical regions of the Peak of, 1241

Thaumatrope, why do the figures on appear to dance, 869

Thaw, why is it colder when a thaw takes place, 164

Thermometer, what is the, 709

Thermometer, why does it indicate degrees of heat, 710

Thermometer, why are there Reaumur's and Fahrenheit's, 711

Thermometer and barometer, what is the difference, 712

Thermometer, in what season of the year does it vary most, 715

Thunder, what is it, 614

Thunder-peal, why is it sometimes loud and continuous, 616

LESSON LXXII.

Thunder-peal, why is it sometimes broken and unequal, 617

Thunder-peal, why is it sometimes a low, grumbling noise, 618

Thunder-peal, why does it sometimes follow immediately after the flash of lightning, 620

Thunder, through what distance will the sound travel, 621

Thunderbolt, what is a, 628

Tinder-box, the history of a, 340

Toasting-fork, why has it a wooden handle, 124

Tops, why do they stand erect while they spin, but fall when they stop, 814

Tops, why do they "sleep", 816

Tops, why do they cease to spin, 817

Touch, in what part of the body is the sense of most perfect, 1005

Toxicologists, what are they, 61

Trap and ball, why is the ball propelled upward, when the trigger is struck, 866

Trees, what are the estimated ages of, 1214

Trees, what are the northern limits of, 1237

Trees, why are they covered with bark, 1270

LESSON LXXIII.

Trees, why have those with large trunks a great number of leafy branches, 1274

Trees, why have poplars comparatively few leaves, 1275

Trees, why had the mammoth comparatively few leaves, 1276

Trees, why have oaks an abundance of leaves, 1277

Trees, why are their trunks generally round, 1278

Twilight, what is the cause of, 482

Vacuum, what is a, 649

Vacuum, is it possible to obtain a perfect, 650

Vegetables, have they any heat, 90

Vegetable structures, of what do they consist, 1151

Vegetables, why do farmers sow different crops in rotation, 1164

Vegetables, why are the hearts of cabbages, &c., pale yellow, 1182

Vegetable productions, why are they so widely diffused, 1189

Vegetable eaters, why do they feed so continually, 1091

Vegetation, geographical distribution of, 1208

Vegetation, what are the characteristics of tropical, 1218

Vegetation, what are the changes in on quitting the tropics, 1232

LESSON LXXIV.

Vegetation, what are the characteristics of, upon approaching the polar zones, 1234

Vegetation, what are the characteristics of mountain, 1238

Vines, where are their favourable climates, 1233

Walking, why does it make us warmer, 26

Water, what becomes of that formed by combustion, 58

Water, is it a good or bad conductor of heat, 151

Water, why does it extinguish fire, 306

Water, why, when a blacksmith thrusts a hot iron into a tank, do we perceive a peculiar smell, 309

Water, why does it freeze, 324

Water, why does it become steam, 326

Water, how many degrees of latent heat are hidden in its several states, 327

Water, why does it expand when freezing, while bodies generally contract with cold and expand with heat, 341

Water, why does it never freeze to a great depth, 342

Water, how much deeper is it than it appears to be, 480

LESSON LXXV.

Water, why has the exceptional law, by which it expands when freezing, been ordained, 343

Water, why can we seldom succeed in the first attempt to touch anything lying at the bottom of, 481

Water, what causes it to flow from a pump, 651

Water, why does it run through a syphon, 655

Water, what is it, 748

Water, why does it become solid when it freezes, 751

Water, why does it boil, 753

Water, what proportion of the earth's surface is covered by it, 554

Water, why does it dissolve substances, 758

Water, why does hot dissolve more readily than cold, 759

Water, why is it sometimes hard, 760

Water, why is rain-water soft, 761

Water, why is it difficult to wash in hard, 763

Waters, why are some impregnated with mineral matters, 767

Water, why does stagnant become putrid, 769

Water, is there danger attending drinking it on account of animalculæ, 770

Water, what are the means by which it may be purified, 771

LESSON LXXVI.

Water, what is the pressure of, 775

Water, why will a drop upon the blade of a knife leave a dark spot, 777

Water, why does it roll in agitated globules when dropped upon hot iron, 796

Water, why does oil float upon it, 797

Water, why is spring fresh and invigorating, 803

Water, why is boiled flat and insipid, 804

Weather, why does a yellow sunset foretell wet, 534

Weather, why does a red sunset foretell fine, 535

Weather, why does a red sunrise foretell wet, 536

Weather, why does a grey sunrise foretell dry, 537

Weather, barometrical indications of, 708

Whales, why have they a large development of oily matter about their heads, 1068

Whale, why has it feathery bones extending from its jaws, 1095

Whales, why are, their eyes provided with very thick coats, 1131

Wheat, why do the ears stand up by day and turn down by night, 1175

Wheat, what is it, 1199

LESSON LXXVII.

Wheat, what is the northern limit of, 1235

Wheat, why is silica diffused over its stem, 1261

Wheat-crops, why do they greatly exhaust the soil, 1263

Whirlwinds, what are they, 678

Why do we see, 963

Why do we wink, 975

Why do we weep, 977

Why do we hear, 983

Why do we taste, 986

Why do we smell, 992

Why do we feel, 1004

Why do we sleep, 1019

Why do we dream, 1020

Why do suppers produce dreaming, 1021

Why do we yawn, 1022

Why do we cough, 1023

Why do we sneeze, 1024

Why do we sigh, 1025

Why do we laugh, 1026

Why do we hiccough, 1027

Why do we snore, 1028

Why do we feel hungry, 1337

Why do we feel thirsty, 1338

Wick of a candle, why does it turn black as it burns, 225

Wick of a candle, why is there a spark generally at the end of it, 256

Will-o'-the-wisp, what is it, 318

Winds, what is the cause of, 234

Winds, why are east usually dry, 384

Winds, why are west usually wet, 385

Winds, why are north usually cold and dry, 386

LESSON LXXVIII.

Winds, why are south warm and rainy, 387

Wind, what is it, 666

Winds, what are the velocities of, 667

Winds, what are trade, 668

Winds, what is the cause of trade, 669

Winds, why do trade winds blow from east to west, 670

Winds, what determines the character of, 674

Wind-mills, why do their wings turn round, 690

Windows, why do they reflect the sun's rays in the evening, 523

Windows, why do they not reflect the sun's rays at noon, 524

Wood, why does decayed look luminous, 95

Wood, burning at one end, why does it not feel hot at the other, 118

Wood, why is it a bad conductor of heat, 119

Wood that is green, why does it hiss and steam when burning, 285

Wood, why does it ignite less readily than paper, 297

Woodcocks and snipes, why have they nerves running down to the tip of their bills, 1043

Woodpeckers, why do they "tap" at old trees, 1066

Woollens, why are they worn in winter, 166

Wool, what is it, 1201

Yawn, why do we, 1026

Zoological geography, 1326

[Verse: "God looked down from heaven upon the children of men, to see if there were any that did understand that did see God."--PSALM LIII.]

THE REASON WHY.

CHAPTER I.

1. _Why should we seek knowledge?_

Because it assists us to comprehend the _goodness and power of God_.

And it gives us power over the circumstances and associations by which we are surrounded: the proper exercise of this power will greatly promote our happiness.

2. _Why does the possession of knowledge enable us to exercise power over surrounding circumstances?_

Knowledge enables us to understand that, in order to live healthily, we require to breathe fresh and pure air. It also tells us that animal and vegetable substances, undergoing decay, poison the air, though we may not be able to see, or to smell, or otherwise discover the existence of such poison. Knowing this, we become careful to remove from our presence all such matters as would tend to corrupt the atmosphere. This is only one of the countless instances in which knowledge gives us power over surrounding circumstances.

3. _Name some other instances in which knowledge gives us power._

Knowledge of _Geography_ and of _Navigation_ enables the mariner to guide his ship across the trackless deep, and to reach the sought-for port, though he had never before been on its shores.

Knowledge of _Chemistry_ enables us to separate or to combine the various substances found in nature. Thus we obtain useful and precious metals from what at first appeared to be useless stones; transparent glass from pebbles, through which no light could pass; soap from oily substances; and gas from solid bodies.

[Verse: "Give instruction to a wise man, and he will be yet wiser; teach a just man, and he will increase in learning."--PROVERBS IX.]

Knowledge of _Medicine_ enables the physician to overcome the ravages of disease, and to save suffering patients from sinking prematurely to the grave.

Knowledge of _Anatomy_ and of _Surgery_ enables the surgeon to bind up dangerous fractures and wounds, and to remove, even from the internal parts of bodies, ulcers and diseased formations that would otherwise be fatal to life.

Knowledge of _Mechanics_ enables man to increase his power by the construction of machines. The steam-ship crossing the ocean in opposition to wind and tide, the railway locomotive travelling at 60 miles an hour, and the steam-hammer beating blocks of iron into useful shapes, are evidences of the power which man acquires through a knowledge of mechanics.

Knowledge of _Electricity_ enables man to stand in comparative safety amid the awful war of the elements. Lightning, the offspring of electricity, has a tendency to strike upon lofty objects by which it may be attracted. By its mighty powers churches or houses may be instantly levelled with the dust. But man, knowing that electricity is strongly attracted by particular substances, raises over lofty buildings rods of steel communicating with bars that descend into the ground. The lightning, rushing with indescribable force toward the steeple, is attracted by the bar of steel, and conducted harmlessly to the earth. Man may thus be said to take even lightning by the hand, and to divert its destroying force by the aid of Knowledge. And in countless other instances "Knowledge is Power."

CHAPTER II.

4. _Why do we breathe air?_

Because the air contains _oxygen_, which is necessary to life.

5. _Why is oxygen necessary to life?_

Because it combines with the _carbon_ of the blood, and forms _carbonic acid gas_.

[Verse: "Be not as the horse, or as the mule, which have no understanding: whose mouth must be held with the bit and bridle."--PSALM XXXII.]

6. _Why is this combination necessary?_

Because we are so created that the substances of our bodies are constantly undergoing change, and this resolving of solid matter into a gaseous form, is the plan appointed by our Creator to remove the matter called _carbon_ from our systems.

7. _Why do our bodies feel warm?_

Because, in the union of _oxygen_ and _carbon_, heat is developed.

8. _What is this union of oxygen and carbon called?_

It is called _combustion_, which, in chemistry, means the decomposition of substances, and the formation of new combinations, accompanied by heat; and sometimes by light, as well as heat.

9. _What is formed by the union of oxygen and carbon?_

Carbonic acid gas.

10. _What becomes of this carbonic acid gas?_

It is sent out of our bodies by the compressure of the lungs, and mingles with the air that surrounds us.

11. _Is this carbonic acid gas heavier or lighter than the air?_

Pure carbonic acid gas is the heaviest of all the gases. That which is sent out of the lungs is not pure, because the whole of the air taken into the lungs at the previous inspiration has not been deprived of its _oxygen_, and the nitrogen is returned. Therefore the breath sent out of the lungs may be said to consist of _air_, with a large proportion of _carbonic acid gas_.

12. _What is the composition of air in its natural state?_

It consists of _oxygen_, _nitrogen_, and _carbonic acid gas_, in the proportions of oxygen 20 volumes, nitrogen 79 volumes, and carbonic acid gas 1 volume. It also contains a slight trace of watery vapour.

13. _What is the state of the air after it has once been breathed?_

It has parted with about one-sixth of its oxygen, and taken up an equivalent of carbonic acid. And were the same air to be breathed six times successively, it would have parted with _all_ its oxygen, and could no longer sustain life.

[Verse: "A prudent man forseeth the evil, and hideth himself; but the simple pass on, and are punished."--PROVERBS XXVII.]

14. _Is the impure air sent out of the lungs lighter or heavier than common air?_

At first, being rarefied by warmth, it is _lighter_. But, if undisturbed, it would become _heavier_ as it cooled, and would descend.

15. _Why is it proper to have beds raised about two feet from the ground?_

Because at night, the bed-room being closed, the breath of the sleeper impregnates the air of the room with carbonic acid gas, which, descending, lies in its greatest density near to the floor.

16. _What are the chief sources of carbonic acid gas?_

The vegetable kingdom (as will be hereafter explained), the combustion of substances composed chiefly of carbon, the breathing of animals, and the decomposition of carbonic compounds.

17. _Is breathing a kind of combustion?_

It is. In the breathing of animals, the burning of coals, or of wood, or candles, &c., similar changes occur. The _oxygen_ of the air combines with the _carbon_ of the substance said to be burnt, and forms _carbonic acid gas_, which unfits the air for the purposes of either breathing or of burning, until it has been renewed by admixture with the air.

18. _What is carbon?_

It is one of the elementary bodies, and is very abundant throughout nature. It abounds mostly in vegetable substances, but is also contained in animal bodies, and in minerals. The form in which it is most familiar to us is that of _charcoal_, which is carbon almost pure.

19. _What is meant by an elementary body?_

An elementary body is one of those substances in which chemistry is unable to discover more than one constituent. For instance, the chemist finds that water is composed of _oxygen_ and _hydrogen_. Water is therefore a _compound_ body. But _carbon_ consists of _carbon only_, and therefore it is called a simple, or elementary body.

[Verse: "Where no wood is, there the fire goeth out: so where there is no tale-bearer, the strife ceaseth."--PROVERBS XXVI.]

20. _Why is it dangerous to burn charcoal in rooms?_

Because, being composed of _carbon_ that is nearly pure, its combustion gives off a large amount of _carbonic acid gas_.

21. _What is the effect of carbonic acid gas upon the human system?_

It induces drowsiness and stupor, which, if not relieved by ventilation, would speedily cause death.

22. _What is the reason that people feel drowsy in crowded rooms?_

Because the large amount of carbonic acid gas given off with the breaths of the people, makes the air poisonous and oppressive.

23. _What other causes of drowsiness are there?_

The candles, gas, or fires that may be burning in the rooms where people are assembled. Three candles produce as much carbonic acid gas as one human being; and it is probable that one gas-light produces as much carbonic acid gas as two persons.

24. _Have people ever been poisoned by their own breaths?_

In the reign of George the Second, the Rajah of Bengal took some English prisoners in Calcutta, and put 146 of them into a place which was called the "Black Hole." This place was only 18 feet square by 16 feet high, and ventilation was provided for only by two small grated windows. _One hundred and twenty-three of the prisoners died in the night_, and most of the survivors were afterwards carried off by putrid fevers. Many other instances have occurred, but this one is the most remarkable.

CHAPTER III.

25. _What is oxygen?_

Oxygen is one of the most widely diffused of the elementary substances. It is a gaseous body.

[Verse: "Stand in awe and sin not: commune with your own heart upon your bed and be still"--PSALM IV.]

26. _Why do persons who are walking, or riding upon horseback feel warmer than when they are sitting still?_

Because as they breathe more rapidly, the combustion of the _carbon_ in the blood is increased by the _oxygen_ inhaled, and greater heat is developed.

27. _Why does the fire burn more brightly when blown by a bellows?_

Because it receives, with every current of air, a fresh supply of _oxygen_, which unites with the _carbon_ and _hydrogen_ of the coals, causing more rapid combustion and increased heat.

28. _Why does not the oxygen of the air sometimes take fire?_

Because oxygen, _by itself_, is incombustible. The wick of a candle, which retains the slightest spark, being immersed in oxygen, will instantly burst into a brilliant flame; and even a piece of iron wire made red-hot, and dipped in oxygen, will burn rapidly and brilliantly. Oxygen, though non-combustible of itself, is the most powerful _supporter of combustion_.

29. _Why do we know that oxygen will not burn of itself?_

Because when we immerse a burning substance into a jar of oxygen, it immediately burns with intense brilliancy; but directly it is withdrawn from the oxygen, the intensity of the flame diminishes, and the oxygen which remains is _unaffected_.

30. _Why do we know that oxygen is necessary to our existence?_

Because animals placed in any kind of gas, or in any combination of gases, where oxygen _does not exist_, die in a very short time.

31. _Where is oxygen found?_

It is found in the air, mixed with _nitrogen_; in water combined with _hydrogen_; in the tissues of vegetables and animals; in our blood; and in various compounds called, from the presence of oxygen, _oxides_.

32. _Why is the oxygen of the air mixed so largely with nitrogen?_

Because _oxygen_ in any greater proportion than that in which it is found in the atmosphere, would be too exciting to the animal system. Animals placed in _pure oxygen_ die in great agony from fever and excitement, amounting to madness.

[Verse: "As vinegar is to the teeth, and as smoke to the eyes, so is the sluggard to him that sent him."--PROVERBS X.]

33. _What is nitrogen?_

Nitrogen is an elementary body in the form of gas.

34. _Where is nitrogen found?_

It is chiefly found in the air, of which it constitutes 79 out of 100 volumes. It may be mixed with oxygen in various proportions; but in the atmosphere it is uniformly diffused. It is found in most animal matter, _except fat and bone_. It is not a constituent of the _vegetable acids_, but it is found in most of the _vegetable alkalies_.

35. _What are acids?_

Acids are a numerous class of chemical bodies. They are generally sour. Usually (though there are exceptions) they have a great affinity for water, and are easily soluble therein; they unite readily with most _alkalies_, and with the various _oxides_. All acids are compounds of two or more substances. Acids are found in all the kingdoms of nature.

36. _What are alkalies?_

Alkalies are a numerous class of substances that have a great affinity for, and readily combine with, _acids_, forming _salts_. They exercise peculiar influence upon vegetable colours, turning blues green, and yellows reddish brown. But they will restore the colours of vegetable blues which have been reddened by _acids_; and, on the other hand, the _acids_ restore vegetable colours that have been altered by the _alkalies_. Alkalies are found in all the kingdoms of nature.

37. _Could animals live in nitrogen?_

No; they would immediately die. But a mixture of _oxygen_ and _nitrogen_, in equal volumes, constitutes _nitrous oxide_, which gives a pleasurable excitement to those who inhale it, causing them to be merry, almost to insanity; it has, therefore, been called _laughing gas_.

38. _Why does nitrous oxide produce this effect?_

Because it introduces into the body more _oxygen_ than can be consumed. It, therefore, deranges the nervous system, and being a powerful stimulant, gives an unnatural activity to the nervous centres and the brain.

[Verse: "Lord, make me know mine end, and the measure of my days, that I may know how frail I am."--PSALM XXXIX.]

39. _In what proportions are the atmospheric gases found in the blood?_

The mean quantity of the gases contained in the human blood has been found to be equal to 1-10th of its whole volume. In _venous_ blood, the average quantity of _carbonic acid_ is about 1-18th, that of _oxygen_ about 1-85th, and that of _nitrogen_ about 1-100th of the volume of the blood. In _arterial_ blood their quantities have been found to be _carbonic acid_ about 1-14th, _oxygen_ about 1-38th, and _nitrogen_ about 1-72nd.

40. _Then is nitrogen taken into the blood from the air?_

Such a supposition is highly improbable. It is probably derived from _nitrogenised food_, just as _carbonic acid_ is derived from _carbonised food_.

41. _What is venous blood?_

Venous blood is that which is returning through the _veins_ of the body from the organs to which it has been circulated.

42. _What is arterial blood?_

Arterial blood is that which is flowing from the heart through the _arteries_ to nourish the parts where those arteries are distributed.

43. _What is the difference between venous and arterial blood?_

Venous blood contains _more_ carbonic acid, and _less_ oxygen and nitrogen than arterial blood.

44. _Will nitrogen burn?_

It will not burn, nor will it support combustion.

45. _What is the difference between "burning" and "supporting combustion?"_

Oxygen gas will not burn of itself, but it aids the decomposition by fire of bodies that are combustible. It is therefore called a _supporter of combustion_. But hydrogen gas, _though it burns of itself_ will extinguish a flame immersed in it. It is therefore said to be a body which will _burn_, but _will not support combustion_.

[Verse: "As coals are to burning coals, and wood to fire; so is a contentious man to kindle strife."--PROVERBS XXVI.]

46. _What becomes of the nitrogen that is inhaled with the air?_

It is thrown off with the breath, mixed with _carbonic acid gas_, and flies away to be renewed by a fresh supply of oxygen.

47. _Where does nitrogen find a fresh supply of oxygen?_

In the atmosphere. Nitrogen is said to possess a remarkable tendency to _mix_ with oxygen, without having a positive chemical _affinity_ for it. That is to say, neither the _oxygen_ nor the _nitrogen_ undergoes any change by the union, except that of _admixture_. The oxygen and the nitrogen still possess their own peculiar properties. Oxygen and nitrogen are found in nearly the same proportions in all climates, and at all altitudes.

48. _In combustion does any other result take place besides the union of oxygen and carbon forming carbonic acid gas?_

Yes. Usually _hydrogen_ is present, which in burning unites with _oxygen_, and forms _water_.

CHAPTER IV.

49. _What is hydrogen?_

Hydrogen is an elementary gas, and is the lightest of all known bodies.

50. _Will hydrogen support animal life?_

It will not. It proves speedily fatal to animals.

51. _Will hydrogen support combustion?_

Although it will burn, yielding a feeble bluish light, it will, if pure, extinguish a flame that may be immersed in it. Hydrogen will therefore _burn_, but will not _support combustion_.

52. _Why will hydrogen explode, if it will not support combustion?_

When hydrogen explodes it is always in combination with _oxygen_, or with the common air, which contains _oxygen_. _Two_ measures of hydrogen and _one_ of oxygen form a most explosive compound.

[Verse: "As smoke is driven away, so drive them away: as wax melteth before the fire, so let the wicked perish at the presence of God."--PSALM XLVI.]

53. _Why does hydrogen explode, when mixed with oxygen, upon being brought in contact with fire?_

Because of its strong affinity for _oxygen_, with which, upon the application of heat, it unites to form water.

54. _Where does hydrogen chiefly exist?_

In the form of _water_, where it exists in combination with _oxygen_. _Eleven_ parts of hydrogen, and _eighty-nine_ of oxygen, form water.

55. _Is hydrogen found elsewhere?_

It is never found but in a state of combination; united with oxygen, it exists in _water_; with nitrogen, in _ammonia_; with chlorine, in _hydro-chloric acid_; with fluorine, in _hydro-fluoric acid_; and in numerous other combinations.

56. _Is the gas used to illuminate our streets, hydrogen gas?_

It is; but it is combined with carbon, derived from the coals from which it is made. It is therefore called _carburetted hydrogen_, which means _hydrogen_ with _carbon_.

57. _How is hydrogen gas obtained from coals?_

It is driven out of the coals by heat, in closed vessels, which prevent its union with _oxygen_.

58. _What becomes of the water which is formed by the burning of hydrogen in oxygen?_

It passes into the air in the form of watery vapour. Frequently it condenses, and may be seen upon the walls and windows of rooms where many lights or fires are burning. Sometimes, also, portions of it become condensed in the globes of the glasses that are suspended over the jets of gas. _A large volume of these gases forms only a very small volume of water._

59. _What becomes of the carbonic acid gas which is produced by combustion?_

It is diffused in the air, which should be removed by adequate ventilation.

[Verse: "I will both lay me down in peace and sleep: for thou, Lord, only, makest me dwell in safety."--PSALM IV.]

60. _What proportion of carbonic acid gas is dangerous to life?_

Any proportion over the natural one of 1 per cent. may be regarded as _injurious_. But toxicologists state that _five per cent._ of carbonic acid gas in the atmosphere is _dangerous_ to life.

61. _What are toxicologists?_

Persons who study the nature and effects of poisons and their antidotes.

62. _Which kind of combustible used for lighting tends most to vitiate the air?_

Assuming all the lights to be of the same intensity, the degree in which the substances burnt would vitiate the atmosphere may be gathered from the number of minutes each would take to exhaust a given quantity of air. This has been found to be: rape oil, 71 minutes; olive oil, 72; Russian tallow, 75; town tallow, 76; sperm oil, 76; stearic acid, 77; wax candles, 79; spermaceti candles, 83; common coal gas, 98; canal coal gas, 152. Thus it is shown that rape oil is _most destructive_ of the atmosphere, and that coal gas is the _least destructive_.

63. _Is an escape of hydrogen gas from a gas-pipe dangerous to life?_

It is dangerous, first, by _inhalation_. There are no less than six deaths upon record of persons who were killed by sleeping in rooms near to which there was a leakage of gas.

It is dangerous, secondly, by _explosion_.

In 1848, an explosion of gas occurred in Albany-street, Regent's-park, London. The gas accumulated in a shop for a very short time only. It had been escaping from a crack in the meter for about one hour and twenty minutes. The area of the room was about 1,620 cubic feet. When the gas exploded, it blew out the entire front of the premises, carried two persons through a window into an adjoining yard, and forced another person on to the pavement on the opposite side of the street, where she was killed. The effect of the explosion was felt for more than a quarter of a mile on each side of the house, and most of the windows in the neighbourhood were shattered. The iron railings over the area of the house directly opposite were snapped asunder; and a part of the roof, and the back windows of another house, were carried to a distance of from 200 to 300 yards. The pavement was torn up for a considerable length, and the damage done to 103 houses was afterwards reported to amount to £20,000. Other serious explosions have taken place. The explosions of "_coal damp_," which frequently occur in mines, are of a similar character.

[Verse: "O Lord, our Lord, how excellent is thy name in all the earth! who hast set thy glory above the heavens."--PSALM VIII.]

64. _What proportion of hydrogen gas with atmospheric air will explode?_

According to the researches of Sir Humphrey Davy, _seven_ or _eight_ parts of _air_, to _one_ of _gas_, produce the greatest explosive effect; while _larger_ proportions of gas are less dangerous. A mixture of _equal parts_ of gas and air will burn, but it will not explode. The same is the case with a mixture of _two_ of _air_, or _three_ of _air_, and _one_ of _gas_; but _four_ of _air_ and _one_ of _gas_ begin to be explosive, and the explosive tendency increases up to _seven_ or _eight_ of _air_ and _one_ of _gas_, after which the increased proportion of gas diminishes the force of the explosion.

65. _What is the best method of preventing the explosion of gas?_

Observe the rule, _never to approach a supposed leakage with a light_. Fortunately the gas, which threatens our lives, warns us of the danger by its pungent smell. The first thing to be done is to open windows and doors, and to ventilate the apartment. Then turn the gas off at the main, and wait a short time until the accumulated gas has been dispersed.

66. _Does hydrogen gas rise or fall when it escapes?_

Being _twelve times lighter than common air_ it _rises_, and therefore it would be better for ventilation to open the window at the _top_ than at the _bottom_. But all gases exhibit a strong tendency to _diffuse themselves_, and therefore they do not rise or fall in the degree that might be anticipated.

67. _What proportion of hydrogen in the air is dangerous to life, if inhaled?_

One-fiftieth part has been found to have a _serious effect_ upon animals. The effects it produces upon the human system are those of depression, headache, sickness, and general prostration of the vital powers. It is therefore advisable to observe precautions in the use of gas.

[Verse: "From the place of his habitation he looketh upon all the inhabitants of the earth."--PSALM XXXIII.]

68. _What proportion of gas in the air may be recognised by the smell?_

By persons of acute powers of smelling it may be recognised when there is _one_ part of _gas_ in _five hundred parts of atmospheric air_; but it becomes very perceptible when it forms _one_ part in _a hundred and fifty_. Warning is, therefore, given to us long before the point of danger arrives.

69. _What other sources of hydrogen are there in our dwellings?_

It arises from the decomposition of animal and vegetable substances, containing _sulphur_ and _hydrogen_. These give off a gas called _sulphuretted hydrogen_, from which the fætid effluviam of drains and water-closets chiefly arise. We should, therefore, take every precaution to secure effective drainage, and to keep drain-traps in proper order.

70. _May the use of gas for purposes of illumination be considered highly dangerous?_

Not if it is intelligently managed. The appliances for the regulation of gas are so very simple and perfect, that accidents seldom arise except from neglect. In England 6,000,000 tons of coal are usually consumed in the manufacture of gas, producing 60,000,000,000 cubic feet of gas. And yet accidents are of very uncommon occurrence.

CHAPTER V.

71. _What is heat?_

Heat is a principle in nature which, like light and electricity, is best understood by its _effects_. We popularly call that heat, which raises the temperature of bodies submitted to its influence.

72. _What is caloric?_

Caloric is another term for heat. It is advisable, however, to use the term _caloric_ when speaking of the _cause_ of heat, and of _heat_ as the _effect_ of the presence of _caloric_.

[Verse: "While the earth remaineth, seed-time and harvest, and cold and heat, and summer and winter, and day and night, shall not cease."--GEN. VIII.]

73. _What is the source of caloric?_

The sun is its chief source. But caloric, in some degree, exists _in every known substance_.

74. _What are the effects of caloric?_

Heat which, in proportion to its intensity, acts variously upon all bodies, causing _expansion_, _fusion_, _evaporation_, _decomposition_, _&c._

75. _Why is caloric called a repulsive agent?_

Because its chief effects are to _expand_, _fuse_, _evaporate_, or _decompose_ the substances upon which it acts.

76. _What is an attractive agent, in contradistinction to a repulsive agent?_

Chemical attraction, or affinity, is an attractive agent--as when bodies seek of their own natures to unite and form some new body.

77. _When is a body said to be hot?_

When it holds so much _caloric_ that it diffuses heat to surrounding objects.

78. _When is a body said to be cold?_

When it holds less _caloric_ than surrounding objects, and absorbs heat from them.

79. _How may caloric be excited to develop heat?_

By any means which cause agitation, or produce an active change in the condition of bodies. Thus friction, percussion, sudden condensation or expansion, chemical combination, and electrical discharges, all develope _heat_.

80. _Why do "burning glasses" appear to set fire to combustible substances?_

Because they gather into one point, or _focus_, several rays of _caloric_ as they are travelling from the sun, and the accumulation of caloric developes that intensity of _heat_ which constitutes _fire_.

81. _What is a focus?_

In optics, it is the point or centre at which, or around which, divergent rays are brought into the closest possible union.

[Verse: "Yet man is born to trouble, as the sparks fly upward.--I would seek unto God, and unto God would I commit my cause."--JOB V.]

82. _What is fire?_

It is a violent chemical action attending the combustion of the ingredients of _fuel_ with the _oxygen_ of the air.

83. _What are the properties of fire?_

It imparts heat, which has the effect of expanding both fluids and solids.

It cannot exist without the presence of combustible materials.

It has a tendency to diffuse itself in every direction.

It cannot exist without oxygen or atmospheric air.

84. _What elements take part in the maintenance of a fire?_

Hydrogen, carbon, and oxygen. Hydrogen and carbon exist in the _fuel_, and oxygen is supplied by the _air_.

85. _How does the combustion of a fire begin?_

A match made of phosphorous and sulphur (highly inflammable substances) is drawn over a piece of sand-paper; the _friction_ of the match induces the presence of _caloric_, which developes _heat_, and ignites the match, the burning of which is sustained by the _oxygen_ of the air. The flame is then applied to paper or wood, and the heat of the flame is sufficient to drive out _hydrogen gas_, which unites with the _oxygen_ of the air, and burns, imparting greater heat to the _carbon_ of the coals, which assumes the form of carbonic acid gas by union with _oxygen_, and in a little while all the conditions of _combustion_ are established.

86. _What are the properties of heat?_

It may exist without _fire_ or _light_.

It is not sensible to _vision_.

It makes an impression upon our _feelings_.

It acts powerfully upon _all bodies_.

It has no _weight_.

It attends, or is connected with, _all the operations of nature_.

It radiates from _all bodies_ in straight lines, and in all _directions_.

It strikes most powerfully in _direct lines_.

Its rays may be collected into a _focus_, just as the rays of the sun.

It may be _reflected_ from a polished surface.

It is more easily _conducted_ by some substances than by others.

[Verse: "For my days are consumed like smoke, and my bones are burned as an hearth."--PSALM CII.]

87. _What is animal heat?_

Animal heat is derived from the slow combustion of _carbon_ in the blood of animals with the _oxygen_ of the air which the animals breathe.

88. _What is latent heat?_

Latent heat (or more properly _latent caloric_) is that which exists, in some degree, in all _bodies_, though it may be imperceptible to the _senses_.

89. _Is there latent caloric in ice, snow, water, marble, &c?_

Yes; there is some amount of _caloric_ in all substances.

A blacksmith may hammer a small piece of iron until it becomes _red hot_. With this he may light a match, and _kindle the fire of his forge_. The iron has become more dense by the hammering, and it cannot again be heated to the same degree by similar means, until it has been exposed _in fire_, to _a red heat_. Is it not possible that, by hammering, the particles of iron have been driven closer together, and _the latent heat_ driven out? No further hammering will force the atoms nearer, and therefore no further heat can be developed. But when the iron has _again absorbed caloric_, by being plunged in a fire, it is again charged with latent heat. Indians produce _sparks_ by rubbing together _two pieces of wood_. Two pieces of ice may be rubbed together until sufficient warmth is developed to _melt them both_. The axles of railway carriages frequently become _red hot_ from _friction_.

90. _Have vegetables heat?_

Yes; whenever oxygen combines with carbon to form carbonic acid gas, an extrication of heat takes place, however minute the amount. Such a combination occurs much more extensively during the germination of seeds and the impregnation of flowers, than at any other time. In the germination of barley heaped in rooms, previous to being converted into malt, it is well known that a _considerable amount of heat is developed_.

91. _Has any investigation of this subject ever been carefully made?_

Yes. Lamarck, Senebier, and De Candolle, found the flowers of the _Arum Maculatum_, between three and seven o'clock in the afternoon, as much as 7 deg. Reaum. warmer than the external air. Schultz found a difference of 4 deg. to 5 deg. between the heat of the spathe of the _Canadian pinnatifolium_ and the surrounding air, at six to seven o'clock p.m. Other observations have established differences of as much as 30 deg. between the temperature of the spathe of the _Arum cordifolium_, and that of the surrounding atmosphere.

[Verse: "And there are diversities of operations, but it is the same God which worketh in all."--CORINTHIANS XII.] 92. _Have plants sometimes a temperature lower than that of the surrounding air?_

Yes. It has not only been found that under particular circumstances the heat of certain parts of plants is elevated to a very remarkable degree, but that, under nearly all circumstances, they have a temperature different from that of the external air, being _warmer in winter, and cooler in summer_.

CHAPTER VI.

93. _How many kinds of combustion are there?_

There are _three_, viz., slow oxydation, _when little or no light is evolved_; a more rapid combination, _when the heat is so great as to become luminous_; and a still more energetic action, _when it bursts into flame_.

94. _Why does phosphorous look luminous?_

Because it is undergoing slow _combustion_.

95. _Why do decayed wood, and putrifying fish, look luminous?_

Because they are undergoing slow _combustion_. In these cases the heat and light evolved are at no one time very considerable. But the _total amount of heat_, and probably of _light_, generated through the lengthy period of this slow oxydation, _amounts to exactly the same as would be evolved during the most rapid combustion of the same substances_.

96. _What is flame?_

It is gaseous matter burning at a _very high temperature_.

97. _Why, when we put fresh coals upon a fire, do we hear the gas escaping from the coals without taking fire?_

Because, the fire being slow, the temperature is not high enough to ignite the gas.

[Verse: "I will praise thee, O Lord, with my whole heart; I will show forth thy marvellous work."--PSALM IX.]

98. _What is the gas which escapes from the coals?_

Carburetted hydrogen.

99. _Why, if we light a piece of paper, and lay it where the gas is escaping from the coals, will it burst into flame?_

Because the lighted paper gives a _heat sufficient_ to ignite the gas; and because also hydrogen requires the contact of _flame_ to ignite it.

100. _Why, when the coals have become heated, will the hydrogen burst into flame?_

Because the _carbon_ of the coals, and the _oxygen_ of the air, have begun to combine, and have greatly increased the _heat_, and have produced a rapid combustion, _so nearly allied to flame_, that it _ignites the hydrogen_.

101. _What temperature is required to produce flame?_

That depends upon the nature of the combustible you desire to burn. Finely divided phosphorous and phosphorated hydrogen will take fire at a temperature of 60 deg. or 70 deg.; solid phosphorous at 140 deg.; sulphur at 500 deg.; hydrogen and carbonic oxide at 1,000 deg. (red heat); coal gas, ether, turpentine, alcohol, tallow, and wood, at about 2,000 deg. (incipient white heat). When once inflamed they will _continue to burn_, and will maintain a very high temperature.

102. _What is smoke?_

Smoke consists of small particles of _carbon_ of _hydrogen gas_, and _other volatile matters_, which are driven off by heat and carried up the chimney.

103. _Is it not a waste of fuel to allow this matter to escape?_

It is, as it might all be burnt up by better management.

104. _How may the waste be avoided?_

By putting on only a little coals at a time, so that the heat of the fire shall be sufficient to consume these volatile matters as they escape.

[Verse: "And the strong shall be as tow, and the maker of it as a spark, and they shall both burn together, and none shall quench them."--ISAIAH I.]

105. _Why is there so little smoke when the fire is red?_

Because the _hydrogen_ and the _volatile_ parts of the _coal_ have already been driven off and consumed, and the combustion that continues is principally caused by the _carbon_ of the coals, and the oxygen of the air.

106. _Will carbon, burnt in oxygen, produce flame and smoke?_

It burns brightly, but it produces neither flame nor smoke.

107. _Why do not charcoal and coke fires give flame?_

Because the _hydrogen_ has been driven off by the processes by which charcoal and coke are made.

108. _What is a conductor of heat?_

A conductor of heat is any substance through which heat is _readily transmitted_.

109. _What is a non-conductor of heat?_

A non-conductor is any substance through which heat will _not_ pass readily.

110. _Name a few good conductors._

Gold, silver, copper, platinum, iron, zinc, tin, stone, _and all dense solid bodies_.

111. _Name a few non-conductors._

Fur, wool, down, wood, cotton, paper, and _all substances of a spongy or porous texture_.

112. _How is heat transmitted from one body to another?_

By Conduction, Radiation, Reflection, Absorption and Convection.

113. _What is the Conduction of heat?_

It is the communication of heat from one body to another _by contact_. If I lay a penny piece upon the hob, it becomes hot by _conduction_.

114. _What is the Radiation of heat?_

The transmission of heat by a _series of rays_. If I hold my hand before the fire, the rays of heat fall upon it, and _my hand receives the heat through radiation_.

[Verse: "Sing praises to the Lord, which dwelleth in Zion, declare among the people his doings."--PSALM IX.]

115. _What is the Reflection of heat?_

The reflection of heat is the _throwing back_ of its rays towards the direction whence they came. In a Dutch oven the rays of heat pass from the fire to the oven, and are _reflected_ back again by _the bright surface of the tin_. There is, therefore, considerable economy of heat in ovens, and other cooking utensils constructed upon this plan.

116. _What is the Absorption of heat?_

The absorption of heat is the taking of it up by the body to which it is transmitted or conducted. Heat was conveyed to my hand by _radiation_, and _taken up_ by my hand by _absorption_.

117. _What is the Convection of heat?_

The convection of heat is the transmission of it _through_ a body or a number of bodies, or particles of bodies, by those substances which _first received it_; as when hot water rises from the bottom of a kettle and imparts heat to the cold water lying above it.

CHAPTER VII.

118. _Why does not a piece of wood which is turning at one end, feel hot at the other end?_

Because wood is _a bad conductor of heat_.

119. _Why is wood a bad conductor of heat?_

Because the arrangement of the particles of which it is composed does not favour the transmission of _caloric_.

120. _Why do some articles of clothing feel cold, and others warm?_

Because some are bad conductors of heat, _and do not draw off much of the warmth of our bodies_; while others are _better conductors_, and _take up a larger portion of our warmth_.

[Verse: "The fining pot is for silver, and the furnace for gold: but the Lord trieth the hearts."--PROVERBS XVII.]

121. _Which feels the warmer, the conductor or non-conductor?_

The non-conductor, as it does not readily _absorb_ the warmth of our bodies.

122. _What substances are the best conductors of heat?_

Gold, silver, copper, and most substances of close and hard formation, &c.

123. _What substances are the worst conductors of heat?_

Fur, eider down, feathers, raw silk, wood, lamp-black, cotton, soot, charcoal, &c.

124. _Why has the toasting-fork a wooden handle?_

Because wood is not _so good a conductor_ as metal, therefore the wood prevents the heat from being transmitted _by conduction_ to our hands.

125. _Why has the coffee-pot a wooden handle?_

Because the metal of the coffee-pot would otherwise _conduct the heat to the hand_; but wood, _being a bad conductor_, prevents it.

126. _Why does hot water in a metal jug feel hotter than in an earthenware one?_

Because metal, being a good conductor, _readily delivers heat to the hand_; but _earthenware, being an indifferent conductor_, parts with the heat slowly.

127. _How can we ascertain that wood prevents the conduction of heat to the hand?_

By passing the top of the finger along the wooden handle of the coffee-pot, until it reaches the point where the wood meets the metal. The wooden handle will be found to be _cool_, but the metal will feel _very hot_.

128. _Of what use are kettle-holders?_

Being made of _bad conductors_, such as wood, paper, or woollen cloth, they will not readily _conduct_ the heat from the kettle to the hand.

[Verse: "Wisdom is the principal thing; therefore get wisdom: and with all thy getting get understanding."--PROVERBS IV.]

129. _Will a kettle-holder, being a bad conductor, sometimes conduct heat to the hand?_

Yes. But so slowly that the hand will not _feel the inconvenience of too much heat_.

130. _Why does hot metal feel hotter than heated wool, though they may both be of the same degree of temperature?_

Because metal gives out heat _more rapidly than wool_, by which it is made _more perceptible to our feelings_.

131. _Which would become cold first--the metal or the wool?_

The _wool_, because, although the metal conducts heat more rapidly, to a substance in contact with it, it does not _radiate heat_ as well as a _black and rough substance_.

132. _Why do iron articles feel intensely cold in winter?_

Because iron is one of the best conductors, and draws off heat from the hand very rapidly.

133. _What is the cause of the sensation called cold?_

When we feel cold, heat is being _drawn off from our bodies_.

134. _What is the cause of the sensation called heat?_

When we feel hot, our bodies are _absorbing heat_ from external causes.

The condition here implied is that of health, and of ordinary circumstances. A person in a condition of fever, suffering from intense heat arising from a diseased state of the blood, could not be said to be _absorbing heat_. Nor could such a description apply to a person who, by a very rapid walk, has raised the temperature of his body considerably above its natural state, by the _internal combustion_ which has already been described. A person feeling hot in bed, from excessive clothes, feels hot from the _development of heat internally_, which is not _conducted away_ with sufficient rapidity to maintain the natural temperature of the body.

135. _If a person, sitting before a fire-place, without a fire, were to set one foot upon a rug, and the other upon the stone hearth, which would feel the colder?_

The foot on the stone, because stone is a good conductor, and would _conduct the warmth of the foot away from it_.

[Verse: "The earth is the Lord's, and the fulness thereof; the world, and they that dwell therein."--PSALM XXIV.]

136. _What does the hearth-stone do with the heat that it receives?_

It delivers it to the surrounding air, and to any other bodies with which it may be _in contact_--and as it parts with heat, _it takes up more from any body hotter than itself_.

137. _When there is no fire in a room, what is the relative temperature of the various things in the room?_

They are all of the same temperature.

138. _If all the articles in the room are of the same temperature, why do some feel colder than others?_

Because they differ in their relative powers of _conduction_. Those that are the best conductors feel coldest, as they convey away the heat of the hand most rapidly.

If you lay your hand upon the _woollen table cover_, or upon the _sleeve of your coat_ or mantle, it will feel _neither warm nor cold_, under ordinary circumstances. But if you raise your hand from the table cover, or coat, and lay it on the marble mantel piece, the mantel-piece will feel _cold_. If now you return your hand from the mantel-piece to the table cover or coat, _a sensation of warmth will become distinctly perceptible_. This will afford a good conception of the relative _powers of conduction of wool and marble_.

139. _How long does a substance feel cold or hot to the touch?_

Until it has brought the part touching it to the same temperature as itself.

140. _When do substances feel neither hot nor cold?_

When they are of the same temperature as our bodies.

141. _Why, under these circumstances, do they feel neither hot nor cold?_

Because they neither take heat from, nor supply it to, the body.

142. _Which would feel the warmer, when the fire was lighted, the hearth-rug or the hearth-stone?_

The hearth-stone, because it is a _good conductor_, and would not only _receive heat_ readily, but would _part with it as freely_ (thereby making its heat _perceptible_). But the hearth-rug, _being a bad conductor_, would part with its heat very slowly, and it would therefore be _less perceptible_.

[Verse: "Fire and hail; snow and vapour; stormy wind fulfilling his word."--PSALM CXLVIII.]

143. _Would the hearth-stone feel hotter than the hearth-rug though both were of the same temperature?_

It would feel _hotter than the hearth-rug_, because it would part with its heat so rapidly that it would be the _more perceptible_.

144. _But if the hearth-stone and the hearth-rug were both colder than the hand, which would feel the colder of the two?_

Then the hearth-stone would feel the colder, because, _being a good conductor_, it would _take heat_ from the hand more freely than the hearth-rug, which is a _bad conductor_.

145. _Why would the hearth-stone feel comparatively hotter in the one case, and colder in the other?_

Because, _being a good conductor_, it would conduct heat rapidly _to_ the hand when hot, and take heat rapidly _from_ the hand when cold.

CHAPTER VIII.

146. _Which are the better conductors of heat, fluids or solids?_

Generally speaking, _solids_, especially those of them that are dense in their substance.

147. _Why are dense substances the best conductors of heat?_

Because the heat more readily travels from particle to particle until it pervades the mass.

148. _Why are fluids bad conductors of heat?_

Because of the want of _density_ in their bodies; and because a portion of the imbibed heat always passes off from fluids by _evaporation_.

[Verse: "He casteth forth his ice like morsels: who can stand before his word,"--PSALM CXLVII.]

149. _Why are woollen fabrics bad conductors of heat?_

Because there is a considerable amount of _air_ occupying the spaces of the texture.

150. _Is air a good or a bad conductor?_

Air is a _bad conductor_, and it chiefly transmits heat, as water does, by _convection_.

151. _Is water a good or a bad conductor?_

Water is an indifferent conductor, but it is a _better conductor than air_.

152. _Why, when we place our hands in water, which may be of the same temperature as the air, does the water feel some degrees colder?_

Because water, _being a better conductor than air_, takes up the warmth of the hand _more rapidly_.

153. _Why, when we take our hands out of water do they feel warmer?_

Because the air does not abstract the heat of the hand so rapidly as the water did, and the change in the degree of rapidity with which the heat is abstracted _produces a sensation of increased warmth_.

154. _Why do we see blocks of ice wrapped in thick flannel in summer time?_

Because the flannel, being a non-conductor, prevents the _external heat_ from _dissolving the ice_.

Flannel wrapped around a _warm_ body _keeps in its heat_; and wrapped around a _cold_ body, prevents heat from _passing into it_.

155. _How do we know that air is not a good conductor of heat?_

Because, _in still air_, heat would travel to a given point much more rapidly, and in greater intensity, through even an indifferent _solid conductor_, than it would through the _air_.

156. _How do we know that water is not a good conductor of heat?_

Because in a deep vessel containing _ice_, and with heat applied at the top, some portion of the water may be made to boil _before the ice, which lies a little under the surface, is melted_.

[Verse: "As snow in summer, and as rain in harvest; so honour is not seemly for a fool."--PROV. XXVI.]

157. _Why would you apply the heat at the top, in this experiment?_

Because in heating water it _expands and rises_. The boiling of water is caused by the heated water _ascending from the bottom_, and the colder water descending to occupy its place. If the heat were not applied at the top, it would be distributed quickly by _convection_, but not by _conduction_.

158. _Why are bottles of hot water, used as feet-warmers, wrapped in flannel?_

Because the flannel, _being a bad conductor_, allows the heat to _pass only gently_ from the bottle, and preserves the warmth for a _much longer time_.

159. _Why are hot rolls sent out by the bakers, wrapped up in flannel?_

Because the flannel, _being a bad conductor_, does not _carry off rapidly the heat of the rolls_.

160. _Why is it said that snow keeps the earth warm?_

Because snow is a _bad conductor_, and prevents the frosty air from _depriving the earth of its warmth_.

161. _Why are snow huts which the Esquimaux build found to be warm?_

Because snow, _being a bad conductor_, keeps in _the internal heat of the dwelling_, and prevents the _cold outer air from taking away its warmth_.

162. _Why is snow, being composed of congealed water (and water being a better conductor than air), so good a non-conductor?_

Because in the process of congealation it is frozen into crystalline forms, which, being collected into a mass, form a woolly body, thus proving the truthfulness of the Bible simile, which says, God "giveth snow like wool."

[Verse: "He giveth snow like wool: he scattereth the hoar frost like ashes."--PSALM CXLVII.]

[Illustration: CRYSTALS OF SNOW, AS SEEN THROUGH A MICROSCOPE. FIG. 1.]

163. _Why does it frequently feel warmer after a frost has set in?_

Because, in the act of congealation a great deal of heat is given out, and _taken up by the air_, and thus _the severity of the cold is in some degree moderated_.

164. _Why is it frequently colder when a thaw takes place?_

Because, in the process of thawing, a certain amount of heat is _withdrawn from the air_, and enters the thawed ice.

165. _What benefit results from these provisions of Nature?_

They moderate both the _severity of frosts_, and _the rapidity of thaws_, which, in changeable climates, would be seriously detrimental to _life_, and to _vegetation_.

166. _Why are furs and woollens worn in the winter?_

Because, being non-conductors, they prevent the warmth of the body from being _taken up by the cold air_.

167. _Why are the skins of animals usually covered with fur, hair, wool, or feathers?_

Because their coverings, being _non-conductors of heat_, preserve the warmth of the bodies of the animals.

[Verse: "He sendeth out his word, and melteth them: he causeth his wind to blow, and the waters to flow."--PSALM CXLVII.]

168. _How is the greater warmth of animals provided for in the winter?_

It is observed that, as winter approaches, there comes a short woolly or downy growth, which, _adding to the non-conducting property of their coats_, confines their animal warmth.

In small birds during winter, let the external colour of the feathers be what it may, there will be found a kind of _black_ down next their bodies. Black is the _warmest colour_, and the purpose here is to _keep in the heat_, arising from the respiration of the animal.

169. _How is warmth provided for in animals that have no such coats?_

They are furnished with a layer of _fat_, which lies underneath the skin. Fat consists chiefly of _carbon_, and is a _non-conductor_.

170. _Why are summer breezes said to be cool?_

Because, as they pass over the heated surface of the body, they bear away a part of its heat.

171. _Why is a still summer air said to be sultry?_

Because, being heated by the sun's rays, _and being a bad conductor_, it does not relieve the body by _carrying off its heat_.

172. _Why does fanning the face make it feel cooler?_

Because, by inducing currents of air to pass over the face, a part of the excessive heat is taken up _and carried away_.

173. _Why does perspiration cool the body?_

Because it takes up a part of the heat, and, evaporating, _carries it into the air_.

174. _Why does blowing upon hot tea cool it?_

Because it directs currents of air over the surface of the tea, and these currents take up a part of the heat _and bear it away_.

175. _Why does air in motion feel cooler than air that is still?_

Because each wave of air _carries away a certain portion of heat_ and being followed by another portion of air, _a further amount of heat is borne away_.

[Verse: "Though I walk in the valley of the shadow of death I will fear no evil, for thou art with me."--PSALM XXIII.]

176. _Is the atmosphere ever as hot as the human body?_

Not in this country. On the hottest day it is 10 or 12 deg. _cooler than the temperature of our bodies_.

177. _What is the highest degree of artificial heat which man has been known to bear?_

A man may be surrounded with air raised to the temperature of 300 deg. (the boiling point being 212), and yet not have the heat of his body raised more than two or three degrees above its natural temperature of from 97 deg. to 100 deg.

178. _Why may man endure this degree of heat for a short time without injury?_

Because the skin, and the vessels of fat that lie underneath it, are bad conductors of heat.

And because perspiration passing from the skin and evaporating, would _bear the heat away_ as fast as it was received.

Because, also, the vital principle (life) exercises a mysterious influence in the preservation of living bodies from physical influences.

179. _Is the air ever hot enough, in any part of the world, to destroy life?_

Yes. The hot winds of the Arabian deserts, which are called _simooms_, scatter death and desolation in their track, withering trees and shrubs, and burying them under waves of hot sand. When camels see the approach of a simoom they rush to the nearest tree or bush, or to some projecting rock, where they place their heads in an opposite direction to that from which the wind blows, and endeavour to escape its terrible violence. The traveller throws himself on the ground on the lee side of the camel, and screens his head from the fiery blast within the folds of his robe. But frequently both man and beast _fall a prey to the terrible simoom_.

180. _Why are these hot winds so terrible in their effects?_

Because, being in motion, they search their way to every part of the body, and passing over it _leave some portion of their heat behind_, which is again followed by _additional heat from every fresh blast of wind_.

[Verse: "The fear of the Lord is the beginning of knowledge: but fools despise wisdom and instruction."--PROVERBS I.]

CHAPTER IX.

181. _What is Radiation?_

The radiation of heat is a _motion of the particles_, in a series of rays, diverging in every direction from a heated body.

182. _What is this phenomena of Radiation understood to arise from?_

From a strongly repulsive power, possessed by particles of heat, by which they are excited to recede from each other with great velocity.

183. _What is the greatest source of Radiation?_

The sun, which sends forth rays of _both light and heat_ in all directions.

184. _When does a body radiate heat?_

When it is surrounded by a medium which is _a bad conductor_.

185. _When we stand before a fire, does the heat reach us by conduction or by radiation?_

By radiation.

186. _What becomes of the heat that is radiated from one body to another?_

It is either _absorbed_ by those bodies, or transmitted through them and passed to other bodies by _conduction_, or diffused by _convection_, or returned by _reflection_.

187. _How do we know that heat is diffused by radiation?_

If we set a metal plate (or any other body, though metal is best for the experiment) before the fire, _rays of heat will fall upon it_. If we turn the plate at a slight angle, and place another object in a line with it, we shall find that the plate will _reflect the rays it has received by radiation_, on to the object so placed; but if we place an object _between the fire and the plate_, we shall find that the rays of heat _will be intercepted_, and that the latter can no longer _reflect heat_.

[Verse: "The fear of the Lord is the beginning of wisdom: a good understanding have all they that do his commandments."--PSALM CXI.]

188. _Does the agitation of the air interfere with the direction of rays of heat?_

It has been found that the agitation of the air does _not_ affect the direction of rays of heat.

189. _Why, then, if a current of air passes through a space across which heat is radiating, does the air become warmer?_

Because it takes up _some portion of the heat_, but it does not alter the direction of the rays.

This is clearly illustrated by reference to _rays of light_ which are seen under many circumstances. But they are never bent, moved, nor in any way affected by the wind.

190. _Why will not a current of air disturb the rays of heat, just as it would a spider's web, or threads of silk?_

Because heat is an _imponderable_ agent, that is, something which cannot be acted upon by the ordinary physical agencies. It has _no weight_, presents no _substantial body_, and is, in these latter respects, similar to _light and electricity_.

191. _What other sources of radiation of heat are there besides the sun and the fire?_

The _earth_, and all _minor bodies_, are, in some degree, _radiators of heat_.

192. _What substances are the best radiators?_

All _rough_ and _dark_ coloured substances and surfaces are the _best radiators of heat_.

193. _What substances are the worst radiators of heat?_

All _smooth_, _bright_, and _light coloured_ surfaces are _bad radiators of heat_.

Dr. Stark, of Edinburgh, has proved, by a series of experiments, the influence which the _colours_ of bodies have upon the _velocity of radiation_. He surrounded the bulb of a thermometer successively with equal weights of _black_, _red_, and _white_ wool, and placed it in a glass tube, which was heated to the temperature of 180 deg. by immersion in hot water. The tube was then cooled down to 50 deg. by immersion in cold water; the _black_ cooled in 21 minutes, the _red_ in 26 minutes, and the _white_ in 27 minutes.

[Verse: "Say unto wisdom, Thou art my sister; and call understanding thy kinswoman."--PROVERBS VII.]

194. _If you wished to keep water hot for a long time, should you put it into a bright metal jug, or into a dark earthenware one?_

You should put it into a _bright metal_ jug, because, _being a bad radiator,_ it would not part readily with the heat of the water.

195. _Why would not the dark earthenware jug keep the water hot as long as the bright metal one?_

Because the particles of earthenware being rough, and of dark colour, _they radiate heat freely_, and the water would thereby be quickly cooled.

CHAPTER X.

196. _But if (as stated in the Lessons upon Conduction) metal is a better conductor of heat than stone or earthenware, why does not the metal jug conduct away the heat of the water sooner than the earthenware jug?_

It would do so, _if it were in contact with another conductor_; but, being surrounded by air, _which is a bad conductor_, the heat must pass off _by radiation_, and as bright metal surfaces are bad radiators, the metal jug would retain the heat of the water _longer than the earthenware one_.

197. _Supposing a red-hot cannon ball to be suspended by a chain from the ceiling of a room, how would its heat escape?_

Almost entirely by _radiation_. But if you were to rest upon the ball a cold bar of iron, a part of the heat would be drawn off by _conduction_. Warm air would rise from around the ball, and, moving upwards, would distribute some of the heat by _convection_. And some of its rays, falling upon a mirror, or any other bright surface, might be diffused by _reflection_.

[Verse: "I will teach you by the hand of God; that which is with the Almighty will I not conceal."--JOB XXVII.]

198. _Do some substances absorb heat?_

Yes; those substances which are _the best radiators_ are also _the best absorbers_ of heat.

199. _Why does scratching a bright metal surface increase its power of radiation?_

Because every irregularity of the surface acts as a point of radiation, or _an outlet_ by which the heat escapes.

200. _Why does a bright metal tea-pot produce better tea than a brown or black earthenware one?_

Because bright metal _radiates but little heat_, therefore the water is kept hot much longer, _and the strength of the tea is extracted by the heat_.

201. _But if the earthenware tea-pot were set by the fire, why would it then make the best tea?_

Because the dark earthenware tea-pot is a good _absorber of heat_, and the heat it would _absorb_ from the fire would more than counterbalance the loss by _radiation_.

202. _How would the bright metal tea-pot answer if set upon the hob by the fire?_

The bright metal tea-pot would probably _absorb less heat_ than it would radiate. Therefore it would not answer so well, _being set upon the hob_, as the earthenware tea-pot.

203. _Why should dish covers be plain in form, and have bright surfaces?_

Because, being bright and smooth, they will not allow heat to escape _by radiation_.

204. _Why should the bottoms and back parts of kettles and saucepans be allowed to remain black?_

Because a _thin_ coating of soot acts as a _good absorber of heat_, and overcomes the _non-absorbing_ quality of the _bright surface_.

[Verse: "And the foolish said unto the wise, Give us of your oil, for our lamps are gone out."]

205. _But why should soot be prevented from accumulating in flakes at the bottom and sides of kettles and saucepans?_

Because, although soot is a _good absorber_ of heat, it is a _very bad conductor_; an accumulation of it, therefore, would cause a waste of fuel, by _retarding the effects of heat_.

206. _Why should the lids and fronts of kettles and saucepans be kept bright?_

Because bright metal _will not radiate heat_; therefore, the heat which is taken up readily through the _absorbing_ and _conducting_ power of the bottom of the vessel, is kept in and economised by the _non-radiating_ property of the bright top and front.

207. _Does cold radiate as well as heat?_

It was once thought that _cold radiated_ as well as _heat_. But a mass of ice can only be said to radiate cold, _by its radiating heat in less abundance than that which is emitted from other bodies surrounding it_. It is, therefore, _incorrect_ to speak of the _radiation of cold_.

CHAPTER XI.

208. _Why, if you hold a piece of looking-glass at an angle towards the sum, will light fall upon an object opposite to the looking-glass?_

Because the rays of the sun are _reflected_ by the looking-glass.

209. _Why, when we stand before a mirror, do we see our features therein?_

Because the rays of light that fall upon us are _reflected_ upon the bright surface of the mirror.

210. _Why, if a plate of bright metal were held sideways before a fire, would heat fall upon an object opposite to the plate?_

Because rays of heat may be _reflected_ in the same manner as the rays of light.

[Verse: "But the wise answered saying, Not so; lest there be not enough for us and you: but go ye rather to them that sell, and buy for yourselves."--MATT. XXV.]

211. _Why would not the same effect arise if the plate were of a black or dark substance?_

Because black and dark substances are not _good reflectors of heat_.

212. _What are the best reflectors of heat?_

Smooth, light-coloured, and highly polished surfaces, especially those of _metal_.

213. _Why does meat become cooked more thoroughly and quickly when a tin screen is placed before the fire?_

Because the bright tin reflects the rays of heat back again to the meat.

214. _Why is reflected heat less intense than the primary heat?_

Because it is impossible to collect all the rays, and also because a portion of the caloric, imparting heat to the rays, is absorbed by the air, and by the various other bodies with which the rays come in contact.

215. _Can heat be reflected in any great degree of intensity?_

Yes; to such a degree that inflammable matters may be ignited by it. If a cannon ball be made red hot, and then be placed in an iron stand between two bright reflectors, inflammable materials, placed in a proper position to catch the reflected rays, _will ignite from the heat_.

There is a curious and an exceptional fact with reference to _reflected_ heat, for which we confess that we are unable to give "_The Reason Why_." It is found that snow, which lies near the trunks of trees or the base of upright stones, melts before that which is at a distance from them, though the sun may shine equally upon both. If a blackened card is placed upon ice or snow under the sun's rays, the frozen body underneath it will be thawed before that which surrounds it. But if we _reflect_ the sun's rays from a metal surface, the result is _directly contrary_--the exposed snow is the first to melt, leaving the card standing as upon a pyramid. Snow _melts_ under heat which is _reflected_ from the trees or stones while it withstands the effect of the _direct solar rays_. In passing through a cemetery this winter (1857), when the snow lay deep, we were struck with the circumstance that the snow in front of the head-stones facing the sun was completely dissolved, and, in nearly every instance, the space on which the snow had melted assumed a coffin-like shape. This forced itself so much upon our attention that we remained some time to endeavour to analyse the phenomena; and it was not until we remembered the curious effect of _reflected heat_ that we could account for it. It is obvious that the rays falling from the upper part of the head-stone on to the _foot_ of the grave would be less powerful than those that radiated from the _centre_ of the stone to the centre of the grave. Hence it was that the heat dissolved at the foot of the grave only a narrow piece of snow, which widened towards the centre, and narrowed again as it approached the foot of the head-stone, where the lines of radiation would naturally decrease. Such a phenomena would prove sufficient to raise superstition in untutored minds.

[Verse: "The light of the righteous rejoiceth, but the lamp of the wicked shall be put out."--PROVERBS XIII.]

216. _Are good reflectors of heat also good absorbers?_

No; for reflectors at once _send back_ the heat which they receive, while absorbers _retain it_. It is obvious, therefore, that _reflectors_ cannot be good _absorbers_.

217. _How do fire-screens contribute to keep rooms cool?_

Because they turn away from the persons in the room rays of heat which would otherwise make the warmth excessive.

218. _Why are white and light articles of clothing cool?_

Because they _reflect_ the rays of heat.

White, as a _colour_, is also a bad _absorber_ and _conductor_.

219. _Why is the air often found excessively hot in chalk districts?_

Because the soil _reflects_ upon objects near to it the heat of the solar rays.

220. _How does the heat of the sun's rays ultimately become diffused?_

It is first _absorbed_ by the earth. Generally speaking, the earth _absorbs_ heat by day, and _radiates_ it by night. In this way an equilibrium of temperature is maintained, which we should not otherwise have the advantage of.

221. _Does not the air derive its heat directly from the sun's rays?_

Only partially. It is estimated that the air absorbs only _one-third_ of the caloric of the sun's rays--that is to say, that a ray of solar heat, entering our atmosphere at its most attenuated limit (a height supposed to be about _fifty miles_), would, in passing through the atmosphere to the earth, part with only one-third of its calorific element.

[Verse: "As for the earth, out of it cometh bread; and under it is turned up as it were fire."--JOB XXVIII.]

222. _What becomes of the remaining two-thirds of the solar heat?_

They are _absorbed_ chiefly by the _earth_, the great medium of calorific _absorption_; but some portions are taken up by _living things_, both animal and vegetable. When the _rays of heat_ strike upon the earth's surface, they are passed from particle to particle into the interior of the earth's crust. Other portions are distributed through the air and water by _convection_, and a third portion is thrown back into space by _radiation_. These latter phenomena will be duly explained as we proceed.

223. _How do we know that heat is absorbed, and conducted into the internal earth?_

It is found that there is a given depth beneath the surface of the globe at which an equal temperature prevails. The depth increases as we travel south or north from the equator, and corresponds with the shape of the earth's surface, _sinking under the valleys, and rising under the hills_.

224. _Why may we not understand that this internal heat of the earth arises, as has been supposed by many philosophers, from internal combustion?_

Because recent investigations have thrown considerable and satisfactory light upon the subject. It has been ascertained that the internal temperature of the earth _increases_ to a certain depth, _one degree in every fifty feet_. But that below that depth the temperature _begins to decline_, and continues to do so with every increase of depth.

225. _Do plants absorb heat?_

Yes. They both _absorb_ and _radiate_ heat, under varying circumstances. The majestic tree, the meek flower, the unpretending grass, all perform a part in the grand alchemy of nature.

[Verse: "Consider the lilies of the field, how they grow; they toil not, neither do they spin."]

When we gaze upon a rose it is not its beauty alone that should impress us: every moment of that flower's life is devoted to the fulfilment of its part in the grand scheme of the universe. It decomposes the rays of solar light, and sends the red rays only to our eyes. It absorbs or radiates heat, according to the temperature of the ærial mantle that wraps alike the flower and the man. It distills the gaseous vapours, and restores to man the vital air on which he lives. It takes into its own substance, and incorporates with its own frame, the carbon and the hydrogen of which man has no immediate need. It drinks the dew-drop or the rain-drop, and gives forth its sweet odour as a thanksgiving. And when it dies, it preaches eloquently to beauty, pointing to the end that is to come!

CHAPTER XII.

226. _How do we know that plants operate upon the solar and atmospheric heat?_

A delicate thermometer, placed among the leaves and petals of flowers, will at once establish the fact, not only that flowers and plants have a temperature differing from that of the external air, but that the temperature varies in different plants according to the hypothetical, or supposed requirements, of their existences and conditions.

227. _What is the chief cause of variation in the temperature of flowers?_

It is generally supposed that their temperature is affected by their _colours_.

228. _Why is it supposed that the colour of a flower influences its temperature?_

Because it is found by experiment that the _colours_ of bodies bear an important relation to their properties respecting _heat_, and hold some analogy to the relation of _colours_ to _light_.

If when the ground is covered with snow, pieces of woollen cloth, of equal size and thickness, and differing only in colour, are laid upon the surface of the snow, near to each other, it will be found that the relation of _colour_ to temperature will be as follows:--In a few hours the _black_ cloth will have dissolved so much of the snow beneath it, as to sink deep below the surface; the _blue_ will have proved nearly as warm as the black; the _brown_ will have dissolved less of the snow; the _red_ less than the brown; and the white the _least_, or none at all. Similar experiments may be tried with reference to the _condensation of dew_, &c. And it will be uniformly found that the _colour_ of a body materially affects its powers of _absorption_ and of _radiation_.

[Verse: "And yet I say unto you, that even Solomon, in all his glory, was not arrayed like one of these."--MATT. VI.]

229. _Why do we know that these effects are not the result of light?_

Because they would occur, in just the same order, in the absence of light.

230. _Why are dark coloured dresses usually worn in winter, and light in summer?_

Because black _absorbs_ heat, and therefore becomes warm; while _light colours_ do _not_ absorb heat in the same degree, and therefore they remain cool.

231. _Why do iron articles, even when near fire, usually feel cool?_

Because they are bad absorbers, and do not take up heat freely, unless they are _in contact_ with a hot body.

232. _How is heat diffused through the atmosphere?_

By _convection_. The warmth radiating from the surface of the earth warms the air in contact with it; the air expands, and becoming lighter, flies upwards, bearing with it the caloric which it holds, and diffusing it in its course.

233. _How do the waters of the ocean become heated?_

Chiefly by _convection_. Nearly all the heat which the sun sheds upon the ocean is borne away from its surface by evaporation, or is radiated back into the atmosphere. But the ocean gathers its heat by _convection_ from the earth. It girdles the shores of tropical lands where, being warmed to a high degree of temperature, it sets across the Atlantic from the Gulf of Mexico, and exercises an important influence upon the temperature of our latitude.

234. _What is the cause of winds?_

Currents of air, and winds, are the result of _convection_. The air, heated by the high temperature of the tropics, _ascends_, while the colder air of the temperate and the frigid zones _blows towards the equator_ to supply its place.

[Verse: "Give unto the Lord the glory due unto his name; worship the Lord in the beauty of holiness."--PSALM XXIX.]

235. _What is the cause of sea breezes?_

Sea breezes are also the result of _convection_. The land, under the heat of the day's sunshine, becomes of a high temperature, and the expanded air on its surface _flies away towards the ocean_. As the sun goes down, the earth cools again, and the air _flies back_ to find its equilibrium.

Many countries by the sea are subjected to these periodical breezes, known as either "land" or "sea breezes," according to their direction. About eight o'clock in the morning an ærial current begins to flow from the sea towards the land, and continues until about three o'clock in the day; then the current takes a reverse direction, flowing from the land to the sea. This it continues to do throughout the night, until the time of sunrise, when a temporary calm ensues.

236. _Why does a soap bubble ascend in the air?_

Because, being filled with _warm_ air, it is _lighter_ than the surrounding medium, and therefore ascends.

237. _Why does the bubble fall after it has been in the air some time?_

Because the air contained in it has become cool, and, as it contains carbonic acid gas, it is _heavier_ than the air.

238. _What became of the warmth at first contained in the bubble?_

It has been _distributed in the air_ through which the bubble passed.

239. _What does this simple illustration of the distribution of warmth explain?_

It explains the law of _convection_, or _heat distribution_, over the surface of the globe.

240. _Why does air ascend the chimney?_

Because, being heated, it becomes _lighter_ than the surrounding medium, and therefore flies upwards, through the outlet provided for it.

241. _Why does air fly from the doors and windows towards the fire-place?_

Because, as the warm air flies away, cold air rushes in to occupy its place.

[Verse: "How much better is it to get wisdom than gold? and to get understanding rather to be chosen than silver."--PROVERBS XVI.]

242. _What does this example of the motion of the air in our rooms explain?_

It explains the movement of volumes of air by _convection_, and illustrates the origin of _breezes_ and _winds_.

243. _What is the chief effect of this law of convection?_

Under its influence air and water are the great _equalisers of solar heat_, rendering the earth agreeable to living things, and suited to the laws of their existence.

Owing, also, to this law of _convection_, the constituents of the air are equalised. The breath of life, supplied by the purer oxygen of the "sunny south," is diffused in salubrious gales over the wintry climes of the north. And the waters, evaporated from the bosom of the central Atlantic Ocean and the Pacific, are borne across vast continents, and poured down in fertilising showers upon distant lands.

To the educated mind, nothing is too simple to merit attention. To the ignorant, few things are sufficiently attractive to excite curiosity. Knowledge enables us to estimate the varied phenomena that are hourly arising around us, and to see, even in the most trifling effects, illustrations of those great causes and consequences that govern with mighty power the material world. Man, sitting by his fire-side, is enabled to witness the operation of some of nature's grandest laws: _light_ and _heat_ are around him; _conduction_, _radiation_, _reflection_, _absorption_, and _convection_ of heat are all going on before him; little winds are sweeping by his footstool, and warm currents, with miniature clouds folded in their arms, are passing upward before his view. Chemical changes are going on; the solid rock of coal disappears, flying away as an invisible gas. The little "hills are melted," and hard stones have been converted into "fervent heat." Although some of these changes are imperceptible to the _eye_, they are manifest to the educated _mind_; and the pleasures of philosophical observation are as sweet as a poet's dreams.

CHAPTER XIII.

244. _Why will a piece of paper, held three or four inches over the flame of a candle, become scorched?_

[Verse: "Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house."--MATT. V.]

Because the hot air and gas produced by the burning of the candle _ascends_ rapidly.

245. _Why will a piece of paper held about an inch below the flame of a candle scarcely become warmed?_

Because the heat _ascends_; and only a little of it falls upon the paper, and that by _radiation_.

[Illustration: Fig. 2.--DIAGRAM SHOWING THE COMBUSTION OF A CANDLE.]

246. _Why does the lower part of the flame of a candle_ (D) _burn of a blue colour?_

Because the _hydrogen_ of the tallow, having a stronger affinity for the _oxygen_ of the air than _carbon_ has, ignites first. Pure hydrogen burns with a bluish flame.

247. _Why does the middle of the flame_ (C) _look dark?_

Because it is occupied with gaseous vapours, derived from the tallow, which have not yet _ignited_.

248. _Why does the upper part of the flame_ (B) _produce a bright yellow light?_

Because it is in this part of the flame that the _hydrogen_ of the candle, and the _oxygen_ of the air, combine, and there is just sufficient _carbon_ mixed with the _hydrogen_ to improve its _illuminating power_.

249. _Why is there a fringe of pale light_ (A) _around the upper part of the flame?_

Because some of the _carbon_ escapes in a state of _incandesence_, and as soon as it reaches the air it combines with _oxygen_, and so forms _carbonic acid gas_.

If any dark body, such as the blade of a knife, be held between the eye and the flame of the candle, so as to shut off the light of the more luminous part, the pale fringe around the flame will be found distinctly perceptible. _Incandesence_ means _heated to whiteness_.

[Verse: "How oft is the candle of the wicked put out? and how oft cometh their destruction upon them?"--JOB XXI.]

250. _Why does the flame terminate in a point?_

Because cold air rushes towards the flame in every direction, and is carried upward. At the point where the flame terminates the cold currents have so _reduced the temperature_ that combustion can no longer be sustained.

251. _Why, if you hold anything immediately over the flame, will the flame lengthen?_

Because, by preventing the rapid escape of the heated air, you maintain a temperature which _increases the combustion_ at the point of the flame.

252. _Why should persons whose clothes take fire, throw themselves down?_

Because flame spreads most rapidly in an _upward_ direction.

253. _Why should persons whose clothes are on fire roll slowly about when they are down?_

Because they thereby _press out_ the fire.

254. _Why does pressing a flame or a spark put it out?_

Because it prevents the contact of the flame or spark with the _oxygen_ of the air.

Extinguishers put out the flame of candles in the same manner. A person dies from "suffocation" through the absence of oxygen; and it is literally practicable to _"suffocate" a fire_.

255. _Why does the wick turn black as it burns?_

Because it consists principally of _carbon_.

256. _Why, when the point of the wick turns out and meets the air, does it exhibit a bright spark?_

Because the _carbon_ of the wick comes into immediate contact with the _oxygen_ of the air.

257. _Why does holding a candle "upside down" put it out?_

Because the melted grease runs down too rapidly, and at too low a temperature to undergo combustion. It therefore _reduces the heat_, and extinguishes the flame.

[Verse: "Lord, what is man that thou takest knowledge of him! or the son of man, that thou makest account of him."--PSALMS CXLIV.]

258. _Why is it more difficult to blow out the flame of a candle with a cotton wick than one with a rush wick?_

Because the cotton wick imbibes more of the combustible materials, and holds in its loose texture the inflammable gases in a state ready for combustion.

259. _Why does blowing sharply at a candle flame put it out?_

Because the breath drives away the vapour of the grease which, becoming gaseous, supports the flame.

And because too rapid a flow of cold air reduces the temperature below the point at which combustion can be maintained.

260. _Why will a gentle puff of breath, if given speedily after the flame is extinguished, rekindle it?_

Because the _oxygen_ of the air combines with the _carbon_ and _hydrogen_ that are still escaping from the _heated wick_, and re-lights it.

261. _Why will not a similar puff rekindle the flame of a rushlight?_

Because its wick retains but _little heat_, and holds a comparatively small amount of combustible matter in a _volatile state_.

262. _Why is a fire, when it is very low, sometimes put out by blowing it?_

Because the too rapid flow of cold air _reduces the temperature_ of the burning mass.

263. _Why will a piece of paper twisted like an extinguisher put out a candle?_

Because, before the flame of the candle can ignite the paper, _the oxygen contained within it is consumed_, and the flame is suffocated.

[Verse: "When his candle shined upon my head, and when by his light I walked through darkness."--JOB XXIX.]

264. _Why do tallow candles require snuffing?_

Because the _oxygen_ of the air cannot reach the wick through the body of flame--therefore the _unconsumed carbon_ accumulates upon the wick.

265. _Why do composite and wax candles not require snuffing?_

Because their wicks are made by a series of plaits, by which they are bent to meet the _oxygen_ of the air, and consumed.

266. _Why does setting a glass upon a lamp increase its brilliancy, though it shortens the flame?_

Because it conducts an increase of air to the flame, and the greater supply of _oxygen_ causes the escaping vapour of oil to be all rapidly consumed.

267. _Why does a candle burn dimly when the wick has become loaded with carbon?_

Because the carbon _radiates_ the heat, and disperses it, and reduces the heat of the flame below that temperature which is essential to its _luminosity_.

268. _What differences characterise the combustion of carbon and of hydrogen?_

The combustion of _carbon_ takes place without the production of flame. The charcoal (or carbon in any other form) being heated to redness, enters directly into combination with the _oxygen_ of the surrounding air, and the carbonic acid gas, being invisible, passes away unobserved.

But in the combustion of _hydrogen_ the heat developed is so intense as to render _the gas itself luminous_, just as iron may be heated to a red or white heat.

269. _What has become of the candle when it has been burnt?_

It has been resolved partly into _carbonic acid gas_ which, though unperceived, has diffused itself through the surrounding air; and partly into _water_, which escaped in the form of thin vapour.

270. _Has any part of the candle been consumed or lost?_

No; there is no such thing as "loss" in the operations of nature. Every particle of the candle, now invisible, exists either in the form of _gas_, _vapour_, or _water_, with, perhaps, a few solid particles that may be called _ashes_, but which are too minute to excite attention.

[Verse: "I know that whatsoever God doeth, it shall be for ever: nothing can be put to it, nor anything taken from it; and God doeth it that men should fear before him."--ECCLES. III.]

The economy of nature should teach us a very impressive lesson--_nothing is suffered to be wasted_, not even the slightest atom. As soon as any body has fulfilled its purpose in one state of being, it is passed on to another. The candle, existing no longer as a candle, is flying upon the wings of the air as _carbonic acid gas_, and as _water_. These probably find their way to the garden or the field, where the carbonic acid gas forms the _food of the plant_, and the water affords it a refreshing _drink_. And can it be supposed that the Almighty Being, who has thus economised the existence of the _material_ creation, should be less mindful of the immaterial _soul_ of man? There _is_ an eternity before us, the certainty of which is evidenced even by the laws of the material creation.

CHAPTER XIV.

271. _What is coal?_

Coal is a "_vegetable fossil_."

272. _What is meant by a vegetable fossil?_

It is a substance _originally vegetable_, which, by pressure and other agencies within the earth, has been brought to a condition approaching that of _mineral_ or earthy matter.

273. _Why do we know that coal is of vegetable origin?_

By the _chemical components_ of its substance; and also by the _vegetable forms_ that are found abundantly in coal beds.

Professor Buckland, in his _Bridgewater Treatise_, speaking of the impressions of plants found in the coal mines, says; "The finest example I have ever witnessed is that of the coal mines of Bohemia. The most elaborate imitations of living foliage upon the painted ceilings of Italian palaces bear no comparison with the beauteous profusion of extinct vegetable forms with which the galleries of these instructive coal mines are overhung. The roof is covered as with a canopy of gorgeous tapestry, enriched with festoons of most graceful foliage, flung in wild irregular profusion over every part of its surface. The effect is heightened by the contrast of the coal-black colour of these vegetables with the light ground-work of the rock to which they are attached. The spectator feels himself transported, as if by enchantment, into the forests of another world; he beholds trees, of forms and characters now unknown upon the surface of the earth, presented to his senses almost in the beauty and vigour of their primeval life; their scaly stems and bending branches, with their delicate apparatus of foliage, are all spread forth before him, little impaired by the lapse of countless ages, and bearing faithful records of extinct systems of vegetation which began and terminated in times of which these relics are the infallible historians."

[Verse: "Surely every man walketh in a vain show; surely they are disquieted in vain: he heapeth up riches, and knoweth not who shall gather them."--PS. XXXIX.]

274. _What are the chemical components of coal?_

They consist of _carbon_, _hydrogen_, _oxygen_, and _nitrogen_. The proportions of these elements vary in different kinds of coal. Carbon is the chief component; and the proportions may be stated to be, generally, _carbon_, 90 per cent.; _hydrogen_, from 3 to 6 per cent.; the other elements enter into the compound in such small proportions, that, for all ordinary purposes, it is sufficient to say that coal consists of _carbon_ and _hydrogen_, but chiefly of _carbon_.

275. _What is charcoal?_

Charcoal consists almost entirely of _carbon_. It is made from _wood_ by the application of heat, without the admission of air. The hydrogen and oxygen of the wood are expelled, and that which remains is charcoal, or _carbon_ in one of its purest states.

276. _What is animal charcoal?_

Animal charcoal, like vegetable charcoal, consists of _carbon_ in a state approaching purity. It is made from the _bones of animals_, heated in iron cylinders. It is commonly called _ivory black_.

277. _What is the purest form of carbon known?_

The purest form of _carbon_ is the _diamond_, which may be said to be absolutely pure.

Hence we derive another of the beautiful lessons of science--a lesson which teaches us to _despise nothing that God has given_. The soot which blackens the face of a chimney-sweep, and the diamond that glistens in the crown of the monarch, consist of the same element in merely a different atomic condition. What a lesson of humility this teaches to Pride! The haughty beauty as she walks the ball-room, inwardly proud of the radiance of her gems as they rise and fall upon her breast, little thinks or knows that _every breath that is expired around her wafts away the like element of which her treasures are composed_. That even in our own flesh and bones the same abounding substance lies hid; and that the buried tree of the primitive world, and the little flower of to-day, are both the instruments of giving this singular element to man!

278. _What is coke?_

Coke is coal, divested of its hydrogen and other volatile parts, by a similar process to that by which charcoal is produced. It forms the residue after hydrogen gas has been made from coals. It consists almost entirely of _carbon_.

[Verse: "Oh that men would praise the Lord for his goodness, and for his wonderful works to the children of men."--PSALM CVII.]

279. _Why do burning coals produce yellow flame?_

Because the _hydrogen_ which they contain is combined with some proportion of _carbon_, which imparts a bright yellow colour to the flames.

280. _Why do some of the flames of a fire appear much whiter than others?_

Because the quality of coals, and the conditions under which they are burnt, are liable to variation. Some coals yield a _heavy_ hydrogen, called _bi-carburetted hydrogen_, which burns with a much brighter flame than _carburetted hydrogen_.

281. _Why does bi-carburetted hydrogen burn with a whiter flame than the common coal gas?_

Because it is combined with a larger proportion of _carbon_, to which it owes its increased luminosity.

282. _Why do some of the flames of a fire appear blue?_

Because the hydrogen which is escaping where those flames occur is _pure hydrogen_, destitute of carbon.

283. _Why does the fire sometimes appear red, and without flame?_

Because the volatile gases have been driven off and consumed, and combustion is continued by the _carbon_ of the coals and the _oxygen_ of the air.

284. _What effect has the burning of a fire upon the composition of the air?_

It is found that in burning 10lb. of coal the oxygen contained in 1,551 cubic feet of air is altogether absorbed. It is therefore necessary to keep the atmosphere of a room, in which a coal fire is burning, fresh and pure, to supply 155 cubic feet of fresh air for every pound of coal that is consumed.

[Verse: "O Lord how manifold are thy works, in wisdom hast thou made them all: the earth is full of thy riches."--PSALM CIV.]

285. _Why does wood which is "green" hiss and steam when it is burnt?_

Because it contains a large amount of water, which must be evaporated before combustion can proceed.

286. _What is the effect of this evaporation?_

A great deal of heat is unprofitably expended in driving off the water of the fuel.

287. _Why does poking a fire cause it to burn more brightly?_

Because it opens avenues through which the air may enter to supply _oxygen_.

288. _Why do "blowers" improve the draft of air through a fire?_

Because, by obstructing the passage of the current of air _over_ the fire, they cause additional air to pass _through_ it, and therefore a greater amount of _oxygen_ is carried to the coals.

289. _What is smoke?_

Unconsumed particles of _coal_, rendered volatile by heat, and driven off.

290. _What is soot?_

_Carbon_ in minute particles, driven off with other volatile matters and deposited on the walls of chimneys.

291. _Why do fresh coals increase the quantity of smoke?_

Because they contain volatile matters which are easily driven off; and because, also, they reduce momentarily the heat, so that those matters that first escape cannot be consumed.

292. _Why do charcoal and coke fires burn clearly and without flame?_

Because the _hydrogen_ has been previously driven off from those substances.

293. _Why is it difficult to light charcoal and coke fires?_

Because they contain no _hydrogen_ to produce _flame_, and assist combustion.

[Verse: "He hath made his wonderful works to be remembered: the Lord is precious and full of compassion."--PSALM CXL.]

A new plan of kindling fires has lately been recommended. Coals are to be laid in the _bottom_ of the fire-place to a considerable depth, then the paper and wood are to be laid on, and then a little coals and cinders over them. This plan of "laying in" the fire is precisely the _reverse_ of that which has been pursued for many years. The theory is, that when the coals in the bottom are ignited, a more even combustion is kept up, whilst the smoke and gas which would otherwise escape, and become as so much waste fuel, is burnt up, and produces heat. We have heard the plan strongly recommended by persons who have tried it, and who testify to the great economy of fuel to which it conduces.

CHAPTER XV.

294. _Why does paper ignite more readily than wood?_

Because its texture is less _dense_ than that of wood; its particles are therefore more _readily heated_ and decomposed.

295. _But if articles of loose texture are bad conductors of heat, why do they so easily ignite?_

The fact that they are _bad conductors_ assists their ignition. The heat which would pass from particle to particle of the dense substance of iron, and be _conducted away_, accumulates in the interspaces of paper, and ignites it.

296. _Why does wood ignite less readily than paper?_

Because its substance is _denser_ than that of paper; it therefore requires a higher degree of heat to inflame its substance.

297. _Why does wood, when ignited, burn longer than paper?_

Because, being a denser substance, it submits a _larger number of particles_, within a given space, to the action of the heat, and the formation of gases.

298. _Why do we, in lighting a fire, first lay in paper, then wood, and lastly coals?_

Because the paper is more easily ignited than wood, and wood than coals; therefore the _paper_ assists the ignition of the _wood_, and the _wood_ assists the ignition of the _coals_.

[Verse: "It is a good thing to give thanks unto the Lord, and to sing praises unto thy name, O Most High."--PSALM XCII.]

299. _Why will not wood ignite by the flame of a match?_

It will do so, unless there is a great disproportion between the size of the wood and the flame of a match. A _thin_ piece of wood will ignite, but a square block will not, because the heat of the flame is insufficient to raise the temperature of a _large surface_ to the point that will drive out its gases.

300. _Why do we place the paper under the wood, and the wood under the coals?_

Because heat and flame, when surrounded by air, have a strong tendency to spread themselves _upwards_.

301. _Would it be possible to light the coals by putting the paper and the wood upon the top?_

It would be possible; but the loss of heat would be so great, that a _much larger quantity_ of paper and wood would be required.

302. _Why does a poker laid across the top of a dull fire revive it?_

Because the poker _radiates_ the heat it receives from the fire downward upon the fuel.

Because, also, it divides the ascending air, and thereby _creates currents_.

The amount of good which the poker does to the fire is very slight indeed. Generally, the housewife stirs the fire first, and blows or brushes away the ashes that prevent the influx of air. She then places the poker upon the top, and the popular mind supposes that the poker "draws" the fire. The custom of placing a poker over the fire is of very remote antiquity. It was once believed that forming _a cross_, by placing the poker over the bars, protected the fire from the hostility of malignant _witches_!

303. _Why should fire-places be fixed as low as possible in rooms?_

Because heat _ascends_, and when the fire-places are high the lower parts of the room are _inadequately warmed_.

Also, as currents of air fly towards the fire, elevated fire-places _cause drafts_ about the persons of the inmates to a much greater extent than they would if they were lower down.

[Verse: "Unto thee, O God, do we give thanks: for that thy name is near thy wondrous works declare."--PSALM LXXV.]

304. _Why, if a piece of paper be laid with its flat surface upon the fire, will it "char," but not ignite?_

Because, as in the case of the proper candle-extinguisher, the _carbonic acid gas_ accumulating beneath it prevents its igniting.

305. _Why, if you direct a current of air towards the paper, will it burst into a blaze?_

Because the carbonic acid gas is displaced by a current of air containing _oxygen_.

306. _Why does water extinguish fire?_

Because it _saturates the fuel_, and prevents the gases thereof from combining with the oxygen of the air.

307. _As water contains oxygen, why does not the oxygen of the water support the fire?_

Because the affinity between the _hydrogen_ and _oxygen_ of the water is so strong that fire cannot separate them.

Water may be decomposed by _heat_, as will be hereafter explained. But the heat of an ordinary fire is insufficient. There is, however, some reason for believing that, in cases of very large fires, such as the accidental burning of houses, &c., when the supply of water thrown upon the fire is very deficient, the water _does_ become _decomposed_, and add to the fury of the flames.

308. _Why does the blacksmith sprinkle water upon the coals of his forge?_

The blacksmith uses _small coals_ because the small pieces thereof are more easily ignited than large lumps would be, and they convey heat better by completely surrounding the articles put into the fire. He sprinkles water on the coal dust _to hold its particles together by cohesion_, until the heat forms it into a cake. A strong blast of hot hair drives the vapour of the water away, and leaves a porous mass to the action of the fire.

309. _Why, when the blacksmith thrusts a heated iron into a tankard of water, do we recognise a peculiar smell?_

Because the intense heat disengages a small volume of the gases of which water is formed.

[Verse: "Oh the depth of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out."--ROM. XI.]

310. _Which gas do we (in this instance) recognise by the smell?_

The _hydrogen_ gas. Oxygen gas possesses no odour.

311. _What is Spontaneous Combustion?_

Spontaneous combustion is that which occurs in various bodies when they become highly heated by _chemical changes_.

312. _Why is heat developed during chemical changes?_

Because, as all bodies contain _latent caloric_, the disturbance of the atoms of which those bodies are composed, during the new combinations that constitute _chemical changes_, frequently sets the caloric free, and an _accumulation of caloric_ produces spontaneous combustion.

313. _Does a match ignite spontaneously when drawn over a rough surface?_

No. Because in this case the combustion arises from heat _applied by friction_.

314. _Does phosphorous ignite spontaneously when held in a warm hand?_

Phosphorous will ignite when held in a warm hand, but it does not then produce spontaneous combustion, because it ignites through the agency of _applied heat_.

315. _But if a piece of dry phosphorous be sprinkled with powdered charcoal it will ignite, without the application of heat. Why is this?_

Because the _carbon_ (charcoal) absorbs _oxygen_ from the air, and conveys it to the _phosphorous_. Here are _chemical changes_ which develope heat, and produce _spontaneous combustion_.

316. _Why do hay-stacks sometimes take fire?_

Because the hay, having become damp, decays, and passes on to a state of _fermentation_, in which _chemical changes occur_, during which heat is evolved. Hay, taking fire under these circumstances, would exhibit _spontaneous combustion_.

[Verse: "Who hath woe? who hath sorrow? who hath contentions? who hath babbling? who hath words without cause? who hath redness of the eyes? * * * They that tarry long at the wine."--PROV. XXIII.]

317. _What substances are liable to produce spontaneous combustion?_

All substances which contain sugar, starch, and other components liable to _fermentation_. All bodies that evolve, under low degrees of temperature, _inflammable gases_. And all organic bodies undergoing decay.

Grain, cotton, hemp, flax, coals, oily and greasy substances.

318. _What is the Ignis Fatuus (sometimes called "Will-o'-the-Wisp", "Corpse Candles," and "Jack-o'-Lantern")?_

It is a flame produced by spontaneous combustion, caused by the decay of animal or vegetable bodies, which evolve _phosphoretted hydrogen_ gas, under circumstances attended by a low degree of heat, sufficient to ignite the gases. It is mostly seen over marshy places, and burial-grounds.

Many a "Ghost Story" has owed its origin to these singular but harmless appearances. People, ignorant of the cause, have been terrified at the effect. To the fancy of an affrighted mortal, the simple flame of the _Ignis Fatuus_ has assumed the form of a departed friend, and even found a supernatural voice. If, excited by a momentary daring, the beholder moved towards the light upon which he gazed, it fled from him. If he turned from it and walked away, it followed him, step by step. The darkness of a lonely road, or the sacred solitude of a burial-place, have been sufficient accessories to authenticate the appearance of a spirit. And yet how simple the phenomenon? Matters so volatile as those which produce the _Ignis Fatuus_ would naturally be driven back by the motion in the air caused by an advancing body; and, on the other hand, a body moving from them would create a current in which the _Ignis Fatuus_ would follow. Poisonous gases, escaping from decaying bodies, pass into the air and take fire. They are thereby converted into harmless compounds. Thus we see that the "ghost" which terrifies the mind of the ignorant, becomes a "guardian angel" to the educated.

319. _Has spontaneous combustion ever occurred in living bodies?_

It has occurred in numerous instances to persons habituated to the excessive use of spirits.

320. _Why should spontaneous combustion occur in the case of the drunkard?_

Because spirituous drinks contain a large proportion of ALCOHOL, one of the constituents of which is _hydrogen_. The vital energies of the drunkard, being destroyed by excess, chemical agencies obtain an ascendancy, and it is supposed that the _hydrogen_ of the alcohol combines with the _phosphorous_ of the body to form _phosphoretted hydrogen_, which ignites spontaneously, and literally consumes the living temple.

[Verse: "Drought and heat consume the snow waters; so doth the grave those which have sinned."--JOB XXIV.]

Cases of spontaneous combustion are of rare occurrence. But they are sufficiently well authenticated by high medical authority, in many parts of the world, to present an awful warning to the inveterate drunkard. The cases of which we have read the particulars present details of the most appalling description. How signally the Almighty displeasure at intemperance is expressed, when the very drink which imparts the mad pleasure of intoxication is made the _direct_ instrument by which the drunkard is destroyed!

CHAPTER XVI.

321. _Why does friction produce heat?_

Because all bodies contain _latent heat_, that is, heat that lies hid in their substance, and the rubbings of two bodies against each other _draws the latent heat to the excited surfaces_.

322. _Why does the rubbing of two surfaces together attract latent heat to those surfaces?_

Because it is a law of nature that _heat_ shall always attend _motion_; and it is generally found that the _intensity of heat_ bears a specific relation to the _velocity of motion_.

323. _What are the sources of heat?_

The _rays_ of the _sun_, the _currents_ of _electricity_, the _action_ of _chemicals_, and the _motion_ of _substances_.

324. _Why does water freeze?_

Because its latent heat is partly _drawn off_ by the surrounding air.

325. _Why does ice melt?_

Because the heat, once latent in the water, but drawn off by the air, _has returned_ to it, and restored the water to its former condition.

[Verse: "So teach us to number our days, that we may apply our hearts unto wisdom." PSALM XC.]

326. _Why does water become steam?_

Because a larger amount of heat has entered into it than can remain latent in water. The water therefore expands and rises in the form of vapour, or _water attenuated by heat_.

327. _How many degrees of heat are latent, or hidden, in the different states of water?_

In thawing _ice_, 140 deg. of caloric become latent; and in converting the water into steam, 1,000 deg. more of caloric are be taken up. Therefore, _ice_ requires to take up 1,140 deg. of latent caloric before it becomes steam.

328. _What is the most modern theory of heat?_

It is this--that caloric, which produces heat, is an extremely _subtile fluid_, of so refined a nature that it possesses no weight, yet is capable of diffusing itself among the particles of the most solid bodies.

It is also believed that--all bodies are subject to the action of two opposing forces: one, the _mutual attraction_ of their _particles_; the other, the _repulsive force_ of _caloric_--and that bodies exist in the _æriform_, _fluid_, or _solid state_, _according to the predominance of either the one or the other of these opposing forces_.

329. _How do we measure the quantity of caloric in any substance?_

It is impossible to determine the amount of caloric which any body contains. Our _sensations_ would obviously be deceptive, since, if we dipped the right hand in snow, and held the left hand before the fire, and then immersed both hands in cold water, the water would feel _warm_ to the _right hand_ and _cold_ to the _left hand_.

But, as _caloric_ uniformly expands substances that are under its influence, one of the bodies most sensitive to _calorific_ effects has been selected to be the _indicator_ of the amount of _caloric_. This substance is _quicksilver_; and the scale of measurement, and the apparatus for exhibiting the rise or fall of the quicksilver, constitute the _thermometer_.

330. _If it is impossible to measure the amount of caloric in any substance, how can it be said that ice absorbs_ 140 _deg. in becoming water?_

Those figures simply record the amount of calorie indicated by the _thermometer_. The instrument will show with sufficient accuracy the _relative amount_ of caloric in various bodies, or in the same bodies _under different circumstances_, but it can never determine the _precise amount of caloric_ in any one body.

[Verse: "Great is the Lord, and greatly to be praised in the city of our God, in the mountain of his holiness."--PSALM XLVIII.]

331. _Why, if a hot and a cold body were placed near to each other, would the cold one become warmer, and the hot one cooler?_

Because _free caloric_ (that is, caloric that is not latent,) always exhibits a tendency to establish an _equilibrium_. If twenty bodies, of different temperatures, were placed in the same atmosphere, they would _all soon arrive at the same temperature_. The caloric would leave the bodies of those of the _highest_, and find its way to those of the _lowest_ temperature.

332. _How does caloric travel?_

It travels in _parallel rays_ in all directions with a velocity approximating to that of light; and it passes through various bodies with a rapidity proportionate to their power of _conduction_.

333. _Why does melted metal run like a stream of fluid?_

Because _caloric_ has passed into its substance, and, repelling its particles, has separated them to that degree which produces fluidity.

334. _How do we know that it is caloric passing into the substance of the metal which produces this effect?_

Because, as soon as a bar of metal begins to be heated, it _expands_ and _lengthens_. It continues to do so, until the heat arrives at that point which _causes the metal to melt_.

335. _Why does the iron of an ironing-box sometimes become too large for the box to receive it?_

Because _caloric_ has passed into the substance of the iron, and _repelled its particles_, by which it has become expanded.

336. _Why does the iron enter the box when it has become partially cooled?_

Because a portion of the caloric has left the iron, the particles of which have _drawn closer together_, and contracted the mass.

[Verse: "Cast thy burden upon the Lord, and he shall sustain thee; he shall never suffer the righteous to be moved."--PSALM LV.]

This effect is frequently observed by females in domestic life, who, when they are ironing, or using the Italian irons, find that the heated metal has been too much expanded to enter the box or tube. They find it necessary to wait until the cooling of the iron has had the effect of reducing its dimensions. The expansion of bodies by heat is one of the grandest and most important laws of nature. We are indebted to it for some of the most beautiful, as well as the most awful, phenomena. And science has gained some of its mightiest conquests through its aid. Yet frequently, though quite unthought of, in the hands of the humble laundress, will be found a most striking illustration of this wonderful force of caloric.

337. _Are there any instances in which the abstraction of latent heat will reduce the hulk of bodies?_

Yes, there are several. But the most familiar one is that which is exhibited by mixing a _pint_ of the _oil of vitriol_ with a _pint_ of _water_. _A considerable amount of heat will be evolved_; and it will be found that the two pints of fluid _will not afterwards fill a quart measure_.

338. _Is there any latent heat in air?_

Yes: a considerable amount. In a pint measure of air, though in no way evident to our perceptions, there lurks sufficient caloric to raise a piece of metal several inches square to glowing redness.

339. _How do we know that caloric exists in the air?_

It has been positively demonstrated by the invention of a small condensing syringe, by which, through the rapid compression of a small volume of air, a spark is emitted which ignites a piece of prepared tinder.

340. _What is the cause of the spark when a horse's shoe strikes against a stone?_

The _latent heat_ of the iron or the stone is set free by the _violent percussion_. The same effect takes place when _flint_ strikes against _steel_, as in the old method of obtaining a light with the aid of the tinder-box.

[Verse: "The waters are laid as with a stone, and the face of the deep is frozen."--JOB XXXVIII.]

What an eloquent lecture might be delivered upon the old-fashioned tinder-box, illustrated by the one experiment of "striking a light." In that box lie, cold and motionless, the Flint and Steel, rude in form and crude in substance. And yet, within the breast of each, there lies a spark of that grand element which influences every atom of the universe; a spark which could invoke the fierce agents of destruction to wrap their blasting flames around a stately forest, or a crowded city, and sweep it from the face of the world; or which might kindle the genial blaze upon the homely hearth, and shed a radiant glow upon a group of smiling faces; a spark such as that which rises with the curling smoke from the village blackmith's forge--or that which leaps with terrific wrath from the troubled breast of a Vesuvius. And then the tinder--the cotton--the carbon: What a tale might be told of the cotton-field where it grew, of the black slave who plucked it, of the white toiler who spun it into a garment, and of the village beauty who wore it--until, faded and despised, it was cast amongst a heap of old rags, and finally found its way to the tinder-box. Then the Tinder might tell of its hopes; how, though now a blackened mass, soiling everything that touched it, it would soon be wedded to one of the great ministers of nature, and fly away on transparent wings, until, resting upon some Alpine tree, it would make its home among the green leaves, and for a while live in freshness and beauty, looking down upon the peaceful vale. Then the Steel might tell its story, how for centuries it lay in the deep caverns of the earth, until man, with his unquiet spirit, dug down to the dark depths and dragged it forth, saying, "No longer be at peace." Then would come tales of the fiery furnace, what Fire had done for Steel, and what Steel had done for Fire. And then the Flint might tell of the time when the weather-bound mariners, lighting their fires upon the Syrian shore, melted silicious stones into gems of glass, and thus led the way to the discovery of the transparent pane that gives a crystal inlet to the light of our homes; of the mirror in whose face the lady contemplates her charms; of the microscope and the telescope by which the invisible are brought to sight, and the distant drawn near; of the prism by which Newton analysed the rays of light; and of the photographic camera in which the sun prints with his own rays the pictures of his own adorning. And then both Flint and Steel might relate their adventures in the battle-field, whither they had gone together; and of fights they had seen in which man struck down his fellow-man, and like a fiend had revelled in his brother's blood. Thus, even from the cold hearts of flint and steel, man might learn a lesson which should make him blush at the "glory of war;" and the proud, who despise the teachings of small things, might learn to appreciate the truths that are linked to the story of a "tinder-box."

CHAPTER XVII.

341. _Since all bodies expand by heat and contract by cold, why does water, when it reaches the freezing point, expand?_

Because, in freezing, water undergoes crystallization, in which its particles assume a new arrangement occupying _greater space_.

342. _Why does water never freeze to a great depth?_

Because the covering of ice which is formed upon the surface of the water prevents the cold air from continuing to draw off the _caloric_ of the water.

[Verse: "For he saith to the snow, Be thou on the earth; likewise to the small rain, and to the great rain of his strength."--JOB XXXVII.]

343. _Why has this exceptional law of the expansion of water, when freezing, been ordained?_

Because, but for this, deep waters might be frozen through their whole depth. This would destroy the myriads of fish and other living things that inhabit the water. Parts of the earth, now clad in verdure, would be lost in eternal winter; and even in the most temperate zones it would take months to effect a thaw; and thawing would be attended with such floods and subterranean commotion as are terrible to contemplate.

344. _Why are bed-room windows sometimes covered with crystalline forms on winter mornings?_

Because the vapour of the breaths of the inmates has condensed upon the window-panes, and formed water. The water has frozen with the cold, and exhibits the beautiful crystalline forms into which its particles are arranged.

Here we have another domestic illustration of the great laws of nature. It is the same law which locks the arctic regions in ice and decorates our window-panes. This beautiful phenomenon is usually witnessed by us on frosty mornings when we rise from our beds. It has a story which the observer of nature may read in its sparkling eyes. It tells that, although without the air is biting cold, God has wrapped a mantle around the face of nature to keep it from injury; and that the earth and the waters, though looking chilled and dead, have still the warmth of life preserved in their bosoms.

345. _What is dew?_

Dew is _watery vapour_ diffused in the air, _condensed_ by coming in contact with bodies _colder than the atmosphere_.

346. _Why does the air become charged with watery vapour?_

Because, during the day, under the influence of the sun's rays, vapours are _exhaled_ from all the moist and watery surfaces of the earth. These vapours are _held in suspension_ in the atmosphere until, by a change in the temperature of the earth, and of bodies on the surface of the earth, they are _condensed_, and deposited in translucid drops.

347. _What causes the decline of temperature that favours the deposition of dew?_

The earth, which during the day _received heat_ from the solar rays, _radiates the heat_ back into the air, and therefore becomes itself colder. All the various objects upon the face of the earth also _radiate heat_ in a greater or lesser degree. And dew will be found to be deposited upon the surfaces of such bodies in proportion to the fall of their temperature through _radiation_.

[Verse: "The Lord is my shepherd, I shall not want. He maketh me to lie down in green pastures."--PSALM XXIII.]

348. _Why is there little or no dew when the nights are cloudy?_

Because clouds act as secondary radiators; and when the _earth_ radiates its heat towards the _clouds_, the clouds again _radiate it back to the earth_.

[Illustration: Fig. 3.--ILLUSTRATING THE FORMATION OF DEW.]

If plates of glass be laid over grass-beds, as in the engraving Fig. 3, no dew will be deposited on the grass underneath the glass plates, although all around the grass will be completely wetted. The explanation is that the glasses, being radiators of heat, act in the same manner as the clouds, returning the heat to the bodies underneath them, and preventing the formation of dew thereon.

349. _Why does dew form most abundantly on cloudless nights?_

Because the heat which is radiated by the earth does not return to it. The temperature of the earth, and the air immediately upon its surface, is therefore lowered, and dew is formed.

It has been observed that sheep that have lain on the grass during the formation of dew have their backs completely saturated with it, but that underneath the line where their bodies turn to the earth, their coats will be dry. In the same manner glass globes suspended in the air, on dew forming nights, will be found loaded with globules of dew upon the top, but there will be no appearance of moisture underneath.

[Verse: "Dost thou know the balancings of the clouds, the wondrous works of him which is perfect in knowledge."--JOB XXXVII.]

350. _Why are star-lit nights usually colder than cloudy nights?_

Because heat is _radiated_ from the earth, and passes away into the utmost regions of the atmosphere.

351. _Why is there little dew under branches of thick foliage?_

Because the foliage _acts as a screen_, which prevents the radiated heat of the earth from passing away.

352. _Why is there no dew formed on windy nights?_

Because, as winds generally consist of dry air, they _absorb and bear away_ the atmospheric moisture.

353. _Why are valleys and low places chiefly subject to dew?_

Because the elevated lands around them _prevent the disturbance of the air_ in which the moisture is held.

354. _What bodies are most likely to be covered with dew?_

All bodies that are _good radiators of heat_, such as wool, swansdown, grass, leaves of plants, wood, &c.

355. _What bodies are likely to receive little dew?_

All _bad radiators of heat_, such as polished metal surfaces, smooth stones, and polished surfaces generally. Dew will be found to lie more abundantly upon rough and woolly leaves than upon smooth ones.

356. _At what period of the night is the largest amount of dew usually formed?_

It is generally supposed that dew is formed _most copiously_ in the _mornings_ and _evenings_. But _such is not the case_. It is deposited at all hours of the night, but _most plentifully after midnight_.

357. _Why is dew formed most plentifully after midnight?_

Because, as _radiation_ has been going on for some time, the temperature of the earth, and of various bodies upon it, has been _considerably reduced_.

[Verse: "Out of whose womb came the ice? and the hoary frost of heaven, who hath gendered it?"--JOB XXXVIII.]

358. _In what parts of the world is the maximum of dew formed?_

In warm lands near the sea, or in the vicinity of rivers or lakes, as the localities of the Red Sea, the Persian Gulf the coast of Coromandel, in Alexandria, and Chili.

359. _In what parts of the world is the minimum of dew formed?_

It is quite absent in arid regions, in the interior of continents, such as Central Brazil, the Sahara, and Nubia.

360. _Why is dew seldom formed at sea?_

Because of the defective _radiating_ quality of the surface of _water_.

361. _Why is a heavy dew regarded as the precursor of rain?_

Because a heavy formation of dew indicates that the air is _saturated with moisture_.

362. _What is hoar-frost?_

Hoar-frost is frozen dew.

363. _Why is hoar-frost said to foretell rain?_

Because it shows that the air is saturated with moisture, and the temperature of the air being low, the vapours are _likely to condense_, and produce _showers_.

364. _What is honey-dew?_

Honey-dew is the name applied to a _sweet and sticky moisture_ occasionally deposited upon the leaves of plants. It is, however, an error to call it _dew_, as it is procured by a class of _insects_ termed _aphides_.

365. _What are fogs?_

Fogs are _clouds_ formed near the earth's surface_;_ but London fogs are distinguished from clouds by the fact that they embrace in their vaporous folds the _smoke_ and _volatile matters_ imparted to the air by the operations of man. This is also the case with fogs generally that arise near large towns.

[Verse: "Hath the rain a father? or who hath begotten the drops of dew?"--JOB XXXVIII.]

366. _Why are certain coasts liable to almost perpetual fogs?_

Because of local or geographical agencies which contribute to their production. The coasts of California are almost constantly wrapped in fog; and, almost as constantly, the western coast of the American continent, as far south as Peru. Newfoundland, Nova Scotia, and Hudson's Bay, are all subject to dense and frequent fogs arising from the condensation of vapour from the water flowing from the hot Gulf-stream, coming in contact with the colder air.

367. _What are dry fogs?_

Dry fogs are characterised by a dull opaque appearance of the atmosphere. They are most common in certain parts of North America, though they sometimes occur in Germany and in England. They are generally referred to the _electrical state of the atmosphere_, but the theory of them is still a matter of doubt.

368. _What is a mist?_

The term _mist_ is generally applied to vapours that rise over _marshy places_, or the surfaces of _water_, and roll or move over the land.

369. _What is the difference between a mist and a fog?_

Fogs, as they are known to us, generally arise over the _land_, and are usually mingled with the smoke of large towns. Mists generally arise over water, or wet surfaces.

370. _Why do mists and fogs disappear at sunrise?_

Because the condensed vapours are again _expanded_ and _dispersed_ by the heat of the sun's rays.

371. _Why do fogs frequently rise in the morning and fall again in the evening?_

Because, warmed by the sun's rays, they become more rarefied, and fly away at an altitude where they _appear_ to be altogether dispelled; but at night, when the earth _cools by radiation_, the vapours near the earth _again condense_, and settle in the _form of fog_.

372. _Why do fogs sometimes rest upon a given locality for several days together, and then disappear?_

They are probably kept near to the surface of the earth by a superstratum of cold air. A cold air lying _above_, or a cold air lying _below_, might equally contribute to keep a fog near the surface of a particular part of the earth, until a _flow of wind_, or a _fall of rain_, altered the atmospheric condition.

[Verse: "He bindeth up the waters in his thick clouds; and the cloud is not rent under them."--JOB XXVI.]

There are many interesting facts connected with the history of dew. It has attracted the attention of natural philosophers in all ages. But its true theory was never understood until recently. The ancients imagined that dews were shed from the stars; and the alchemists and physicians of the middle ages believed that the dew distilled by night possessed penetrating and wonder-working powers. The ladies of those times sought to preserve their beauty by washing in dew, which they regarded as a "celestial wash." They collected it by placing upon the grass heaps of wool, upon the threads of which the magic drops clustered.

CHAPTER XVIII.

373. _What are clouds?_

Clouds are volumes of _vapour_, usually elevated to a considerable height.

[Illustration: Fig. 4--CIRRO-CUMULUS, OR SONDER CLOUD.]

374. _Whence do clouds arise?_

From the _evaporation of water_ at the earth's surface.

375. _Why do we not see them ascend?_

We do, sometimes, in the form of what we call _mists_, but generally the vapours that rise and contribute to the formation of clouds are so thin that they are _invisible_.

[Verse: "With clouds he covereth the light, and commandeth it not to shine by the cloud that cometh betwixt."--JOB XXXVI.]

376. _Why, if they are invisible when they rise, do they became visible when they have ascended?_

Because the vapours become _cooled_ in passing through the air, and form a denser body.

377. _Why, when they are condensed, do they not follow the course of gravitation, and descend?_

Because the vapours form into _minute vesicles_, which we may call _vapour bubbles_, and these, being warmed by the sun, are specifically _lighter than the air_.

Because, also, the lower parts of clouds _do partially_ descend, but again becoming more _rarefied_ by meeting with a _warmer atmosphere_, they again ascend, and are thus _poised_ upon the air.

Because, also, there is always a degree of atmospheric motion _upward_, caused by the _convection of heat_ from the earth's surface. And, although there must also be downward movements of the air to supply the place of that which has ascended, still _the heat_ of the ascending air, _combined with its upward movement_, expands and floats the vapour of the clouds.

378. _At what height do clouds usually fly?_

They fly at every degree of altitude; but clouds of _specific character_ are said to fly at given altitudes, or to occupy certain _ranges of altitude_. We will give their probable altitudes when speaking of the specific clouds.

[Illustration: Fig. 5.--CIRRUS, OR CURL CLOUD.]

[Verse: "Who giveth rain upon the earth, and sendeth waters upon the fields."--JOB V.]

379. _How many descriptions of clouds are there?_

There are _seven_.

1. The _Cirrus_ (Fig. 5), estimated range of altitude from 10,000 to 24,000 feet.

2. The _Cumulus_ (Fig. 7), from 3,000 to 10,000 feet.

3. The _Stratus_, an extended continuous level sheet of cloud, increasing from beneath. They fly very low.

4. The _Nimbus_ (Fig. 10), 1,500 to 5,000 feet.

5. The _Cirro-cumulus_ (Fig. 4), from 3,000 to 20,000 feet.

6. The _Cirro-stratus_ (Fig. 6), from 5,000 to 10,000 feet.

7. The _Cumulo-stratus_ (Fig. 9), from 3,000 to 10,000 feet.

[Illustration: Fig. 6--CIRRO-STRATUS, OR WANE CLOUD.]

The estimated heights given must be looked upon as very conjectural, although they have been derived from the best existing authorities. It is sufficient to know that the range of the altitude of the various clouds is from that of the _Nimbus_, or _thunder cloud_, 1,500 feet, to that of the _Cirrus_, 24,000 feet, the others being intermediate. The first three of the clouds above enumerated constitute what are called the _primary forms_. The remaining four are called _secondary forms_, because they arise, as their names generally indicate, out of combinations of the _primary forms_. Although, from the frequent mingling of clouds, it is not always practicable to identify them by the adopted classification, still, as there is generally a prevalence of one type of cloud over another, the observer would be able to distinguish a _"Cirrus sky,"_ or _"Cirro-cumulus sky,"_ &c. Upon some occasions the typical characters of the clouds are beautifully defined; and the contemplation of their forms, and the laws of their formation, affords infinite pleasure to the observer. The advantages of scientific knowledge are such, that whether you look downwards, to the earth, or upwards to the sky, you have still the writing of God to read.

380. _What produces the various shapes of clouds?_

1. The state of the _atmosphere_.

2. The _electrical_ condition of the clouds.

3. The _movements_ of the atmosphere.

4. The _season of the year_.

[Verse: "Behold, he withholdeth the waters, and they dry up; also he sendeth them out, and they overturn the earth."--JOB XII.]

381. _What are the dimensions of clouds?_

A single cloud has been estimated to have as many as _twenty square miles of surface_, and to be _above a mile in thickness_, while others are no larger than a _house_, or a man's _hand_.

[Illustration: Fig. 7.--CUMULUS, OR PILE CLOUD.]

382. _How are clouds affected by winds?_

If _cold winds_ blow upon the clouds, the cold condenses the vapour, turning the clouds into _rain_. But if _warm dry winds_ blow upon the clouds, they _rarefy the vapour_ to a greater degree, and temporarily _disperse the clouds_.

383. _How do winds affect the shapes of clouds?_

When winds are _mild and gentle_, the clouds break into _small patches_, and rise to a considerable height. But when the winds are cold and blustering, the clouds fly low, and roll along in _heavy masses_.

384. _Why are east winds usually dry?_

Because in coming towards England they pass over vast continents of land, and comparatively little ocean. Hence they are not loaded with _vapours_.

385. _Why do west winds generally bring rain?_

Because they come across the _Atlantic_, and are heavily charged with _vapour_.

386. _Why are north winds generally cold and dry?_

Because they come from the arctic ocean, over vast areas of _ice and snow_.

[Verse: "Terrors are turned upon me: they pursue my soul as the wind; and my welfare passeth away as a cloud."--JOB XXX.]

387. _Why are south winds warm and rainy?_

Because they come from the southern regions, heated by the _hot earth and sands_, and as they cross the sea they _absorb a large amount of vapour_.

[Illustration: Fig. 9.--CUMULO-STRATUS, OR TWAIN CLOUD.]

388. _Why are clouds said to indicate the changes of the weather?_

Because, as it is the _state of the clouds_ that, to a great extent, determines the _state of the weather_, the formation of the clouds must predicate approaching changes.

389. _What do cirrus clouds foretell?_

_Cirrus_ clouds foretell _fine_ weather, when they fly high, and are thin and light.

They foretell _light rain_ when, after a long continuance of fine weather, they form fleecy lines stretched across the sky.

They foretell a _gale of wind_ when, for some successive days, they gather in the same quarter of the heavens, as if denoting the point from which to expect the coming gale. (Fig. 5).

390. _What do cumulus clouds foretell?_

_Cumulus_ clouds, when they are well defined, and advance with the wind, foretell _fine weather_.

When they are thin and dull, and float against the wind, or in opposition to the lower currents, they _foretell rain_.

When they increase in size, and become _dull and grey at sunset_, they predict a _thunder-storm_. (Fig. 7.)

[Verse: "When he made a decree for the rain, and a way for the lightning and the thunder."--JOB XXVIII.]

391. _What do stratus clouds foretell?_

_Stratus_ clouds foretell _damp and cheerless weather_.

392. _What do nimbus clouds foretell?_

_Nimbus_ clouds foretell _rain_, _storm_, and _thunder_. (Fig. 10.)

393. _What do cirro-cumulus clouds foretell?_

_Cirro-cumulus_ clouds, in summer, foretell _increasing heat_ attended by _mild rain_, and a _south wind_; but in winter they commonly precede the _breaking up of a frost_, and the setting in of _foggy and wet weather_. (Fig. 4.)

394. _What do cirro-stratus clouds foretell?_

_Cirro-stratus_ clouds foretell _rain_ or _snow_, according to the season of the year.

These clouds extend in long horizontal streaks, thinning away at their base, and in parts becoming wavy or patchy.

When they are thus defined in the heavens they are a certain indication of _bad weather_. (Fig. 6.)

395. _What do cumulo-stratus clouds foretell?_

_Cumulo-stratus_ clouds usually foretell a _change of weather_--from rain to fine, or from fine to rain. (Fig. 9.)

[Illustration: Fig. 10.--NIMBUS, OR STORM CLOUD.]

[Verse: "Behold, I will put a fleece of wool in the floor; and if the dew be on the fleece only, and it be dry upon all the earth beside, then shall I know that thou wilt save Israel." * * *]

CHAPTER XIX.

396. _Why are cloudy days colder than sunny days?_

Because the clouds intercept the _solar rays_ in their course towards the earth.

397. _Why are cloudy nights warmer than cloudless nights?_

Because the clouds _radiate back to the earth_ the heat which the earth evolves?

Because, also, the clouds radiate to the earth the heat they have _derived from the solar_ rays during a cloudy day.

398. _Why is the earth warmer than the air during sunshine?_

Because the earth freely _absorbs the heat of the solar rays_; but the air derives _comparatively little heat_ from the same source.

399. _Why does the earth become colder than the air after sunset?_

Because the earth _parts with its heat freely by radiation_; but the air does not.

400. _Why do glasses, mats, or screens, prevent the frost from hitting plants?_

Because they prevent the _radiation of heat from the plants_, and also from the earth beneath them.

401. _Why are the screens frequently covered with dew on their exposed sides?_

Because they radiate heat from _both their surfaces_. A piece of glass, laid horizontally over the earth, would radiate heat both _upwards_ and _downwards_. But on its lower surface it would _receive_ the radiated heat of the earth, while from its upper surface it would _throw off its own heat_ and become cool. Therefore dew would be deposited upon the _upper_, but not on the _under_ surface.

402. _Why does dew rest upon the upper surfaces of leaves?_

Because the under surfaces receive the _radiated warmth of the earth_.

[Verse: "And it was so: for he rose up early on the morrow, and thrust the fleece together, and wringed the dew out of the fleece, a bowl full of water."]

403. _Why are cultivated lands subject to heavier dews than those that are uncultivated?_

Because cultivation breaks up the hard surface of the earth, and thus _its radiating power is increased_.

404. _Why is the gravel walk through a lawn comparatively dry while the grass of the lawn is wet with dew?_

Because gravel is a _bad radiator_, but grass is a _good radiator_.

405. _What benefit results from this arrangement?_

In cultivated lands, where moisture is required, it is _induced_ by the very necessity which demands it; while in rocky and barren places, where it would be of no good, dew _does not form_.

406. _Why does little dew form at the base of hedges and walls, and around the trunks of trees?_

Because those bodies in some degree _counteract the radiation_ of heat from the earth; and they also _radiate heat_ from their own substances.

407. _Why do heavy morning dews and mists usually come together?_

Because they both have their origin in the _humidity of the atmosphere_. The temperature of the earth having fallen, dew has been deposited; but, at the same time, the condensation of the vapour in the air _has formed a screen over the surface of the earth_, which has checked _the further radiation of heat_, and, consequently, _the further formation of dew_. The sun rises, therefore, upon an atmosphere charged with visible vapour at the earth's surface, and his first sloping rays, _having little power to warm the atmosphere_, the mist _continues visible for some time_.

408. _What effect have winds upon the formation of dew?_

Winds, generally, and especially when rapid, prevent the formation of dew. But those winds that are moist, and _contribute to the formation of clouds_, indirectly aid the formation of dew _through the formation of clouds_, and also by the _moisture they impart to the air_.

[Verse: "And Gideon said unto God, * * * Let it now be dry only upon the fleece, and upon all the ground let there be dew."]

409. _Why does the humidity of the atmosphere sometimes form clouds, and at others form fogs, mists, dews, &c.?_

The result depends upon the varying _temperature_, _motion_, and _direction_ of the atmosphere.

A _warm light atmosphere_, of a few day's duration, will elevate the vapours to the region where they are formed into _clouds_.

A _chill air_, lying upon the surface of the warmer earth, will occasion _mists_ or _fogs_.

A _cold earth_, acting upon the vapours contained in a _warmer atmosphere_, will condense them and occasion _dews_.

410. _Why are frosty mornings usually clear?_

Because, in the cold atmosphere which preceded the frost, _there was but little evaporation_; and now that the frost has set in, the vapours that existed have become _frozen_ in the form of _hoar-frost_.

411. _Why are clear nights usually cold?_

Because the "screen" afforded by the clouds does not exist; therefore the heat of the earth escapes, while the vapours of the air are abstracted from it by condensation into dew, thereby imparting great _clearness to the nights_.

412. _Why are hoar-frosts, or, as they are termed, "white frosts," so frequent, and "black frosts" so unusual?_

Because white, or _hoar frosts_, result from the _coldness of the earth_, which, from its great radiating power, is always varying. But _black-frosts_ result from the _coldness of the air_, which is liable to less variation of temperature than the earth.

413. _What is a black-frost?_

A _black-frost_ results from the _coldness of the atmosphere_, which is at the time overshadowed by a dull cloud, giving a darkness to everything, and a leaden appearance to the _frozen surface of water_.

414. _Why are black-frosts said to last?_

Because as they result from the temperature of the air, which is less likely to vary than that of the earth, there is a probability that the coldness thereof _will last for some time_.

[Verse: "And God did so that night: for it was dry upon the fleece only, and there was dew on all the ground,"--JUDGES VI.]

415. _What benefits result from the radiation of heat, &c.?_

But for the _radiation of heat_, we should be subjected to the most unequal temperatures. The setting of the sun would be like _the going out of a mighty fire_. The earth would become _suddenly cold_, and its inhabitants would have to bury themselves in warm covering, to wait the return of day. By the _radiation_ of heat, an _equilibrium of temperature_ is provided for, without which we should require a new order of existence.

The amount of heat which our earth _receives from the sun_, and the economy of that heat by the laws of _radiation_, _reflection_, _absorption_, and _convection_, are exactly proportionate to the necessities of our planet, and the living things that inhabit it. It is held by philosophers that any change in the orbit of our earth, which would either increase or decrease the amount of heat falling upon it, would, of necessity, be followed by the _annihilation of all the existing races_. The planets Mercury and Venus, which are distant respectively 37 millions of miles, and 63 millions of miles, from the great source of solar heat, possess a temperature which would _melt our solid rocks_; while Uranus (1,800 millions of miles), and Neptune (whose distance from the sun has not been determined), must receive so small an amount of heat, that water, such as ours, would become as solid as the hardest rock, and our atmosphere would be resolved into a liquid! Yet, poised in the mysterious balance of opposing forces, our orb flies unerringly on its course, at the rate of 63,000 miles an hour; preserving, in its wonderful flight, that precise relation to the sun, which takes from his life-inspiring rays the exact degree of heat, which, being shared by every atom of matter, and every form of organic existence, _is just the amount needed to constitute the heat-life of the world_!

CHAPTER XX.

416. _What is rain?_

Rain is the _vapour of the clouds_ which, being condensed by a fall of temperature, forms drops of water that descend to the earth.

It is the _return to the earth_ in the form of _water_, of the moisture _absorbed by the air_ in the form of _vapour_.

417. _Does rain ever occur without clouds?_

It sometimes, but rarely happens, that a sudden transition from warmth to cold will _precipitate the moisture of the air_, without the formation of _visible clouds_.

[Verse: "Canst thou lift up thy voice to the clouds, that abundance of waters may cover thee?"--JOB XXXVIII.]

418. _Why are drops of rain sometimes large and at other times small?_

Because the drops, in falling, _meet and unite_, and also gather _moisture_ in their descent. The greater the height from which a rain drop has descended, _the larger it is_, provided that its whole course lay through a _rainy atmosphere_.

The size of the drops is also influenced by the _amount of moisture in the atmosphere_, the _degree of cold_, and the _rapidity_ of the _change of temperature_, by which the drops are produced.

419. _In what seasons of the year are rains most prevalent?_

Throughout _Central Europe_ rains are most prevalent in _summer_, but in _Southern Europe_ the preponderance is on the side of _winter rains_.

420. _In what months of the year does it rain most frequently in this country?_

It rains more frequently _from September to March_, than from _March to September_; but the _heaviest rains_ occur from _March to September_.

421. _Why are there more rainy days from September to March?_

Because the temperature of the air is more frequently lowered to that degree which _precipitates its vapours_.

_Months in the order of their comparative wetness_:--1. October. 2. February. 3. July. 4. September. 5. January. 6. December.

_Months in the order of their comparative dryness_:--1. March. 2. January. 3. May. 4. August. 5. April. 6. November.

422. _In what part of the world does the greatest quantity of rain fall?_

The greatest _quantity_ of rain falls near the _equator_, and the amount _decreases towards the poles_.

[Verse: "Who can number the clouds in wisdom? or who can stay the bottles of heaven."--JOB XXXVIII.]

423. _In what part of the world do the heaviest rains occur?_

The _heaviest_ rains occur in the _tropics_, during the hot season. The drops of rain in the tropical regions are so large, and the force with which they descend so great, that their splash upon the skin causes a _smarting sensation._

424. _In what parts of the world do the least rains occur?_

There are some parts of the earth which are _rainless_, such as Egypt, the desert of Sahara, the table lands of Persia and Montgolia, the rocky flat of Arabia Petræ, &c.

425. _How many rainy days are there in a year?_

The frequency of rainy days is greatest in countries near the sea, and their number decreases the further we journey from the sea-border towards the inland. In England it rains on an average 152 to 155 days in the year.

426. _In what part of England does the greatest amount of rain fall?_

In the town of _Keswick_, in Cumberland, where 63 inches of rain fall in a year; Kendal, in Westmoreland, 58 inches; Liverpool, 34 inches; Dublin, 25 inches; Lincoln, 24 inches; London, 21 inches.

427. _Why do the heaviest rains occur at the tropics?_

Because the _hot air_ absorbs a large amount of vapour, and rises into the higher regions of the atmosphere, where the vapours are _suddenly condensed into heavy rains_, by cold currents from the poles.

428. _Why does the greatest quantity of rain fall at the equator?_

Because the _hot air_ absorbs a large amount of vapour, and as the atmosphere is usually calm, there is an absence of currents, by which the saturated air would be removed. In this, which is called "_the Region of Calms_," rain falls almost daily.

429. _Why are some parts of the earth rainless?_

Because, being situated in tropical or torrid latitudes, and at a distance from the ocean, the atmosphere above them is always in a _dry state_.

[Verse: "Thou, O God, didst send a plentiful rain, whereby thou didst confirm thine inheritance, when it was weary."--PSALM LXVIII.]

430. _When is air said to be saturated with vapour?_

When it cannot take up _a larger quantity_ than that which it already holds.

When common salt is dissolved in water, until the water can take up no more, the water is then said to be _saturated with salt_.

431. _What proportion of water is air capable of sustaining in the form of vapour?_

The amount of water held in suspension by the air averages the following proportion: one thousand _cubic feet of air_ contain as much vapour as, were it condensed to water, would yield about _two fifths of a pint_.

But _one thousand cubic feet of air_ are capable of holding _half-a-pint of water_; and this may be regarded as the _point of saturation_.

Thus, in a room ten feet square and ten feet high, the air, _at the point of saturation_, would hold in the form of vapour, _half-a-pint of water_. It must not be forgotten, however, that the point of saturation necessarily varies with the _temperature of the air_.

432. _Why are cloudy days and nights not always wet?_

Because the air has not reached the state of _saturation_.

433. _Why does rain purify the air?_

Because it produces motion in the particles of the air, by which they are _intermixed_. And it precipitates noxious _vapours_, and cleanses the face of the earth from _unhealthy accumulations_.

434. _Why are mountainous localities more rainy than flat ones?_

Because the mountains _attract the clouds_; and because the clouds that are flying low are borne against the sides of the mountains and directed upwards, where they meet with _cold currents of air_.

435. _Why does more rain fall by night than by day?_

Because by night the temperature of the air, heated during the day, falls to that degree which condenses _its vapours into rain_.

[Verse: "As the hart panteth after the water brooks, so panteth my soul after thee O God."--PSALM XLII.]

436. _Why do bunches of dried sea-weed indicate the probability of coming rain?_

Because they readily imbibe moisture, and when they become soft and damp they show that the air is _approaching the point of saturation_.

437. _Why does the weather-toy, called the "weather-cock," foretell the probability of rain?_

Because it is made with a piece of cat-gut which swells with moisture, and as it swells, _shrinks_. The cat-gut is so applied that when it _shrinks_, it turns a rod which sends the _man_ out of the house, and when it _dries_ it sends the _woman out_. Therefore, when the _man_ appears, it is a sign of _wet_, and when the _woman_ appears it is a sign of _dry weather_.

There is another toy, called the Capuchin, which is made upon the same principle. The figure lifts a hood over its head when wet is approaching, and takes it off when the weather is becoming dry. In this case, a piece of cat-gut is also employed. Various weather-toys may be made upon this principle--among others, a little umbrella, which will open on the approach of wet, and close on the return of fine weather.

A gentleman once made a wooden horse, which he declared should of itself walk across a room, without machinery of any kind. The assertion was discredited; but the horse was placed in a room close to the wall on one side. The room was locked, and otherwise fastened, so that no one could interfere with the experiment. After a time the door was opened, and it was found that the horse had actually crossed the floor, and stood on the opposite side. The horse was made from wood of a peculiar kind, liable to great expansion in wet weather, and cut in a manner to produce the greatest elongation. The fore hoofs were so made that where they were set they would remain, so that the contracting parts should draw up from behind. It is easy to understand how, in this way, the wooden horse crossed the apartment.

438. _Why does ladies' hair drop out of curl upon the approach of damp weather?_

Because the hair _absorbs moisture_, which causes its spirals to relax and unfold.

439. _Why is it said in mountainous countries that rain is coming, because the mountains are "putting their night-caps on?"_

Because the clouds descend when they are _heavy with vapour_, and being attracted to the mountain tops they are said to "_cap the mountains."_

[Verse: "Hast thou entered into the treasures of the snow; or hast thou seen the treasures of the hail."--JOB XXXVIII.]

CHAPTER XXI.

440. _What is snow?_

Snow is _congealed vapour_, which would have formed _rain_; but, through the coldness of the air, has been _frozen_ in its descent into _crystalline forms_. (Fig. 1.)

441. _Why is snow white?_

Because it reflects all the component rays of _light_.

442. _Why is snow said to be warm, while white garments are worn for coolness?_

Snow is _warm_ by virtue of its light and woolly texture. But it is also warm on account of its _whiteness_; for, had it been _black_, it would have _absorbed the heat of the sun_, which would have _thawed the snow_. Instead of which, it _reflects heat_; and the reflected heat _falls upon_ bodies above the snow, while the _warmth of the earth_ is preserved _beneath it_. _White clothing is cool_, because it reflects _from_ the body of the wearer the heat of the sun. _White snow_ is _warm_, because it _reflects the sun's heat upon bodies_.

There are few persons but have felt the effect of the sun's rays _reflected_ by the white snow on a clear wintry day. And, as regards the warmth of snow towards the earth, by preventing the radiation of heat, it has been found that a thermometer buried four inches deep in snow has shown a temperature of _nine degrees_ higher than at the surface.

443. _Why are lofty mountains always covered with snow?_

Because the _upper regions_ of the atmosphere are _intensely cold_.

444. _Why are the upper regions of the atmosphere intensely cold?_

Because the _atmosphere_ retains but _little of the heat of the sun's rays_ as they pass to the earth. Because at high altitudes the air is _greatly rarefied_. And because the _radiation of heat from the earth_ does not materially affect such _high regions_.

[Verse: "He causeth the vapours to ascend from the ends of the earth: he maketh lightnings for the rain: he bringeth the wind out of his treasuries."--PS. XXXV.]

445. _What is meant by the snow line?_

The _snow line_ is the estimated altitude in _all countries_ where _snow would be formed_. Even at the equator, at an altitude of 15,000 to 16,000 feet from the level of the sea, snow is found upon the mountain summits, where it perpetually lies. As we proceed north or south from the equator the _snow line lessens in altitude_. Had we in England a mountain 6,000 feet high, it would be perpetually _crowned with snow_.

446. _Why do we hear of red snow?_

Red snow is the name given to the snow in the arctic regions upon which a minute vegetable (probably the _Protoccus nivalis_) grows, imparting to the snow a red colour. Recent microscopic investigations have shown it to consist of a minute vegetable cell, which secretes a red colouring matter.

Snow is found to be of greater importance to man than is generally supposed. But, although in this country we are enabled to recognise the hand of Providence in the gift, there are latitudes wherein the blessing thus conferred is more deeply felt. In such countries as Canada, Sweden, and Russia, the falling of snow is looked for with glad anticipations, quite equalling those which herald the "harvest-home" of England, or the "vintage" of France. No sooner is the ground covered with snow, than cranky old vehicles that had been jolting over rough roads, and sticking fast in deep ruts of mud, are wheeled aside, and swift sledges take their place. Towns distant from each other find an easy mode of communication; the markets are enlivened, and trade thrives. Snow supplies a kind of railroad, covering the entire face of the country, and sledges glide over it, almost with the speed of the locomotive.

447. _What is sleet?_

_Sleet_ is snow which, in falling, has met with a _warmer current of air_ than that in which it congealed. It therefore partially melts and forms a kind of _wet snow_.

448. _What is hail?_

_Hail_ is also the _frozen moisture of the clouds_. It is probably formed by _rain drops_ in their descent to the earth, meeting with an _exceedingly cold current of air_ by which they become _suddenly frozen into hard masses_.

It is also supposed that the _electrical_ state of the air and of the clouds influences the formation of _hail_.

[Verse: "If the clouds be full of rain, they shall empty themselves upon the earth."--ECCLES. XI.]

449. _Why is it supposed that the electrical state of the air and the clouds affects the formation of hail?_

Because hail is more common in the _summer_ than at other seasons, and is frequently attended by storms of _thunder and lightning_.

450. _Why do hail-storms most frequently occur by day?_

Because the clouds, being charged with vapour to saturation, favour the formation of hail by _sudden_ electrical or atmospheric changes. In the gradual cooling of night, the clouds would expend themselves in rain.

Astonishing facts respecting hail-storms are upon record. In 1719 there fell at Kremo, hailstones weighing six pounds. In 1828 there was a fall of ice at Horsley, in Staffordshire, some of the pieces of which were three inches long, by one inch broad; and other solid pieces were about three inches in circumference. Hail storms are most frequent in June and July, and least frequent in April and October. Hail clouds float much lower in the sky than other clouds; their edges are marked by frequent heavy folds; and their lower edges are streaked with white, the other portions being massive and black. (Fig. 10.)

CHAPTER XXII.

451. _What is light?_

Light, according to Newton, is the effect of luminous particles which dart from the surfaces of bodies in all directions. According to this theory, the solar light which we receive would _depart from the sun and travel to the earth_.

According to Huyghens, light is caused by an _infinitely elastic ether, diffused through all space_. This ether, existing everywhere, is _excited into waves, or vibrations, by the luminous body_.

The theory of light is so undetermined that neither the views of Newton, nor those of Huyghens, can be said to be exclusively adopted. Writers upon natural philosophy seize hold of either or both of those theories, as they present themselves more or less favourably in the explanation of natural phenomena. In "_The Reason Why,"_ as we have to speak of the _effects_ of light rather than of its _cause_, we shall avoid, as far as possible, the doubtful points. But let no one be discouraged by the fact that the theory of light, as, indeed, of all the imponderable agents, is imperfectly understood. Rather let us rejoice that there are vast fields of discovery yet to be explored; and that light, the most glorious and inspiring element in nature, invites us from the sun, the moon, and the stars, and from the face of every green leaf and variegated flower, to search out the wonders of its nature, and further to exemplify the goodness and wisdom of God.

[Verse: "And God said, Let there be light: and there was light."]

452. _What is the distance of the sun from the earth?_

Ninety five millions of miles.

453. _At what rate of velocity does light travel?_

At the rate of 192,000 miles in a _second_, through our _atmosphere_; and 192,500 miles in a _second_ through a _vacuum_.

454. _How long does light take to travel from the sun to the earth?_

Eight minutes and thirteen seconds.

455. _What is the constitution of the sun?_

It is a spherical body, 1,384,472 times larger than the earth.

456. _From what does the luminosity of the sun arise?_

From a luminous atmosphere, or, as M. Arago named it, _photosphere_, which completely surrounds the body of the sun, and which is probably _burning with great intensity_.

457. _What are the minor sources of light?_

Light may be produced by _chemical action_, by _electricity_, and by _phosphoresence_, in the latter of which various agencies unite.

458. _What is a ray of light?_

A _ray_ of light is the _smallest portion_ of light which we can recognise.

459. _What is a medium?_

A _medium_ is a body which affords _a passage for the rays_ of light.

460. _What is a beam of light?_

A _beam_ of light is a _group of parallel rays_.

461. _What is a pencil of light?_

A _pencil_ of light is a body of rays which _come from or move towards a point_.

[Verse: "And God saw the light, that it was good: and God divided the light from the darkness."--GEN. I.]

462. _What is the radiant point?_

The _radiant point_ is that _from which diverging rays of light are emitted_.

463. _What is the focus?_

The _focus_ is the point to which _converging rays are directed_.

_Diverging_, starting from a point, and separating. _Converging_, drawing together towards a point.

464. _What is the constitution of a ray of light?_

A ray of _white light_, as we receive it from the sun, is composed of _a number of elementary rays_, which, with the aid of a triangular piece of glass, called a _prism_, may be separated, and will produce under refraction the following colours:--

1. An _extreme red_ ray--a mixture of _red_ and _blue_, the red _predominating_.

2. _Red._

3. _Orange_--red passing into and combining with yellow.

4. _Yellow_--the most luminous of all the rays.

5. _Green_--yellow passing into and combining with the blue.

6. _Blue._

7. _Indigo_--a dark and intense blue.

8. _Violet_--blue mingled with red.

9. _Lavender grey_--a neutral tint.

10. Rays called _fluorescent_, which are either of a _pure silvery blue_, or a _delicate green_.

465. _Why is a ray of light, which contains these elementary rays, white?_

Because the colour of light is governed by the _rapidity of the vibrations of the ether-waves_. When a ray of light is refracted by, or transmitted through a body, its _vibrations are frequently disturbed and altered_, and thus a _different impression_ is made upon the _eye_.

Light which gives 37,640 vibrations in _an inch_, or 458,000,000,000,000 in a _second of time_, produces that sensation upon the eye which makes the object that directs the vibrations appear _red_. _Yellow_ light requires 44,000 vibrations _in an inch_, and 535,000,000,000,000 in a _second of time_. And the other colours enumerated (_see_ 464) all require different _velocities of vibration_ to produce the colours by which they are distinguished.

[Verse: "The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light."--MATT. V.]

Accepting the theory of vibrations, and applying it to the elucidation of the phenomena of light--it is unnecessary, we think, to believe that a ray of _white_ light _contains_ rays in _a state of colour_. It is said that if we divide a circular surface into parts, and paint the various colours in the order and proportions in which they occur in the refracted ray, and then spin the circle with great velocity, the colours will blend and appear _white_. But such is not the case; the result is in some degree an illusion, arising out of the sudden removal of the impression made upon the eye by the colours; and if a piece of white paper be held by the side of the coloured circle in motion, the latter will be found to be _grey_. When it is remembered that in colouring a white surface with thin colours, the white materially qualifies the colours, it must be admitted that the experiment fails to support the assertion that the colours of the spectrum produce white. But there can be no difficulty in understanding that a ray of light undergoing _refraction_, becomes divided into minor rays, which _differing in their degrees of refrangibility_, vary also in the _velocity of their vibrations_, and produce the several sensations of colour.

466. _Why is a substance white?_

Because it reflects the light that falls upon it _without altering its vibrations._

467. _Why is a substance black?_

Because it _absorbs the light_ and _puts an end to the vibrations_.

468. _Why is the rose red?_

Because it imparts to the light that falls upon it that _change in its vibratory condition_, which produces on our eyes the _sensation of redness_.

469. _Why is the lily white?_

Because it reflects the light without altering its vibrations.

470. _Why is the primrose yellow?_

Because, though it receives white light, it alters its vibrations to 44,000 in an inch, and 535,000,000,000,000 in a second, and this is the velocity of vibration which produces upon the eye a _sensation_ of _yellow_.

[Verse: "But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness."--MATT. V.]

471. _Why are there so many varieties of colour and tint in the various objects in nature?_

Because every surface has a peculiar constitution, or atomic condition, _by which the light falling upon it is influenced_. In tropical climates, where the brightness of the sun is the most intense, there the colours of natural objects are the richest; the foliage is of the darkest green; the flowers and fruits present the brightest hues; and the plumage of the birds is of the most gaudy description. In the temperate climates these features are more subdued, still bearing relation to the degree of light. And at a certain depth of the ocean, where light penetrates only in a slight degree, the objects that abound are nearly colourless.

It has been held by many philosophers (and the theory is so far conclusive that it cannot be dispensed with) that there is an analogy between the vibratory causes of _sound_, and the vibratory causes of _colour_. Any one who has seen an Æolian harp, and listened to the wild notes of its music, will be aware that the wires of the harp are swept by accidental currents of air; that when those currents have been strong, the notes of the harp have been raised to the highest pitch, and as the intensity of the currents has fallen, the musical sounds have deepened and softened, until, with melodious sighing, they have died away. No finger has touched the strings; no musical genius has presided at the harp to wake its inspiring sounds; but the vibration imparted to the air, as it swept the wires, has alone produced the chromatic sounds that have charmed the listener. If, then, the varied vibrations of the _air_ are capable of imparting dissimilar sensations of _sounds_ to the _ear_, is it not only possible, but probable, that the different vibrations of _light_ may impart the various sensations of _colours_ to the _eye_?

CHAPTER XXIII.

472. _What is the refraction of light?_

When rays of light fall _obliquely_ upon the surface of any _transparent medium_, they are slightly diverted from their course. This alteration of the course of the rays is called _refraction_, and the degree of refraction is influenced by the difference between the _densities_ of the mediums _through which light is transmitted_.

[Verse: "Let your light so shine before men, that they may see your good works, and glorify your father which is in heaven."--MATT. V.]

473. _If a ray of light falls in a straight line upon a transparent surface, is it then refracted?_

In that case the ray pursues its course--_there is no refraction_.

474. _Is the direction in which the rays are bent, or refracted, influenced by the relative densities of the media?_

A ray of light falling slantingly upon a _window_, in passing through it is slightly brought to the _perpendicular_; and if it then falls upon the surface of water, it is still further brought to the perpendicular in _passing through the water_.

475. _Is light refracted in passing from a dense medium to a thinner one?_

It is; but the _direction of the refraction_ is just the opposite to the instance just given; a ray of light passing through _water_ into _air_, does not take a more _perpendicular course_, but becomes more _oblique_.

[Illustration: Fig. 11.]

476. _Why, if a rod or a spoon be set in an empty basin, will it appear straight, or of its usual shape?_

Because the rays of light that are reflected from it all pass through the same medium, the _air_.

477. _Why if water be poured into the basin will the rod or spoon appear bent?_

Because the rays of light that pass through the _water_ are _reflected in a different degree_ to those that pass through the air.

[Verse: "Evening, and morning, and at noon, will I pray, and cry aloud; and he shall hear my voice."--PSALM LV.]

Place in the bottom of an empty basin (Fig. 11.) a shilling; then stand in such a position at the point B that the line of sight, over the edge of the basin, just excludes the shilling from view. Then request some one to pour water into the basin, until it is filled to C (Fig. 12.), keeping your eye fixed upon the spot. The shilling will gradually appear, and will soon come entirely in view. Not only will the shilling be brought in view, but also portions of the basin before concealed. This is owing to the rays of light passing from the bottom through the water in a direction _more perpendicular_ than they would have done through the air; but on leaving the water they become more _oblique_, and hence they convey the image of the shilling _over the edge of the basin_, which otherwise would have obstructed the view.

[Illustration: Fig. 12.]

478. _Why is it that in cloudy and showery days we see the sun's rays bursting through the clouds in different directions?_

Because, in passing through clouds of _different densities_ the rays are _bent out of their course_.

479. _Why is the apparent depth of water always deceptive?_

Because the light reflected from the objects at the bottom is _refracted_ as it leaves the water.

480. _How much deeper is water than it appears to be?_

About _one-third_. A person bathing, and being unable to swim, should calculate before jumping into the water, that if it _looks two feet deep_, it is quite _three feet_.

481. _Why can we seldom at the first attempt touch anything lying at the bottom of the water with a stick?_

Because we do not allow for the _different refractive powers_ of water and of air.

[Verse: "I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth."]

482. _Why do we see the sun before sunrise, and after sunset?_

Because of the refractive effects of the atmosphere. Rays of light, passing obliquely from the sun through the air to the earth, are refracted three or four times by the varying density of the medium. Each refraction bends the rays towards the _perpendicular_; and hence we see the sun _before it rises_ and _after it sets_.

[Illustration: Fig. 13.--DIAGRAM EXHIBITING THE REFRACTION OF THE SUN'S RAYS IN PASSING THROUGH THE ATMOSPHERE.]

483. _Why do figures, viewed through the hot air proceeding from furnaces, and from lime-kilns, appear distorted and tremulous?_

Because the ever varying density of the air which is flying away in hot currents, and succeeded by cold, _constantly changes the refractive power_ of the medium through which the figures are viewed.

484. _Why do the stars twinkle?_

Because their light reaches us through _variously heated and moving currents of air_. In this case the earth is the _kiln_, and the _stars_ the _object_ that is _viewed through the refractive medium_.

485. _Why does much twinkling of the stars foretell bad weather?_

Because it denotes that there are _various ærial currents_ of different temperatures and densities, producing _atmospheric disturbance_.

[Verse: "And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud."--GENESIS IX.]

486. _What causes the rainbow?_

The _refraction_ of the sun's rays by the _falling rain_.

487. _Why does the rainbow exhibit various colours?_

The colours belong to the _elementary rays of light_; and these rays having _different degrees of refrangibility_, some of them are bent more than others; they are therefore separated into _distinct rays of different colours_.

488. _Why are there sometimes two rainbows?_

Because the rays of _refracted_ light, reflected upon other drops of rain, are _again_ refracted, and then _reflected again_, forming a secondary bow.

489. _Why are the colours of the secondary bow arrayed in the reverse order of the primary bow?_

Because the secondary bow is _a reflection_ of the primary bow, and, like all reflections, is reversed.

490. _Why are reflections reversed?_

Because those rays which _first reach_ the reflecting surface are the _first returned_. If you hold your open hand towards the looking-glass, the light passing from the point of your finger will reach the reflector and be returned before the rays that pass from the back parts of the hand. Hence the image of the hand will present the reflection of the finger point towards the point of the finger.

491. _Why are the colours of the secondary rainbow fainter than those of the primary?_

Because they are derived from the _refraction and reflection_ of rays which have _already_ been refracted and reflected, and thereby _their intensity has been diminished_.

492. _What is a lunar rainbow?_

A _lunar rainbow_ is caused by the light of the _moon_, in the same manner as the _solar rainbow_ is caused by the light of the _sun_.

[Verse: "I am come a light into the world, that whosoever believeth in me should not abide in darkness."--JOHN XIII.]

493. _Why is the lunar rainbow fainter than a solar rainbow?_

Because the _light of the moon_ is the _reflected light of the sun_, and is therefore _less intense_.

494. _What is a halo?_

A halo is a _luminous ring_, which forms between the eye of the observer and a luminous body.

Haloes may appear around the disc of the sun, moon, or stars. But in this country the _lunar_ haloes are the most remarkable and frequent.

495. _What is the cause of the luminous ring?_

The _refraction of light_ as it passes through an intervening _cloud_, or a stratum of _moist_ and _cold air_.

496. _Why are haloes sometimes large and at other times small?_

Because they are sometimes formed _very high_ in the atmosphere, at other times _very low_. Being high, and farther removed from the spectator, and nearer the source of light, they appear _smaller_; while the nearer they are, the _larger they appear_.

497. _Why do haloes foretell wet weather?_

Because they show that there is a great amount of atmospheric moisture, which will probably form _rain_.

498. _Why do glass lustres and chandeliers exhibit "rainbow colours"?_

Because they _refract the rays of light_ in the same manner as the rain drops.

499. _Why does a soap bubble show the prismatic colours?_

Because, like a large rain drop, it _refracts the rays of light_, and shows the elementary rays.

500. _What causes the rich tints displayed by "mother-of-pearl?"_

The _refraction of the light_ that falls upon the surface of the pearl.

[Verse: "Light is sown for the righteous, and gladness for the upright in heart."--PSALM XCVII.]

501. _What causes the brilliant colours of the diamond?_

The _refraction_ of the rays of light by the various _facets_ of the diamond.

The refraction of light, and the production of prismatic colours, surrounds us with most interesting phenomena. The laundress, whose active labours raise over the wash-tub a soapy froth, performs inadvertently one of the most delicate operations of chemistry--the chemistry of the imponderable agents--and the result of her manipulations manifests itself in the delicate colours that dance like a fairy light over the glassy films that follow the motion of her arms. The laughing child, throwing a bubble from the bowl of a tobacco pipe into the air, performs the same experiment, and produces a result such as that which filled the philosophic Newton with unbounded joy. The foam of the seashore, the plumage of birds, the various films that float upon the surface of waters, the delicate tints of flowers, and the rich hues of luscious fruits, all combine to remind us, that every ray of light comes like an angelic artist sent from heaven, bearing upon his palette the most celestial tints, with which to beautify the earth, and show the illimitable glory of God.

CHAPTER XXIV.

502. _What is the difference between the refraction and the reflection of light?_

_Refraction_ is the deviation of rays of light from their course through the interference of a _different_ medium; _reflection_ is the return of rays of light which, having fallen upon a surface, are repelled by it.

503. _What is the radiation of light?_

The _radiation_ of light is its _emission in rays_ from the surface of a _luminous body_.

504. _Do all bodies radiate light?_

All bodies radiate light; but those that are not in themselves primary sources of light, are said to _reflect it_.

505. _Do black bodies reflect any light?_

Black bodies _absorb_ the light that falls upon them. But they reflect a _very small_ degree of light.

506. _Why is glass transparent?_

Because its atoms are so arranged that they allow the vibrations of light to continue through their substance.

[Verse: "As in water face answereth to face, so the heart of man to man."--Proverbs xxvii.]

507. _Does glass obstruct the passage of any portion of light?_

Glass _reflects_ (sends back) a very small portion of light. This may be observed by holding a piece of paper, or a hand, a few inches from a window, when a faint reflection of it will be visible. Probably the small amount of light _reflected by transparent glass, which gives a passage to the greater part of the rays_, may serve to illustrate the small amount of light reflected from _black surfaces_, which _absorbs the greater portion of light_.

Instead of a piece of white paper, hold a piece of _black cloth_ two or three inches from the window-pane, and you will have two reflections so weak that the image of the cloth will be almost lost. The first reflection is that of the very small amount of light from the black surface on to the glass, and the second reflection is that of the inconceivably small amount returned by the glass, and by which the faint image of the black cloth is produced. But put the black cloth outside of the window-pane, and then hold an object before them, and you will find that the _two weak reflectors, acting together_, produce an improved image, or reflection.

508. _Why, if a book is held between a candle-light and the wall, does a shadow fall upon the wall?_

Because the rays of light are _intercepted_ by the book.

509. _Why do the rays pass over the edges of the book in a direct line with the flame of the candle?_

Because light always travels in _straight lines_.

510. _Why is there some amount of light even where shadows fall?_

Because, _as all objects reflect light_, some of them throw their light into the field of the shadow.

511. _Why are some substances opaque to light?_

Because the arrangement of their particles will not admit of the _vibrations of the luminous ether_ passing through them.

Opaque--impervious to rays of light.

512. _Why do we see our faces reflected in mirrors?_

Because the rays of light from our faces are _reflected_ by the surface of the _quicksilver_ at the back of the glass.

[Verse: "The day is thine, the night also is thine: thou hast prepared the light and the sun."--PSALM LXXIV.]

513. _Why does the quicksilver reflect the rays of light?_

Because, being _densely opaque to light_, and presenting also a bright surface, it is a good reflector, and it _throws back the whole of the rays_.

514. _What has the glass to do with the reflection?_

The glass has _nothing to do with the reflection_, except that it affords a field upon which the reflecting surface of the quicksilver is spread; and it keeps the air and dirt from _dulling the quicksilver_.

The parts of a mirror from which the quicksilver is rubbed away give no reflection that could assist the reflecting power of the quicksilver. That the surface of the glass does not reflect the image, is shown by the fact, that if you put the point of any object against the glass, the thickness between the point and the place where the reflection of it begins, will _show the exact thickness of the glass_.

515. _Why does a compound mirror (a multiplying mirror) exhibit a large number of images of one object._

Because all objects reflect rays of light in _every direction_, and therefore the different mirrors, being at _various angles_, receive _each a reflection_ of the same object.

516. _Why does a window-pane appear to be a better reflector by candle-light than by day-light?_

The reflecting power of glass is precisely the same by night as by day, and is always very feeble. But it appears to be greater by night, _because the surrounding darkness increases the apparent strength of the reflection_.

517. _How do we know that objects reflect light in every direction?_

Because if we _prick a hole in a card with a pin_, and then look through that small hole upon a _landscape_, we can see some miles of country, and some thousands of objects; every part of every object throughout the whole scene, must have sent rays of light the small hole pricked in the card.

[Verse: "Such knowledge is too wonderful for me; it is high, I cannot attain unto it."--PSALM CXXXIX.]

At one extremity of the landscape, viewed through the hole in the card, there may be a forest of trees; in the distance there may be hills bathed in golden light, and overhung with glittering clouds; in the mid-distance there may be a river winding its course along, as though it loved the earth through which it ran, and wished, by wandering to and fro, to refresh the thirsty soil; in the foreground may be a church, covered by a million ivy leaves; and grouping towards the sacred edifice may be hundreds of intending worshippers, old and young, rich and poor; flowers may adorn the path-ways, and butterflies spangle the air with their beauties; yet every one of those objects--the forest, the hills, the clouds, the river, the church, the ivy, the people, the flowers, the butterflies--must have sent rays of light, which found their way through the little hole in the card, and entered to paint the picture upon the curtain of the eye.

This is one of the most striking instances that can be afforded of the wonderful properties of light, and of the infinitude of those luminous rays that attend the majestic rising of the sun. Not only does light fly from the grand "ruler of the day" with a velocity which is a million and a half times greater than the speed of a cannon-ball, but it darts from every reflecting surface with a like velocity, and reaches the tender structure of the eye so gently that, as it falls upon the little curtain of nerves which is there spread to receive it, it imparts the most pleasing sensations, and tells its story of the outer world with a minuteness of detail, and a holiness of truth. Philosophers once sought to _weigh_ the _sunbeam_; they constructed a most delicate balance, and suddenly let in upon it a beam of light; the lever of the balance was so delicately hung that the fluttering of a fly would have disturbed it. Everything prepared, the grave men took their places, and with keen eyes watched the result. The sunbeam that was to decide the experiment had left the sun eight minutes prior to pass the ordeal. It had flown through ninety-five millions of miles of space in that short measure of time, and it shot upon the balance with unabated velocity: but the lever moved not, and the philosophers were mute.

CHAPTER XXV.

518. _Why, when we move before a mirror, does the image draw near to the reflecting surface as we draw near to it, and retire when we retire?_

Because the lines and angles of _reflection_ are always equal to the lines and angles of _incidence_.

519. _What is the line of incidence?_

If a person stands in a direct line before a mirror, the line through which the light travels from him to the mirror is _the line of incidence_.

_Incidence_--falling on.

[Verse: "Blessed be the Lord, who daily loadeth us with benefits, even the God of our salvation."--PSALM LXVIII.]

520. _What is the line of reflection?_

The _line of reflection_ is the line in which the rays of light are returned from the image formed in the glass to the eye of the observer.

_Reflection_--a turning back.

521. _What is the angle of incidence?_

The _angle_ of incidence is the angle which rays of light, falling on a reflecting surface, make with a line perpendicular to that surface.

[Illustration: Fig. 14.--EXPLAINING THE LINES AND ANGLES OF INCIDENCE AND OF REFLECTION.]

522. _What is the angle of reflection?_

The _angle_ of reflection is the angle which is formed by the returning rays of light, and a line perpendicular to the reflecting surface. It is always _equivalent_ to the angle of incidence.

Take a marble and roll it across the floor, so that it shall strike the wainscot obliquely. Let A in the diagram represent the point from which the marble is sent. The marble will not return to the hand, nor will it travel to the line B, but will bound off, or be _reflected_, to C. Now B is an imaginary line, _perpendicular to the reflecting surface_; and it will be found that the path described by the marble in _rolling to the surface and rebounding from it_, form, with the line B, two angles that are _equal_. These represent the angles of _incidence_ and of _reflection_, and explain why the reflection of a person standing at A before a mirror, would be seen by another person standing at C. This simple law in optics explains a great many interesting phenomena, and therefore it should be clearly impressed upon the memory.

[Verse: "And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also."--GEN. I.]

523. _Why do windows reflect the sun in the evening?_

Because the eye of the observer is in the _line of the reflection_.

524. _Why do windows not reflect the sun at noon?_

They do, but our eyes are not then in the _line of the reflection_.

[Illustration: Fig. 15.--SHOWING THE LINES OF INCIDENCE AND REFLECTION OF THE SUN'S RAYS AT NOON AND AT EVENING.]

It is obvious from the foregoing diagram that the evening rays of reflection fall upon the eyes of spectators, while the reflections at noon are so perpendicular that they are lost.

525. _Why do the sun and moon appear smaller when near the meridian, than when near the horizon?_

Because, when near the horizon, they are brought into _comparison with the sizes of terrestrial objects_; but when near the meridian they occupy the centre of a vast field of sky, and as there are no objects of comparison surrounding them, they _appear smaller_.

[Verse: "There is no darkness nor shadow of death, where the workers of iniquity may hide themselves."--JOB XXXIV.]

This is one "Reason Why," assigned by some observers. But there is also another reason to be found in the fact that, when the sun or moon is near the horizon, we view it through a _greater depth of atmosphere_ than we do when at the meridian. (_See_ Fig. 13.) A straight line passed upward through the air, would not be so long as that which passes to S. Consequently, as the air is generally impregnated with moisture, at the time when these effects are observed, the rays of light are caused to diverge more, and the disc of the sun or moon _appears magnified_. Probably both of these reasons contribute to the effect. This latter reason also explains why the disc of the sun or moon may sometimes appear _oval_ in shape, the lower stratum of air being more loaded with moisture than that through which we view the upper part of the disc.

526. _Why do our shadows lengthen as the sun goes down?_

Because light travels only in _straight lines_, and as the sun descends, the direction of his rays becomes more _oblique_, thereby causing longer shadows.

527. _What is the cause of the optical illusions frequently observed in nature?_

There are various kinds of natural optical illusions:--

The _mirage_, in which landscapes are seen reflected in burning sands.

The _fata morgana_, in which two or three reflections of objects occur at the same time.

The _ærial spectra_, or ærial reflections, &c.

[Illustration: Fig. 16.--ILLUSTRATING THE APPEARANCE OF PHANTOM SHIPS.]

The optical illusions above enumerated owe their origin to various atmospheric conditions, in which _refractions_ and _reflections_ are _multiplied_ by the different densities of atmospheric layers. They chiefly occur in hot countries, where, from the varying effects of heat, the conditions of atmospheric refraction and reflection frequently prevail in their highest degree.

[Verse: "In the morning ye say, it will be foul weather to-day, for the sky is red and lowering."--MATT. XVI.]

528. _Why do we have twilight mornings and evenings?_

Because the coming and the departing rays of the sun are _refracted_ and _reflected_ by the upper portions of the atmosphere. (_See_ Fig. 13.)

529. _How long before the sun appears above the horizon does the reflection of his light reach us?_

The time _varies_ with the refracting and reflecting power of the atmosphere, from _twenty minutes_ to _sixty minutes_. But the sun's position is usually _eighteen degrees_ below the horizon when twilight begins or ends.

530. _Why is the sky blue?_

The white light of the sun falls upon the earth without change; it is then reflected back by the earth, and as it passes through the atmosphere portions of it are again returned to us, and this double reflection produces a _polarised_ condition of light which imparts to vision the sensation of a _delicate blue_. (_See_ 549.)

531. _Why do the clouds appear white?_

Because they reflect back to us the solar beam _unchanged_.

532. _Why does the sky appear red at sunset?_

Because the light vapours of the air, which are condensed as the sun sets, refract the rays of light, and produce red rays. The refraction which produces _red_ requires only a _moderate degree of density_.

533. _Why do the clouds sometimes appear yellow?_

Because there is a larger amount of vapour in the air, which produces a different degree of refraction, _resulting in yellow_.

534. _Why does a yellow sunset foretell wet weather?_

Because it shows that the air is heavy with vapours. The refraction that produces _yellow_ requires a greater degree of density.

[Verse: "When it is evening ye say it will be fair weather, for the sky is red."--MATT. XVI.]

535. _Why does a red sunset foretell fine weather?_

Because the redness shows that the vapours in the air _towards the West_, or wet quarter, are _light_, as is evidenced by the degree of refraction of the sun's rays.

536. _Why does a red sunrise foretell wet?_

Because it shows that _towards the East_, or dry quarter, the air is charged with vapour, and therefore probably at other points the air has reached _saturation_.

537. _Why does a grey sunrise foretell a dry day?_

Because it shows that the vapours in the air are _not_ very dense.

538. _Why is "a rainbow in the morning the shepherd's warning?"_

Because it shows that _in the West_, or wet quarter, the air is _saturated_ to the rain point.

539. _Why is "a rainbow at night the shepherd's delight?"_

Because it shows that the _rain is falling in the East_, and as that is a dry quarter, it will _soon be over_. Rainbows are always seen in opposition to the sun.

CHAPTER XXVI.

540. _What is the difference between light and heat?_

The most obvious distinction is, that light acts upon _vision_, and heat upon _sensation_, or feeling.

Another distinction is, that _heat expands all bodies_, and alters their atomic condition; while _light_, though usually attended by heat, does not display the same expansive force, but produces various effects which are _peculiar to itself_.

[Verse: "Ye are the light of the world. A city that is set on a hill cannot be hid."--MATTHEW V.]

541. _Are light and heat combined in the solar ray?_

Yes. A ray of light, as well as containing elementary rays that produce colours under refraction, contains also _chemical rays_, and _heat rays_.

542. _How do we know that light and heat are separate elements?_

Because we have _heat rays_, as from dark hot iron, from various chemical actions, and from friction, which are _unattended by the development of light_. And we have light, or luminosity, such as that of _phosophoresence_, which is unaccompanied by any appreciable degree of heat.

But, besides this confirmation, further proof is afforded by the fact, that in passing rays of solar light through media that are _transparent to heat_, but not to _light_, the heat rays may be _separated_ from the luminous rays, and _vice versa_.

Black glass, and black mica, which are nearly _opaque to light_, are _transparent to heat_ to the extent of ninety degrees out of a hundred. While pale green glass, coloured by oxide of copper, and covered with a coating of water, or a thin coating of alum, will be perfectly _transparent to light_, but will be almost quite _opaque to heat_. These remarks apply, in a greater or less degree, to various other substances.

543. _In what respects are light and heat similar?_

Both heat and light have been referred to minute vibratory motions which occur, under exciting causes, in a very subtile elastic medium.

They are both united in the sun's rays.

They are both subject to laws of absorption, radiation, reflection, and refraction.

They are both essential to life, whether animal or vegetable.

Both may be developed in their greatest intensity by electricity.

They are both imponderable.

[Verse: "When I consider thy heavens, the work of thy fingers, the moon and the stars which thou hast ordained:"]

544. _In what respects are light and heat dissimilar?_

Heat frequently exists without light.

Light is usually attended with heat.

Light may be instantly extinguished, but Heat can only be more gradually reduced, by diffusion.

The solar rays deliver heat to the earth by day, and the heat remains with the earth when the light has departed.

Heat diffuses itself in all directions.

Light travels only in straight lines.

The colours that absorb and radiate both light and heat do not act in the same degree upon them both. Black, which does not radiate light, is a good _radiator of heat_, &c., &c.

The oxy-hydrogen _light_ emits a most intense heat, but glass which will transmit the rays of light, will afford no passage to the rays of the _heat_.

Heat is latent in all bodies, but no satisfactory proof has been found that light is latent in substances.

These are only a few of the analogies and distinctions that exist between the two mysterious agents, light and heat. But they are sufficient to supply the starting points of investigation.

The importance of the heat that attends the solar rays may be illustrated by the experiments performed a few years ago, by Mr. Baker, of Fleet-street, London, who made a large burning lens, three feet and a half in diameter, and employed another lens to reduce the rays of the first to a focus of half an inch in diameter. The heat produced was so great that iron plates, gold, and stones were _instantly melted_; and sulphur, pitch, and resinous bodies, _were melted under water_.

545. _What is the point of heat at which bodies become luminous?_

The point of heat at which the eye begins to discover luminosity has been estimated at 1,000 deg.

546. _What is the velocity of artificial light?_

The light of a fire, or of a candle, or gas, travels with the same velocity as the light of the sun,--a velocity which would convey light eight times round the world while a person could count "one."

547. _At what rate of velocity does the light of the stars travel?_

At the same velocity as all other light. And yet there are stars so distant that, although the light of the sun reaches the earth in eight minutes and a half, it requires _hundreds of years_ to bring their light to us.

[Verse: "What is man, that thou art mindful of him? and the son of man that thou visitest him?"--PSALM VIII.]

548. _What is the relative intensity of primary and reflected light?_

The intensity of a reflection depends upon the power of the reflecting surface. But, taking the sun and moon as the great examples of primary and reflected light, the intensity of the _sun's light_ is 801,072 times _greater than that of the moon_.

549. _What is polarized light?_

_Polarized light_ is light which has been subjected to _compound refraction_, and which, after polarization, exhibits a new series of phenomena, differing materially from those that pertain to the primary conditions of light.

550. _What are the chief deductions from the phenomena observed under the polarization of light?_

The polarization of light appears to confirm in a high degree the vibratory _theory of light_; and to show that the vibrations of light have two planes or directions of motion. The mast of a ship, for instance, has two motions: it progresses _vertically_ as the ship is impelled forward, and it rolls _laterally_ through the motion of the billows.

Something like this occurs in the vibrations of light, only the _vertical vibration_ is the condition of _one ray_, and the _lateral vibration_ is the condition of another ray, and the vibrations of these two rays intersect each other in the solar ray. When these vibrations occur together, the ray has certain properties and powers. But by polarization the rays may be _separated_, and the result is two distinct rays, having _different vibrations_.

It then appears that various bodies are transparent to these polarized rays _only in certain directions_. And this fact is supposed to show that bodies are made up of their atoms arranged in certain planes, through or between which the _lateral_ or the _vertical_ waves of light, together or singly, can or cannot pass; and that the transparency or the opacity of a body is determined by the _relation of its atomic planes_ to _the planes of the vibrations of light_.

_Ordinary light_, passing through transparent media, produces no very remarkable effect in its course; but _polarized light_ appears to illuminate every atom of the permeated substance, and by surrounding it with a prismatic clothing, to afford an illustration of its _molecular arrangement_.

[Verse: "A man that is called Jesus made clay, and anointed mine eyes, and said unto me, Go to the pool of Siloam, and wash: and I went and washed, and I received sight."--JOHN IX.]

551. _Why are two persons able to see each other?_

Because rays of light _flow from their bodies to each other's eyes_, and convey an impression of their respective conditions.

In some popular works that have come under our notice, we find that the student is told that "we cannot absolutely see each other--we only _see the rays of light reflected from each other_." The statement is erroneous as expressed. We do not see the _rays_ of light, for if we did so, the effect of vision would be destroyed, and all bodies would _appear_ to be in a state of _incandesence_, or of _phosphoresence_. Rays of light, which are in themselves _invisible_, radiate from the objects we look upon, enter the pupil of the eye, and impress the seat of vision in a manner which conveys to the mind a knowledge of the form, colour, and relative size and position of the figure we look upon. If this is not seeing the object--_what is_? It would be just as reasonable to say, that we cannot _hear_ a person speak--that we only hear the _vibrations of the air_. But as the vibrations are imparted to the air by the organs of voice of the speaker, as he sets the air in motion, and makes the air his messenger to us, we certainly hear _him_, and can dispense with any logical myths that confound the understanding, and contribute to no good result.

552. _What is actinism?_

_Actinism_ is the chemical property of light.

_Actinism_--ray power.

553. _Why does silver tarnish when exposed to light?_

Because of the _actinic_, or chemical power of the rays of the sun.

554. _Why do some colours fade, and others darken, when exposed to the sun?_

Because of the _chemical_ power of the sun's rays.

555. _Why can pictures be taken by the sun's rays?_

Because of the actinic powers that accompany the solar light.

556. _What is the particular chemical effect of light exhibited in the production of photographic pictures?_

Simply the _darkening of preparations of silver, by the actinic rays_.

557. _Why are photographic studios usually glazed with blue glass?_

Because blue glass obstructs many of the luminous rays, but it is perfectly transparent to _actinism_.

[Verse: "The hay appeareth, and the tender grass showeth itself, and herbs of the mountain are gathered."--PROV. XXVII.]

558. _Why do plants become scorched under the unclouded sun?_

Because the heat rays are in excess. The clouds shut off the scorching light; but, like the blue glass of the photographer's studio, they transmit _actinism_.

559. _What effect has actinism upon vegetation?_

It quickens the germination of seeds; and assists in the formation of the colouring matter of leaves. Seeds and cuttings, which are required to germinate quickly, will do so under the effect of blue glass (which is equivalent to saying, the effect of an increased proportion of _actinism_), in half the time they would otherwise require.

560. _In what season of the year is the actinic power of light the greatest?_

In the _spring_, when the germination of plants demands its vitalising aid. In _summer_, when the maturing process advances, _light_ and _heat_ increase, and _actinism_ relatively declines. In the _autumn_, when the ripening period _arrives_, _light_ and _actinism_ give way to a greater ratio of _heat_.

[Verse: "But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him."--CORINTH. BOOK I., II.]

We shall have frequently, in the progress of our lessons, to refer to _light_ in its connection with the chemistry of nature, and with organic life. But let us now invite the student to pause, and for a moment contemplate the wonders of a sunbeam. How great is its velocity--how vast its power--how varied its parts--yet how ethereal! First, let us contemplate it as a simple beam in which _light_ and _heat_ are associated. How deep the darkness of the night, and how that darkness clings to the recesses of the earth. But the day beams, and darkness flies before it, until every atom that meets the face of day is lit up with radiance. That which before lay buried in the shade of night is itself now a radiator of the luminous fluid. Mark the genial warmth that comes as the sister of light; then stand by the side of the experimentalist and watch the point on which he directs the shining focus, and in an instant see iron melt and stones run like water, under the fervent heat! Now look upward to the heavens, where the falling drops of rain have formed a natural prism in the rainbow, and shown that the beam of pure whiteness, refracted into various rays, glows with all the tints that adorn the garden of nature. These are the visible effects of light. But follow it into the crust of the earth, where it is, by another power, which is neither light nor heat, quickening the seed into life; watch it as the germ springs up, and the plant puts forth its tender parts, touching them from day to day with deeper dyes, until the floral picture is complete. Follow it unto the sea, where it gives prismatic tints to the _anemone_, and imparts the richest colours to the various _algae_. Think of the millions of pictures that it paints daily upon the eyes of living things. Contemplate the people of a vast city when, attracted by some floating toy in the air, a million eyes look up to watch its progress. The sun paints a million images of the same object, and each observer has a perfect picture. It makes common to all mankind the beauties of nature, and paints as richly for the peasant as for the king. The Siamese twins were united by a living cord which joined their systems, and gave unity and sympathy to their sensations. In the great flood of light that daily bathes the world, we have a bond of union, giving the like pleasures and inspirations to millions of people at the same instant. And that which floods the world with beauty, should no less be a bond of unity and love.

CHAPTER XXVII.

561. _What is electricity?_

Electricity is a property of _force_ which resides in all matter, and which constantly seeks to establish an _equilibrium_.

562. _Why is it called electricity?_

Because it first revealed itself to human observation through a substance called, in the Greek language, _electrum_. This substance is known to us as _amber_.

563. _In what way did electrum induce attention to this property of force in matter?_

Thales, a Greek philosopher, observed that, by briskly rubbing _electrum_, it acquired the property of _attracting_ light particles of matter, which moved towards the amber, and attached themselves to its surface, evidently under the influence of a _force_ excited in the amber.

564. _What is amber?_

It is a _resinous_ substance, hard, bitter, tasteless, and glossy. It has been variously supposed to be a vegetable gum, a fossil, and an animal product. It is probably formed by a _species of ant_ that inhabit pine forests. The bodies of ants are frequently found in its substance.

[Verse: "He made darkness his secret place: his pavilion round about him were dark waters and thick clouds of the skies."]

565. _Why does the rubbing of a stick of sealing-wax cause it to attract small particles of matter?_

Because it excites in the sealing wax that _force_ which was first observed in the _amber_. Sealing-wax, therefore, is called an _electric_ (_amber-like_) body.

566. _Why do we hear of the electric fluid?_

Simply because the term _fluid_ is the most convenient that can be found to express our ideas when speaking of the _phenomena of electric force_. But of the nature of electricity, except through its observed _effects_, nothing is known.

567. _What substances are electric?_

All substances in nature, from the _metals_ to the _gases_. But they differ very widely in their electrical qualities.

568. _What is positive electricity?_

Electricity, when it exists, or is excited, in any body, to an amount which is _in excess_ of the amount natural to that body, is called _positive_ (called also _vitreous_).

569. _What is negative electricity?_

Electricity, when it exists, or is excited, in any body, in an amount which _is less_ than is the amount natural to that body, is called _negative_ (called also _resinous_).

570. _Why is "positive" electricity called also "vitreous," and "negative" electricity called also "resinous"?_

Because some philosophers believe that there is but _one electricity_, but that it is liable to variations of _quantity_ or _state_, which they distinguish by _positive_ and _negative_; while other philosophers believe that there are _two electricities_, which they name _vitreous_ and _resinous_, because they may be induced respectively from _vitreous_ and _resinous_ substances, and they display forces of attraction and repulsion.

571. _Upon what do the electrical phenomena of nature depend?_

Upon the tendency of _electricity_ to find an _equilibrium_ between its _positive_ and _negative_ states (assuming there to be but _one_ fluid); or upon the tendency of _vitreous electricity_ to seek out and combine with _resinous electricity_ (assuming that there are _two_ fluids).

[Verse: "The Lord also thundered in the heavens, and the Highest gave his voice; hailstones and coals of fire."]

572. _How does the equilibrium of electricity become disturbed?_

By changes in the condition of matter. As electricity resides in all substances, and is, perhaps, an essential ingredient in their condition, so every change in the state of matter--whether from heat to cold, or from cold to heat; from a state of rest to that of motion; from the solid to the liquid, or the æriform condition, or _vice versa_; or whether substances combine chemically and produce new compounds--in every change _the electrical equilibrium is disturbed_; and, in proportion to the degree of disturbance, is the force exerted by electricity to resume its balance in the scale of nature.

573. _How does electricity seek to regain equilibrium?_

By passing through substances that are favourable to its diffusion; therefore they are called _conducting_ or _non-conducting_ bodies, according as they favour or oppose the transmission of the electrical current.

574. _What substances are conductors of electricity?_

Metals, charcoal, animal fluids, water, vegetable bodies, animal bodies, flame, smoke, vapour, &c.

575. _What substances are non-conductors?_

Rust, oils, phosphorous, lime, chalk, caoutchouc, gutta percha, camphor, marble, porcelain, dry gases and air, feathers, hair, wool, silk, glass, transparent stones, vitrefactions, wax, amber, &c. These bodies are also called _insulators_. Some of these substances, as chalk, feathers, hair, wool, silk, &c., though non-conductors when _dry_, become conductors when _wetted_.

_Insulating_--preventing from escaping.

576. _Why are amber and wax classed among the non-conductors, when they have been pointed out as electrics, and used to illustrate electrical force?_

It is _because_ they are _non-conductors_ that they have displayed, under excitement, the attractive force shown in respect to the particles of matter which were drawn towards their substances. If a bar of _iron_ were excited, instead of a stick of wax, electricity would be equally developed; but the iron, _being a good conductor_, would pass the electricity to the hand of the operator as fast as it accumulated, and the equilibrium would be undisturbed.

[Verse: "Yea, he sent out his arrows, and scattered them; and he shot out lightnings and discomfited them."--PSALM XVIII.]

577. _What is the effect when electricity, in considerable force, seeks its equilibrium, but meets with insulating bodies?_

The result is a violent action in which, _intense heat and light_ are developed, and in the evolution of which _the electric force becomes expended_.

578. _What is the cause of electric sparks?_

The electric force, passing through a conducting body to find its _equilibrium_, is checked in its course by an insulator, and emits a spark.

579. _What produces the electric light?_

Currents of electricity pass towards each other along wires at the ends of which two charcoal points are placed. As long as the charcoal points remain in contact, the electric communication is complete, and no light is emitted, but, when they are drawn apart, intense heat and light are evolved.

[Illustration: Figs. 17 & 18.--SHOWING THE EFFECT OF THE UNION AND THE SEPARATION OF THE CHARCOAL POINTS.]

580. _What is the cause of lightning?_

Lightning is the result of _electrical discharges_ from the _clouds_.

581. _What develops electricity in the clouds?_

Evaporations from the surface of the earth; changes of temperature in the atmospheric vapour; chemical action upon the earth's surface; and the friction of volumes of air of different densities against each other.

[Verse: "His lightnings enlightened the world: the earth saw and trembled."--PSALM XCVII.]

582. _Why do these phenomena produce electricity?_

Because they disturb the equilibrium of the electric force, and produce _positive_ and _negative_ states of electricity.

583. _When does lightning occur?_

When clouds, charged with the _opposite electricities_ approach, the forces rush to each other, and combine in a state of equilibrium.

584. _Why does lightning attend this movement of the forces of electricity?_

Because the atmosphere, being unable to convey the great charges of electricity as they rush towards each other, _acts as an insulator_, and lightning is caused by the _violence of the electricity in forcing its passage_.

585. _Does lightning ever occur when the conducting power is equal to the force of the electricity?_

No; electricity passes invisibly, noiselessly, and harmlessly, whenever it finds a sufficient source of _conduction_.

CHAPTER XXVIII.

586. _Why does lightning sometimes travel through a "zigzag" course?_

Because the electricity, being resisted in its progress by the air, _flies from side to side_, to find the readiest passage.

587. _Why does lightning sometimes appear forked?_

Because, being resisted in its progress by the air, the electricity divides into two or more points, and _seeks a passage in different directions_.

588. _Why is lightning sometimes like a lurid sheet?_

Because the flash is distant, and therefore we see only the _reflection_.

[Verse: "He directeth it under the whole heavens, and his lightning unto the ends of the earth."]

589. _When is the flash of lightning straight?_

When the distance between the clouds whose electricities are meeting, is small.

590. _What is the cause of the aurora borealis?_

The _mingling of the electricities_ of the higher regions of the atmosphere.

591. _When does the flash of lightning appear blue?_

When the degree of electrical excitement is intense, and _general throughout the atmosphere_.

592. _Why does lightning sometimes appear red, at others yellow, and at others white?_

Because of the varying humidity, which affects the _refracting power_ of the atmosphere.

593. _Does lightning ever pass upwards from the earth to the clouds?_

Yes; when the earth is charged with a _different electricity_ to that which is in the clouds.

594. _Does lightning ever pass directly from the clouds to the earth?_

Yes; when the electricity of the clouds seeks to combine with the _different electricity_ of the earth.

The mingling of the electricities of the earth and the air must be continually going on. But _lightning_ does not attend the phenomena, because all natural bodies, vapours, trees, animals, mountains, houses, rocks, &c., &c., act more or less as conductors between the earth and the air. It is only when there is a great disturbance of the electrical forces, that _terrestrial lightning_ is developed. When lightning strikes the earth with great force, it sometimes produces what are called _fulgurites_ in sandy soils; these are hollow tubes, produced by the melting of the soil.

595. _What is the extent of mechanical force of lightning?_

Lightning has been proved, in one instance, to have struck a church with a force equal to more than 12,000 horse-power. A single horse-power, in mechanical calculations, is equivalent to raising a weight of 32,000 lbs. one foot in a minute. The force of lightning, therefore, has been proved to be equal to the raising of 384,000,000 lbs. one foot in a minute. This is equal to the united power of twelve of our largest steamers, having collectively 24 engines of 500 horse-power each. The velocity of electricity is so great that it would travel round the world _eight times in a minute_.

[Verse: "After it a voice roareth: he thundereth with the voice of his excellency; and he will not stay them when his voice is heard."--JOB XXXVII.]

The church alluded to was St. George's church, Leicester, a new edifice, which was completely destroyed on the 1st of August, 1846, by a thunder-storm. The steeple was rent asunder, and massive stones were hurled to a distance of thirty feet. The vane rod and top part of the spire fell down perpendicularly and carried with it all the floors of the tower. A similar disaster occurred to St. Bride's church, Fleet-street, London, about 100 years ago. The lightning first struck upon the metal vane of the steeple, and then ran down the rod and attacked the iron cramps, smashing the large stones that lay between them. The church was nearly destroyed. By the same wonderful force, ships have been disabled, trees split asunder, houses thrown down, and animals struck dead.

596. _Why is it dangerous to stand near a tree during an electric storm?_

Because the tree is a _better conductor than air_, and electricity would probably strike the tree, and then pass to the person standing near.

597. _If trees are good conductors, why do they not convey the electricity to the ground?_

Trees are only _indifferent conductors_, and the electricity would quit the tree to pass through any _better conductor_.

598. _Why is it dangerous to sit near a fire during an electric storm?_

Because the chimney, being _a tall object_, and smoke a _good conductor_, would probably attract the electricity, and convey it to the body of a person sitting near the fire.

599. _Why is it dangerous to be near water during an electric storm?_

Because water is a _good conductor_, and the vapour arising from it might attract the electricity. Man, _being elevated over the water_, might form the first point attacked by the electricity.

600. _Are iron houses dangerous during an electric storm?_

No; they are _very safe_, because their entire surface is a good conductor, and would convey the electricity harmlessly to the earth.

[Verse: "To him that rideth upon the heavens of heavens, which were of old; lo, he doth send out his voice, and that a mighty voice."--PSALM LXVIII.]

601. _Why does electricity seize upon bell wires and iron fastenings?_

Because copper wires are the _very best_ conductors of electricity; and iron articles are also good conductors.

602. _Supposing electricity to attack a bell wire, where would the point of danger exist?_

At the _extremities of the wire_, where the conducting power of the wire would cease, and the electricity would seek to find _another conductor_.

603. _Are umbrellas, with steel frames, dangerous in an electric storm?_

They are dangerous _in some degree_, because they might convey electricity to the hand, and then transfer it to the body. But, generally speaking, when it rains, the rain itself, _being a good conductor_, relieves the disturbance of electricity by conveying it to the ground.

604. _Are iron bedsteads dangerous in electric storms?_

No, they are safe, because the iron frame, completely surrounding the body, and having _a great capacity for conduction_, would keep the electricity away from the body.

605. _Why is it safe to be in bed during an electric storm?_

Because _feathers_, _hair_, _wool_, _cotton_, &c., especially when dry, are good _insulators_ or _non-conductors_.

606. _What is the safest situation to be in during an electric storm?_

In the centre of a room, _isolated_ as far as possible from surrounding objects; sitting on a chair, and avoiding handling any of the conducting substances. The windows and doors should be closed, to prevent _drafts of air_.

607. _In the open air, what is the safest situation?_

To keep aloof, as far as possible, from elevated structures; and regard the rain, though it might saturate our clothes, as a protection against the lightning stroke, for _wet clothes_ would supply so _good a conductor_, that a large amount of electricity would pass over man's body, through wet garments, and he would be quite unconscious of it.

[Verse: "God thundereth marvellously with his voice: great things doeth he, which we cannot comprehend."--JOB XXXVI.]

During a violent electric storm in the Shetland Islands, a fishing boat was attacked by the electric fluid, which tore the mast to shivers. A fisherman was sitting by the side of the mast at the time, but he felt no shock. Upon taking out his watch, however, he found that the electric current had actually fused his watch into a mass. In this case, it is more than probable that the man was saved through the saturation of his clothes with rain.

608. _Do lightning conductors "attract" electricity?_

Not unless the electric current lies in their vicinity.

609. _Why have lightning conductors sometimes been found ineffective?_

Because they have been unskilfully constructed; have been too small in their dimensions, and have not been properly laid to convey the electricity harmlessly away.

610. _What is the best metal for a lightning conductor?_

_Copper_, the conducting power of which is _five times greater than that of iron_.

611. _Why should a large building have several conductors?_

Because the influence of a conductor over the electricity of the surrounding air does not extend to more than a radius of double the height of the conductor above the building: for instance, a conductor rising _ten feet high_ above the building would influence the electricity _twenty feet all round the conductor_.

612. _Why should conductors have at their base several branches penetrating the earth?_

To facilitate the discharge of the _accumulated electricity_ into the earth.

613. _Why does electricity affect the shapes of clouds?_

Because electricity does not penetrate the _masses_ of _bodies_, but affects generally _their surfaces_. Hence electricity exists in the _surfaces of clouds_, and in its efforts to _find an equilibrium_ it causes the clouds to roll in _heavy masses_, having dark outlines.

[Verse: "All ye inhabitants of the world, and dwellers on the earth, see ye, when he lifteth up an ensign on the mountains; and when he bloweth a trumpet, hear ye."--ISAIAH XVIII.]

The fact that electricity resides in, and is conducted by, the _surfaces_ of bodies, is well established, and should receive due attention in the protective measures adopted to secure life and property against the effects of lightning. A practical suggestion that arises out of this fact is, that _tubes of copper_ would form far more efficient conductors than _bars_ of the same metal. A _copper tube_, of half an inch diameter, would conduct _nearly double_ the amount of electricity which could be conveyed away by a _bar_ of copper of the same diameter. The upper extremity of the tube should be open obliquely, that the electric current might be induced to pass over _both the inner and outer surfaces_.

CHAPTER XXIX.

614. _What is thunder?_

Thunder is the _noise which succeeds the rush_ of the electrical fluid through the air.

615. _Why does noise follow the commotion caused by electricity?_

Because, by the violence of the electric force, vast _fields of air are divided_; great volumes of air are _rarefied_; and _vapours_ are _condensed_, and thrown down as _rain_. Thunder is therefore caused by the _vibrations of the air_, as it collapses, and seeks to restore its own _equilibrium_.

616. _Why is the thunder-peal sometimes loud and continuous?_

Because the electrical discharge takes place near the hearer, and therefore the vibrations of the air are heard in their full power.

617. _Why is the thunder-peal sometimes broken and unequal?_

Because the electrical discharge takes place at a _considerable distance_, and the vibrations are affected in their course by _mountains_ and _valleys_. Because, also, the _forked arms_ of the lightning strike out in different directions, causing the sounds of thunder to reach us from _varying distances_.

[Verse: "Lo, these are parts of his ways; but how little a portion is heard of him? but the thunder of his power who can understand?"--JOB XXV.]

618. _Why has the thunder-peal sometimes a low grumbling noise?_

Because the electrical discharges, though violent, take place _far away_, and the vibrations of the air _become subdued_.

619. _Why does the thunder-peal sometimes follow immediately after the flash of lightning?_

Because the discharge of electricity takes place near the hearer.

620. _Why does the thunder-peal sometimes occur several seconds after the flash?_

Because the discharge takes place far away, and _light_ travels with a much greater velocity than _sound_.

621. _Through what distance will the sound of thunder travel?_

Some _twenty or thirty miles_, according to the _direction of the wind_, and the violence of the peal.

622. _Through what distance will the light of lightning travel?_

The _light_ of lightning, and its reflections, will penetrate through a distance of from _a hundred and fifty to two hundred miles_.

623. _How may we calculate the distance at which the electric discharge takes place?_

Sound travels at the rate of _a quarter of a mile in a second_. If, therefore, the peal of thunder is heard _four seconds_ after the flash of lightning, the discharge took place about a mile off. The pulse of an adult person beats about _once in a second_; therefore, guided by the pulse, any person may calculate the probable distance of the storm:--

2 beats, 1/2 a mile. 3 beats, 3/4 of a mile. 4 beats, 1 mile. 5 beats, 1-1/4 miles. 6 beats, 1-1/2 miles. 7 beats, 1-3/4 miles. 8 beats, 2 miles, &c.

Attention should be paid to the _direction and speed of the wind_, and some modifications of the calculation be made accordingly. Persons between 20 and 40 years of age should count _five beats of the pulse to a mile_; under 20, _six beats_.

[Verse: "The clouds poured out water; the skies sent out a sound; thine arrows also went abroad."]

624. _Why are electric storms more frequent in hot than in cold weather?_

Because of the _greater evaporation_, as the effect of heat; and also of the _effect of heat_ upon the particles of all bodies.

625. _Why do electric storms frequently occur after a duration of dry weather?_

Because _dry air_, being a bad conductor, prevents the _opposite electricities_ from finding their _equilibrium_.

626. _Why is a flash of lightning generally succeeded by heavy rain?_

Because the electrical discharge destroys the _vescicles_ of the vapours. If a number of _small soap-bubbles_ floating in the air were _suddenly broken_ by a violent commotion of the atmosphere, the _thin films_ of the bubbles would form _drops of water_, and fall _like rain_.

627. _Why is an electrical discharge usually followed by a gust of wind?_

Because the equilibrium of the atmosphere is disturbed by the _heat and velocity of lightning_, and the _condensation of vapour_. Air, therefore, rushes towards those parts where a degree of _vacuity_ or _rarefaction_ has been produced.

628. _What is a thunderbolt?_

The name _thunderbolt_ is applied to an electrical discharge, when the lightning appears to be developed with the greatest intensity around a nucleus, or centre, as though it contained a burning body. But there is, in reality, _no such thing as a thunderbolt_.

[Verse: "The voice of the Lord is upon the waters: the God of glory thundereth; the Lord is upon many waters."--PSALM XXIX.]

629. _Why do electric storms purify the air?_

Because they restore the _equilibrium of electricity_ which is essential to the salubrity of the atmosphere; they intermix the _gases of the atmosphere_, by agitation; they _precipitate the vapours_ of the atmosphere, and with the precipitation of vapours, _noxious exhalations_ are taken to the earth, where they become absorbed; they also contribute largely to the formation of _ozone_, which imparts to the air corrective and restorative properties.

630. _What is ozone?_

Ozone is an _atmospheric element_ recently discovered, and respecting which differences of opinion prevail. It is generally supposed to be _oxygen_ in a state _of great strength_, constituting a variety of form or condition.

631. _Why do we know that electricity contributes to the formation of ozone?_

Because careful observations have established the fact that the proportion of _ozone_ in the atmosphere is _relative to the amount of electricity_.

632. _What are the properties of ozone?_

It displays an extraordinary power in the neutralisation of putrefactions, rapidly and thoroughly counteracting noxious exhalations; it is the most powerful of all _disinfectants_.

Schonbien, the discoverer of _ozone_, inclines to the opinion that it is a _new chemical element_. Whatever it may be, there can be no doubt that it plays an important part in the economy of nature. Its _absence_ has been marked by pestilential ravages, as in the _cholera_ visitations; and to its _excess_ are attributed epidemics, such as _influenza_. It was found, during the last visitation of cholera, that the _fumigation of houses with sulphur_ had a remarkable efficacy in preventing the spread of the contagion. The combustion of sulphur ozonised the atmosphere; the same result occurs through the emission of _phosphoric vapours_; ozone is also developed by the electricity evolved by the _electrical machine_, and in the greater _electrical phenomena of nature_. The smell imparted to the air during an electric storm is identical with that which occurs in the vicinity of an electrical apparatus--it is a _fresh_ and _sulphurous_ odour. The opinion is gaining ground that the respiration of animals and the combustion of matter are sources of ozone, and that plants produce it when under the influence of the direct rays of the sun. It is also believed to be produced by water, when the sun's rays fall upon it. The most recent opinion respecting ozone is, that it is _electrized oxygen_. The subject is of vast importance, and opens another field of discovery to the pioneers of scientific truth.

[Verse: "The voice of thy thunder was in the heaven: the lightnings lightened the world, the earth trembled and shook."--PSALM LXVII.]

633. _What is magnetism?_

_Magnetism_ is _the electricity of the earth_, and is characterised by the circulation of _currents of electricity passing through the earth's surface_.

634. _What are magnetic bodies?_

Magnetic bodies are those that exhibit phenomena which show that they are under the influence of _terrestrial electricity_, and which indicate the direction of the _poles_, or _extreme points_, _of magnetic force_.

635. _What is Galvanism?_

Galvanism is the action of _electricity upon animal bodies_, and is so called from the name of its first discoverer, Galvani.

636. _What is Voltaic electricity?_

Voltaic electricity is the electricity that is developed during _chemical changes_, and is so called after Volta, who enlarged upon the theory of Galvani.

637. _What are the differences between mechanical, or frictional electricity, Voltaic electricity, Galvanism, and magnetism?_

_Frictional_ electricity is electricity _suddenly_ liberated under the effects of the _motion_, or the mechanical disturbance of bodies.

_Voltaic_ electricity is a _steady flow_ of an electric current, arising from the _gradual changes_ of _chemical_ phenomena.

_Galvanism_ and _Voltaism_ are almost _identical_, since the latter is founded upon, and is a development of, the former. But the term _Galvanism_ is frequently used when speaking of the development of electricity in _animal bodies_.

_Magnetism_ is the electricity of the _earth_, and is understood to imply the _fixed electricity of terrestrial bodies_.

[Verse: "And I heard as it were the voice of a great multitude, and as the voice of many waters, and as the voice of mighty thunderings, saying Alleluia: for the Lord God omnipotent reigneth."--REV. XIX.]

Man knows not _what electricity is_; yet, by an attentive observance of its _effects_, he avails himself of the power existing in an unknown source, and produces marvellous results. When the Grecian philosopher, Thales, sat rubbing a piece of amber, and watching the attraction of small particles of matter to its surface, he little knew of the mighty power that was then whispering to him its offer to serve mankind. And when Franklin, with the aid of a boy's plaything, drew down an electric current from the clouds, and caught a spark upon the knuckles of his hand, _even he_ little conjectured that the time was so near when that strange element, which sent its messenger to him along the string of a kite, would become one of man's most submissive servants.

So many great results have sprung from the careful observation of the simplest phenomena, that we should never pass over inattentively the most trifling thing that offers itself to our examination. Nature, in her revelations, never seeks to _startle_ mankind. The formation of a rock, and the elaboration of a truth, are alike the work of ages. It was the simple blackening of silver by the sun's rays which led to the discovery of the _chemical agency of light_. It was the falling of an apple which pointed Newton to the discovery of the _laws of gravitation_. It was the force of steam, observed as it issued from beneath the lid of a kettle, that led to the invention of the _steam-engine_. And it is said of Jacquard, that he _invented the loom_ which so materially aided the commerce of nations, while watching the motions of his _wife's fingers_, as she plied her knitting. As great discoveries spring from such small beginnings, who among us may not be the herald of some great truth--the founder of some world-wide benefaction?

That the area of discovery has not perceptibly narrowed its limits, is evident from the fact that the greatest elements in nature are still mysteries to man. And though it may not be within the power of a finite being to unravel the chain of wonders that enfold the works of an infinite God,--still it is evident, from the progress which discovery has made, and from the good which discovery has done, that God _does_ invite and encourage the human mind to contemplate the workings of Divine power, and to pursue its manifestations in every element, and in every direction.

The wonderful force of _electricity_ astonishes us all the more when we view it in contrast with that equally wonderful element, _light_. We have seen that light travels with a velocity of 192,000 miles in a second, but that it falls upon a delicate balance so gently, that it produces no perceptible effect. As far as we know the nature of _electricity_, it is even _more ethereal_ than _light_; yet, while the _ether of light_ falls harmlessly and imperceptibly--even with the momentum of a flight of _ninety-five millions of miles_, the _ether of electricity_, bursting from a cloud only _five hundred yards_ distant, will split massive stones, level tall towers with the dust, strike majestic trees to the ground, and instantly extinguish the life of man! _Why_ does _the one ether_ come divested of all mechanical force, while that which seems to be _even more ethereal_ than it, is capable of exerting the mightiest force over material things? Does it not appear that the Creator of the universe has established these paradoxes of power to testify his Omnipotence--to show to man that with Him all things are possible; and that, in the grand cosmicism of the universe, every attribute of Omnipotence has been fulfilled?

[Verse: "And the seventh angel poured out his vial into the air; and there came a great voice out of the temple of heaven, from the throne, saying, It is done."--REV. XVI.]

Let us now consider man's relation to this Omnipotence. He sees that electricity smites the tall edifice, and observes that in doing so it displays a choice of a certain substance through which it passes harmlessly, and that its violence is manifested only when its path is interrupted. Man, taking advantage of this preference of electricity for a particular conductor, stretches out an arm of that substance, and points it upwards to the clouds; electricity accepts the invitation, and passes harmlessly to the earth. But this not all: man learns by observation that electricity resides in all matter; that it may be collected or dispersed; that it travels along a good conductor at the rate of _half-a-million of miles in a second of time_; he constructs a battery, a kind of scientific fortress, in which he encamps the great warrior of nature; and then, laying down a conducting wire, he liberates the mighty force: but its flight must be on the path which man has defined, and its journey must cease at the terminus which man has decreed, where, by a simple contrivance of his ingenuity (the movements of a magnetic needle), the electric current is made to deliver whatever message of importance he desires to convey. Thus, the element which in an instant might deprive man of life, is subdued by him, and made the obedient messenger of his will.

CHAPTER XXX.

638. _What is the atmosphere?_

The _atmosphere_ is the transparent and elastic body of mixed gases and vapours which envelopes our globe, and which derives its name from Greek words, signifying _sphere_ of _vapour_.

639. _To what height does the atmosphere extend?_

It is estimated to extend to from _forty to fifty miles_ above the surface of the earth.

640. _Why is it supposed that the atmosphere does not extend beyond that height?_

Because it is found, by experiment and observation, that the air becomes _less dense_ in proportion to its altitude from the earth's surface. The gradual decrease of atmospheric density observed in ascending a mountain, or in a balloon, supplies sufficient data to enable us to calculate the height at which the atmosphere would probably _altogether cease_.

At an altitude of 18,000 feet the air is indicated by the barometer to be only _half as dense_ as at the surface of the earth. And as the densities of the atmosphere decrease in a geometrical progression, the density will be reduced to _one-fourth_ at the height of 36,000 feet; and to _one-eighth_ at 54,000 feet. The effects of the decreasing density of the atmosphere are, that the _intensity of light and sound are diminished, and the temperature is lowered_. Persons who have reached a very high elevation, state that the sky above them began to assume the appearance of darkness; and there can be no doubt that, if it were possible to reach an altitude of some fifty to sixty miles, there would be _perfect blackness although the sun's rays might be pouring through the darkened space_, to illuminate the atmosphere. Upon the summit of Mont Blanc, the report of a pistol at a short distance can _scarcely be heard_. When Gay Lussac reached the height of 23,000 feet, he breathed with great pain and difficulty, and felt distressing sensations in his ears, as though they were _about to burst_. Upon the high table-lands of Peru, the lips of Dr. Ischudi cracked and burst; and blood flowed from his eyelids.

[Verse: "For he looketh to the ends of the earth, and seeth under the whole heaven; To make the weight for the winds."--JOB XXVIII.]

641. _What is the amount of atmospheric pressure at the earth's surface?_

The pressure of the atmosphere at the earth's surface is _fifteen pounds_ to every square inch of surface. That is to say, that the column of air, extending fifty miles over a square inch of the earth, presses upon that square inch with a weight equal to _fifteen pounds_.

642. _Is that the weight of dry or moist air?_

That is the weight of air at what is called the _point of saturation_, when it is fully charged with _watery vapour_.

643. _What is the proportion of watery vapour in the atmosphere?_

The proportion _constantly varies_. Evaporation is not a result of accident; it seems an _established law_ that the air shall constantly _absorb vapour_ until it has reached the maximum that it can hold. Experiments have been tried, in which dry air has been pressed upon the surface of water with great force, _but no degree of pressure could prevent the formation of vapour_. (_See_ 431.)

644. _What is the total amount of atmospheric pressure on the earth's surface?_

The total amount of atmospheric pressure on the earth's surface, at 15 lbs. to the square inch, amounts to 12,042,604,800,000,000,000 lbs. This pressure is equal to that of a globe of lead of _sixty miles in diameter_.

645. _What is the pressure of the atmosphere upon the human body?_

Estimating the surface of man's body to be equal to _fifteen square feet_, he sustains an atmospheric pressure of 32,400 lbs., or nearly _fourteen tons and a-half_. The mere _variation of weight_, arising out of the changes in the state of the atmosphere, may amount to as much as a _ton and a-half_.

[Verse: "I therefore so run, not as uncertainly; so fight I, not as one that beateth the air."--CORINTH. IX.]

646. _Why does not man feel this pressure?_

Because the diffusion of air which, _surrounding him in every direction_, and acting upon the _internal_ as well as the _external_ surfaces of his body, and probably _surrounding every atom of his frame_, establishes an equilibrium, in which every degree of pressure _counteracts and sustains itself_.

647. _What is the weight of air relative to that of water?_

A cubic foot of air weighs only 523 grains, a little more than _an ounce_; a cubic foot of water weighs _one thousand ounces_.

648. _What is the greatest height in the atmosphere which any human being has ever reached?_

M. Gay Lussac, in the year 1804, ascended to the height of 23,000 feet.

649. _What is a vacuum?_

A vacuum is a space _devoid of matter_. The term is generally applied to those instances in which air is drawn from within an air-tight vessel.

650. _Is it possible to form a perfect vacuum?_

It is probably _impossible to do so_, even with the most powerful instruments--some portion of air would remain, but in so thin a form that it would be _imperceptible_.

651. _Why does the depression of a pump-handle cause the water to flow?_

Because the putting down of the handle lifts up the piston with its _valve closed_, thereby tending to produce a _vacuum_; but _the pressure of the air_ upon the water _not contained in the pump_, forces more water up into the part where a _vacuum_ would otherwise be formed. Then, when the handle is raised, and the piston forced downwards, _the valve opens_, and the water rushes through.

There is a second valve, below the piston, which closes with the downward movement, to prevent the water from _rushing back again._

[Verse: "The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit."--JOHN II., III.]

652. _How high will atmospheric pressure raise water in the bore of a pump?_

It will raise water to an elevation of _thirty feet_ above its level.

653. _Why will it raise water to an elevation of thirty-feet?_

Because a column of water of _thirty feet high_, nearly balances the weight of _a column of air_ of equal surface, _extending to the whole height of the atmosphere_. When, therefore, water is elevated to the height of thirty feet, the power of the pump is enfeebled, as the air and the water _balance each other_.

654. _How is water raised to a greater elevation when it is required?_

By mechanical contrivances, by which the water is _forced_ to a greater elevation.

655. _Why does water run through the bent tube called a syphon?_

Because the atmospheric pressure upon the water on _the outside of the syphon_ forces it into the tube as fast as the syphon empties itself through its longer arm.

656. _Why does water run through the longer arm of the syphon?_

Because the weight of the water in the longer arm of the syphon _is greater than that in the shorter_; therefore it runs out by its own gravity. And, as in running out, it creates a tendency towards a _vacuum_, the pressure of the outer air comes into operation, and forces the water through the tube.

657. _Why does water issue from the earth in springs?_

Some springs are caused by _natural syphons_ formed in the fissures of rocks, which, communicating with bodies of water, are continually filled by atmospheric pressure, and therefore convey streams of water to the point where they are set free.

[Verse: "Ascribe ye strength unto God: his excellency is over Israel, and his strength is in the clouds."--PSALM LVIII.]

658. _Why, if a wine glass is filled with water, and a card laid upon it, and the whole inverted, will the water remain in the glass?_

Because the pressure of the atmosphere upon the surface of the card counteracts the weight of the water.

659. _What has the card to do with the experiment?_

It forms _a base_ upon which the water may rest, while the glass is being inverted; and it prevents the air from acting upon the _fluidity_ of the water, and forcing it out of the glass.

660. _Why will not beer run out of the tap of a cask until a spile has been driven in at the top?_

Because the pressure of the air upon the opening of the tap counteracts the weight of the beer. But when the spile is driven in, the air enters at the top, _and counteracts its own pressure at the bottom_.

661. _Why does a cup in a pie become filled with juice?_

Because _the heat expands the air_, and drives nearly all of it out of the cup. When the pie is taken out of the oven, and begins to cool, air cannot get into the cup again, because its edges are surrounded by juice. A _partial vacuum_, therefore, exists within the cup, and the pressure of the external air _forces the juice into it_.

662. _Does the cup prevent the juice from boiling over?_

No. So long as the _heat_ exists, the cup remains _empty_; and as it occupies space, the air is driven out of it, into the pie, it rather tends to force the juice over the sides of the dish. It is only _when cooling_ that the juice enters the cup.

663. _Why can flies walk on the ceiling?_

Because their feet are so formed that they can form a _vacuum_, under them; their bodies are therefore sustained in opposition to gravitation by _atmospheric pressure_.

664. _How did Mr. Sands perform the feat of walking across the ceiling?_

By having large discs of wet leather attached to his feet, so that when they were placed upon a smooth surface, the air was excluded, and when he allowed his weight to act upon one of the discs, it formed a _hollow cup_ and a _vacuum_. By forming a vacuum of only _twelve square inches_ he gained a pressure of 180 lbs.; this being more than his weight he could accomplish the feat with no other difficulty than that of remaining in an inverted position. The air was admitted underneath the discs by valves, which were closed by springs, which being pressed by the heels of the performer, let in the air, and _set the feet free_.

[Verse: "And God made a wind to pass over the earth."--GENESIS VIII.]

665. _Why is it difficult to strike limpets from rocks?_

Because they have the means of forming a _vacuum_ under their shells, and are pressed on to the rocks by the weight of the atmosphere.

666. _Why can snails move over plants in an inverted position?_

Because they form a _vacuum_ with the smooth and moist surfaces of their bodies, and are supported by atmospheric pressure.

CHAPTER XXXI.

666. _What is wind?_

Wind is air _in motion_. (_See_ 234.)

667. _What are the velocities of winds?_

A _breeze_ travels ten feet in a second; a _light gale_, sixteen feet in a second; a _stiff gale_, twenty-four feet in a second; a _violent squall_, thirty-five feet in a second; _storm wind_, from forty-three to fifty-four in a second; _hurricane_ of the temperate zone, sixty feet in a second; _hurricane_ of the torrid zone, one hundred and twenty to three hundred feet in a second. When wind flies at one mile an hour, it is scarcely perceptible. When its velocity is one hundred miles an hour, it tears up trees, and devastates its track.

668. _What are trade winds?_

Trade winds are vast currents of air, which _sweep round the globe_ over a belt of some 12,000 miles in width.

[Verse: "They shall be as the morning cloud, and as the early dew that passeth away, as the chaff that is driven with the whirlwind out of the floor, and as the smoke out of the chimney."--HOSEA XIII.]

669. _What is the cause of trade winds?_

The air over the tropical regions becomes heated and ascends; it then diverges in two high currents, one towards the north, and the other towards the south pole, where, being cooled, it again descends, and returns towards the equator to replace the air as it ascends therefrom. There is, therefore, a constant revolution of vast currents of air between the tropics and the poles, producing _north and south winds_.

670. _Why do the trade winds blow from east to west, though, in their origin, their direction is from north to south and from south to north?_

Because, as the north and south winds blow towards the equator, they are affected by the revolution of the earth from _west to east_. As the two winds from the poles approach the equator, they are gradually diverted from their northerly and southerly course, to an easterly direction, by the revolution of the earth.

671. _Why is there a prevalence of calms at the equator?_

Because, as the north and the south winds move towards the equator, they drive before them volumes of atmosphere, which, meeting in opposite directions, resist and counterpoise each other, and abide in a state of stillness between the north and south-easterly winds, one on the north and the other on the south of the equator.

672. _What are monsoons?_

Monsoons are _periodical winds_ which blow at a given period of the year from one quarter of the compass, and in another period of the year from the opposite quarter of the compass.

673. _What is the cause of monsoons?_

Monsoons are caused by changes in the position of the sun. When the sun is in the southern hemisphere, it produces a _north-east wind_, and when it is in the northern hemisphere, a _north-west wind_. The north-east monsoon blows from November to March, and the south-west monsoon from the end of April to the middle of October. The region of monsoons lies a little to the north of the northern border of the trade wind, and they blow with the greatest force, and with most regularity, between the eastern coast of Africa and Hindustan.

[Verse: "He shall blow upon them and they shall wither, and the whirlwind shall take them away as stubble."--ISAIAH XL.]

674. _What determines the character of winds?_

The character of winds is influenced by the condition of _the surfaces over which they blow_. Winds blowing over dry and arid plains and deserts are _dry and hot_. Winds blowing across snow-capped mountains and regions of ice are _cold_. Winds that cross oceans are _wet_; and those that cross extensive continents are _dry_.

675. _What winds are most prevalent in England?_

In England out of a _thousand days_, north winds prevail in 82; north-east, 111; east, 99; south-east, 81; south, 111; south-west, 225; west, 171; north-west, 120.

676. _What is the cause of storms?_

Storms result from violent commotions of the atmosphere, and are chiefly the result of extreme _changes of temperature_.

The _magnetic_ state of the earth, and the _electrical_ state of the atmosphere, also materially influence the phenomena of storms.

By some persons the theory is entertained that storms result from various winds _rushing into a centre_ in which the atmosphere has become extremely condensed. According to this theory, a storm is a mighty whirlwind.

A most violent hurricane occurred in 1780, which destroyed Lord Rodney's fleet, and a vast number of merchant ships. It is said to have killed 9,000 persons in Martinique alone, and 6,000 in St. Lucia. The town of St. Pierre in Martinique was totally destroyed; and only fourteen houses in the town of Kingston, in St. Vincent, were left uninjured.

677. _Why do the most violent storms occur in and near the tropics?_

Because there the temperature is very high, and the cold currents of air rushing towards the equator from the poles, causes great _atmospheric disturbance_.

678. _What are whirlwinds?_

Whirlwinds are produced by violent and contrary currents meeting and striking upon each other, producing _a circular motion_. They generally occur after long calms, attended by much heat.

Whirlwinds occurring at sea, or over the surface of water, sometimes put the water in motion, and as the wind rises upwards it lifts with it a whirling mass of water, producing a _water spout_.

[Verse: "Out of the south cometh the whirlwind; and cold out of the north."--JOB XXXVII.]

[Illustration: Fig. 19.--A WATER SPOUT.]

679. _Why does the chimney smoke when the fire is first lighted?_

Because the air in the chimney is of the same temperature as that in the room, and therefore _will not ascend_.

680. _Why does the smoking (into the room) cease, after the fire has been lighted a little while?_

Because the air in the chimney, being warmed by the fire beneath, becomes lighter and ascends rapidly.

681. _Why does a long chimney create a greater draught than a short one?_

Because the short chimney contains _less air_ than the long one; there is, consequently, less difference of weight between the warm air of the short chimney and the external air; it therefore has not so great an _ascensive power._

[Verse: "And, lo, the smoke of the country went up as the smoke of a furnace."--GEN. XIX.]

682. _Why does smoke issue in folds and curls?_

Because it is _pressed upon_ by the _cold air_ which always _rushes towards a rarer atmosphere_. It thus illustrates the development of _storms_.

683. _Why do some chimneys smoke when the doors and windows are closed?_

Because the draught of air is not sufficient to supply the wants of the fire, and enable it to create an _upward current_.

684. _What is the best method of conveying air to fires?_

Tubes built in the walls, communicating with the outer air, and terminating _underneath the grates_.

685. _Why is this the best method of ventilation?_

Because doors and windows may then be made air-tight, and _draughts across rooms be prevented_.

686. _Why do chimneys that stand under elevated objects, such as hills, trees, and high buildings, smoke?_

Because the wind, striking against the elevated object, _flies back_, and a part of it _rushes downward_.

687. _Why do sooty chimneys smoke?_

Because the accumulation of the soot _diminishes the size of the flue_, and lessens the ascensive power of the draught, by reducing the quantity of _warm air_. It also obstructs the motion of the air, by the _roughness of its surface_.

688. _Why do chimneys smoke in damp and gusty weather?_

Because the ascending air is _suddenly chilled_ by gusts of damp and cold air, and driven down the chimney.

[Verse: "Remember that thou magnify his work, which men behold. Every man may see it; man may behold it afar off."--JOB XXXVI.]

689. _Why does smoke ascend in a straight line in mild and fine weather?_

Because the air is still, and being dry and warm it _does not chill the smoke_, nor drive it out of its course.

690. _Why do the wings of wind-mills turn round?_

Because the wind, striking _at an angle_ upon the wings, forces them aside; and as there are four wings all upon the same angle, and fixed upon the same centre, the _oblique pressure_ of the wind causes the centre to rotate.

There is a world of _miniature phenomena_ which has never been fully recognised, in which we may see the mightier works of nature pleasingly and truthfully illustrated.

When the wind blows into the corner of a street, and whirling around, catches straw, dust, and feathers in its arms, and then wheels away, flinging the troubled atoms in all directions,--it is a miniature of the mightier _whirlwind_, which wrecks ships, uproots trees, and levels houses with the earth.

When a cloud of dust, on a hot summer's day, rises and flies along the thirsty road, making the passenger close his eyelids, and dusting the leaves of wayside vegetation,--it is a miniature of the terrible _simoom_, which blows from the desert sands, scattering death and devastation in its track.

When steam issues from the tea-urn, and becomes condensed in minute drops upon the window-pane,--the miniature is of the _earth's heat_, evaporating the waters, and the cold air of night condensing the vapours into _dew_.

When grass and corn bend before the wind, and are beaten down by its force; when the pond forgets its calm, and rises in troubled waves, casting the flotilla of natural boats that move upon its surface, in rude disorder upon its windward shore,--the little storm is but a miniature of those great _hurricanes_ which wrecked a fleet in the Black Sea, and levelled the encampments of a mighty army.

When the snow that has gathered upon the house-top, warming beneath the smiles of the sun, slips from its bed, and drops in accumulated heaps from the roof,--it is a miniature of those terrible _avalanches_ which in the Pyrenees bury villages in their icy pall, and doom man and beast to death.

When the rivulet hurries on its course, and meeting with obstructions, leaps over them in mimic wrath, overturning some little raft upon which, perchance, a weary fly has alighted,--it is a miniature of those _rapids_ on whose banks the hippopotamus and the alligator yet live; and where, though rarely, man may be seen directing his raft over the troubled current, amid the rush of _debris_ from forests unexplored.

And when, in a basin of the rivulet, two opposing currents meet, and form a little vortex into which insect life and vegetable fragments coming within the sphere of its influence are drawn,--it is a miniature of the roaring _whirlpool_, or the wilder _maelstrom_ of the Norwegian seas.

Nature rehearses all her parts in mild whispers; and for every picture that she paints, she places a first study upon the canvas. Man need not go into the heart of her terrors to understand their laws. Many an unknown Humboldt, sitting by the river's side, may rejoice in the "aspects of nature," and share the bliss of knowledge with the great philosopher.

[Verse: "Can any understand the spreadings of the clouds, or the noise of his tabernacle?"--JOB XXXVI.]

CHAPTER XXXII.

691. _What is a barometer?_

A barometer is an instrument which _indicates the pressure of the atmosphere_, and which takes its name from two Greek words signifying _measurer of weight_.

692. _Why does a barometer indicate the pressure of the atmosphere?_

Because it consists of a tube containing _quicksilver_, closed at one end and open at the other, so that the pressure of the air upon the open end balances the weight of the column of mercury (quicksilver), and when the pressure of the air upon the open surface of the mercury increases or decreases, the mercury _rises or falls_ in response thereto.

693. _Why is a barometer called also a "weather-glass"?_

Because changes in the weather are generally preceded by _alterations in the atmospheric pressure_. But we cannot perceive those changes as they gradually occur; the alteration in the height of the column of mercury, therefore, enables us to know that atmospheric changes are taking place, and, by observation, we are enabled to determine certain rules by which _the state of the weather may be foretold_ with considerable probability.

694. _Why are barometers constructed with circular dials, and an index to denote changes?_

Because that is a convenient mechanical arrangement, by which the alterations of the relative pressures of the air and the mercury are _more clearly denoted than by an inspection of the mercury itself._

[Verse: "Fair weather cometh out of the north: with God is terrible majesty."--JOB XXXVII.]

[Illustration: Fig. 20.--BAROMETER.]

[Illustration: Fig. 21.--TUBE OF BAROMETER, WHEEL, AND PULLEY.]

695. _Why does the hand of the weather dial change its position when the column of mercury rises or falls?_

Because a weight, which _floats upon the open surface of the mercury_, is attached to a string, having a nearly equal weight at the other extremity; the string is laid over a revolving pivot to which the hand is fixed, and _the friction of the string turns the hand, as the mercury rises or falls_.

[Verse: "Thou visitest the earth, and waterest it: thou greatly enrichest it with the river of God, which is full of water: thou preparest them corn, when thou hast so provided for it."--PSALM LXV.]

696. _Why does tapping the face of the barometer sometimes cause the hand to move?_

Because the weight on the surface of the mercury frequently _leans against the sides of the tube_, and does not move freely. And, also, the mercury clings to the sides of the tube by _capillary attraction_; therefore, tapping on the face of the barometer _sets the weight free_, and overcomes the attraction which _impedes the rise or fall of the mercury_.

Fig. 21 illustrates the mechanism at the back of the barometer. A is a glass tube; between A and E there exists a _vacuum_, caused by the weight of the mercury pressing downwards. This space being a vacuum, makes the barometrical column more sensitive, as there is no internal force to resist or modify the effects of the external pressure. E represents the height of the column of mercury; C the open end of the tube; F the weight resting on the surface of the mercury; P the pivot over which the string passes, and upon which the hand turns; W the weight which forms the pulley with the weight F.

697. _Which is the heavier, dry or vaporised air?_

Dry air is _heavier_ than air impregnated with vapours.

698. _Why is dry air heavier than moist air?_

Because of the _extreme tenuity of watery vapours_, the density of which is _less than that of atmospheric air_.

699. _Why does the fall of the barometer denote the approach of rain?_

Because it shows that as the air _cannot support the full weight of the column of mercury_, the atmosphere must be thin with watery vapours.

The fall of the mercury in the long arm of the tube would cause the weight F to be pressed upwards. This would release the string to which the weight W is attached; it would, therefore, fall, and turn the hand down to Rain or Much Rain.

700. _Why does the rise of the barometer denote the approach of fine weather?_

Because the external air becoming dense, and free from highly elastic vapours, presses with increased force upon the mercury upon which the weight F floats; that weight, therefore, sinks in the short tube as the mercury rises in the long one, and in sinking turns the hand to Change, Fair, &c.

[Verse: "He caused an east wind to blow in the heaven; and by his power he brought in the south wind."--PSALM LXXVIII.]

701. _Why does the barometer enable us to calculate the height of mountains?_

Because, as the barometer is carried up a mountain, _there is a less depth of atmosphere above to press upon the mercury_; it therefore falls, and by comparing various observations, it has been found practicable to _calculate the height of mountains by the fall of the mercury in a barometer_.

702. _To what extent of variation is the weight of the atmosphere liable?_

It may vary as much as _a pound and a half to the square inch_ at the level of the sea.

703. _When does the barometer stand highest?_

When there is a _duration of frost_, or when _north-easterly winds_ prevail.

704. _Why does the barometer stand highest at these times?_

Because the atmosphere is exceedingly _dry and dense_, and fully balances the _weight of the column of mercury_.

705. _When does the barometer stand lowest?_

When _a thaw follows a long frost_; or when _south-west winds_ prevail.

706. _Why does the barometer stand lowest at those times?_

Because _much moisture exists in the air_, by which it is rendered less dense and heavy.

707. _What effect has heat upon the barometer?_

It causes the mercury to fall, _by evaporating moisture into the air_.

708. _What effect has cold upon the barometer?_

It causes the mercury to rise, by _checking evaporation_, and _increasing the density of the air_.

[Verse: "For so the Lord said unto me, I will take my rest, and I will consider in my dwelling place like a clear heat upon herbs, and like a cloud of dew in the heat of harvest."--ISAIAH XVIII.]

In noting barometrical indications, more attention should be paid to the _tendency_ of the mercury at the time of the observation, than to the _actual state of the column_, whether it stands _high_ or _low_. The following rules of barometric reading are given as generally accurate, but liable to exceptions:--

_Fair weather_ indicated by the _rise_ of the mercury.

_Foul_ weather by the _fall_ of the mercury.

_Thunder_, indicated by the _fall_ of the mercury in _sultry weather_.

_Cold_, indicated by the _rise_ of the mercury in spring, autumn, and winter.

_Heat_, by the _fall_ of the mercury in summer and autumn.

_Frost_, indicated by the _rise_ of the mercury in winter.

_Thaw_, by the _fall_ of the mercury during a frost.

_Continued bad weather_, when the _fall_ of the mercury has been _gradual_ through several fine days.

_Continued fine weather_, when the _rise_ of the mercury has been _gradual_ through several foul days.

_Bad weather of short duration_, when it sets in quickly.

_Fine weather of short duration_, when it sets in quickly.

_Changeable weather_, when an _extreme_ change has _suddenly_ set in.

_Wind_, indicated by a rapid _rise_ or _fall unattended by a change of temperature_.

The mercury _rising_, and the air becoming _cooler_, promises _fine weather_; but the mercury _rising_, and the air becoming _warmer_, the weather will _be changeable_.

If the top of the column of mercury appears _convex_, or curved upwards, it is an additional proof that the mercury is _rising_. Expect _fine_ weather.

If the top of the column is _concave_, or curved downwards, it is an additional proof that the mercury is _falling_. Expect _bad_ weather.

CHAPTER XXXIII.

709. _What is the thermometer?_

The thermometer is an instrument in which _mercury_ is employed to indicate _degrees of heat_. Its name is derived from two Greek words, meaning _heat measurer_.

710. _Why does mercury indicate degrees of heat?_

Because it _expands_ readily with _heat_, and _contracts_ with _cold_; and as it passes freely through small tubes, it is the most convenient medium for indicating _changes of temperature_.

711. _Why are there Reaumur's Thermometers and Fahrenheit's Thermometers?_

Because their inventors, after whom they are named, adopted a different system of _notation_, or _thermometrical marks_; and as their thermometers have been adopted by various countries and authors, it is now difficult to dispense with either of them.

[Verse: "When ye see a cloud rise out of the west straightway ye say, There cometh a shower; and so it is. And when ye see the south wind blow, ye say there will be heat; and it cometh to pass."--LUKE XIII.]

[Illustration: Fig. 22.--THE THERMOMETERS OF REAUMUR AND FAHRENHEIT COMPARED.]

We have combined the two (_see_ Fig. 22.) The diagram will, we have no doubt, prove exceedingly useful to scientific readers and experimentalists. There is also another system of notation, adopted by the French, called the _centigrade_, but it is not much referred to in Great Britain. In the centigrade thermometer 0 zero is the freezing point, and 100 the boiling point. Fahrenheit's scale is generally preferred. Reaumur's is mostly used in Germany. Of Fahrenheit's scale 32 is the freezing point, 55 is moderate heat, 76 summer heat in Great Britain, 98 is blood heat, and 212 is the boiling point. Mr. Wedgwood has invented a thermometer for testing _high temperatures_, each degree of which answers to l30 degrees of Fahrenheit. According to his scale cast iron melts at 2,786 deg.; fine gold at 2,016 deg.; fine silver 1,873 deg.; brass melts at 1,869 deg.; red heat is visible by day at 980 deg.; lead melts 612 deg.; bismuth melts 476 deg.; tin melts 412 deg.; and there is a curious fact with regard to the three metals, lead, bismuth, and tin, that if they are mixed in the proportions of 5, 8, and 3 parts respectively, the mixture (after previous fusion) will melt at a heat below that of boiling water.

712. _What is the difference between the thermometer and the barometer?_

In the thermometer the column of mercury is much smaller than in the barometer, and is sealed from the air; while in the barometer the column of mercury is open at one end to atmospheric influence.

713. _Why does the mercury in the thermometer, being sealed up, indicate the external temperature?_

Because the heat passes through the glass, in which the mercury is enclosed, and _expanding or contracting the metal within the bulb_, causes the small column above it to _rise_ or _fall_.

[Verse: "Blessed is the people that know the joyful sound: they shall walk, O Lord, in the light of thy countenance."--PSALM LXXXIX.]

714. _When does the thermometer vary most in its indication of natural temperature?_

It varies more in the _winter_ than in the _summer_ season.

715. _Why does it vary more in the winter than in the summer?_

Because the temperature of our climate _differs more from the temperature of the torrid zones in the winter_ than it does in the _summer_, and the _inequalities of temperature_ cause frequent changes in the degree of prevailing heat.

The same remarks (714, 715,) apply to the barometer.

CHAPTER XXXIV.

716. _What is sound?_

Sound is an _impression produced upon the ear_ by _vibrations_ of _the air_.

717. _What causes the air to vibrate and produce sounds?_

The atoms of _elastic bodies_ being caused to _vibrate_ by the application of some kind of force, _the vibrations of those atoms are imparted to the air_, and sound is produced.

718. _How do we know that sounds are produced by the vibrations of the air, induced by the vibrations of the atoms of bodies?_

If we take a tuning fork, and hold it to the ear, we hear _no sound_. If we move it rapidly through the air, or if we blow upon it, it produces _no sound_; but if we _strike it_, _a sound immediately occurs_; the vibration of the fork may be seen, and felt by the hand that holds it; and _as those vibrations cease_, the sound _dies away_.

719. _How do we know that without air there would be no sound?_

Because if a tuning fork were to be struck in a _vacuum_ (as under the receiver of an air pump) _no sound_ would be heard, although the _vibrations_ of the fork could be _distinctly seen_.

[Verse: "And even things without life giving sound, whether pipe or harp, except they give a distinction in the sounds, how shall it be known what is piped or harped."--CORINTH. XIV.]

720. _How are the vibrations of sonorous bodies imparted to the air?_

When a bell is struck, the force of the blow gives an instant agitation to all its particles. The air around the bell is driven back by the impulse of the force, and thus a _vibration of compression_ is imparted to the air; but the air returns to the bell, by its own natural elasticity, thus producing a _vibration of expansion_--when it is again struck, and thus _successive vibrations_ of compression and expansion are _transmitted through the air_.

721. _How rapidly are these vibrations transmitted through the air?_

They travel at a rate of rather more than _a quarter of a mile in a second_, or _twelve miles and three-fourths in a minute_.

722. _Do all sounds travel at the same rate?_

All sounds, whether strong or weak, high or low, musical or discordant, _travel with the same velocity_.

723. _Why are bells and glasses stopped from ringing by touching them with the finger?_

Because the contact of the finger _stops the vibration_ of the atoms of the metal and glass, which therefore _cease to impart vibrations to the air_.

724. _Why does a cracked bell give discordant sounds?_

Because the _connection_ between the atoms of the bell being _broken,_ their vibrations are not uniform: some of the atoms vibrate _more intensely_ than the others; the vibrations imparted to the air are therefore _jarring_ and _discordant_.

725. _Why, when we see a gun fired at a distance, do we see the flash and smoke, before we hear the report?_

Because _light,_ which enables us to _see_, travels at the velocity of 192,000 miles in a _second_; while _sound_, by which we _hear_, travels only at the rate of a quarter of a mile in a _second_.

[Verse: "My heart maketh a noise in me: I cannot hold my peace, because thou hast heard, O my soul, the sound of the trumpet, the alarm of war."--JER. IV.]

726. _Why does the tread of soldiers, when marching in long ranks, appear to be irregular?_

Because the sounds proceeding from _different distances_, reach our ears in _varying periods of time_.

727. _What are the numbers of vibrations in a second that produce the various musical sounds?_

C or Do, 480 vibrations in a second; B or Si, 450 vibrations; A or La, 400 vibrations; G or Sol, 360 vibrations; F or Fa, 320 vibrations; E or Mi, 300 vibrations; D or Re, 270 vibrations; C or Do, 240 vibrations. It is thus seen that the _more rapid_ the vibrations, the _higher_ the note, and _vice versa_.

728. _Why does the length of a wire or string determine the sound that it produces?_

Because the _shorter the string_ the _more rapid_ are its vibrations when struck.

729. _Why does the tension of a wire or string affect its vibrations?_

Because when the string or wire is tight, a touch communicates vibrations to _all its particles_; but when it is loose the vibrations are _imperfectly communicated_.

730. _Why are some notes low and solemn, and others high and quick?_

Because the vibrations of musical strings vary from 32 vibrations in a second, which produces a soft and deep bass, to 15,000 vibrations in a second, which produces the sharpest treble note.

731. _Why can our voices be heard at a greater distance when we speak through tubes?_

Because the vibrations are _confined to the air within the tube_, and are not interfered with by _other vibrations_ or movements in the air; the tube itself is also a _good conductor of sound_.

[Verse: "And I will cause the noise of thy songs to cease; and the sound of thy harps shall no more be heard."--EZEKIEL XXVI.]

732. _Is air a good conductor of sound?_

Air is a _good conductor_, but water is a _better conductor than air_; wood, metals, the earth, &c., are also _good conductors_.

733. _Why can we hear sounds at a greater distance on water than on land?_

For various reasons: because the smooth surface of water is a good conductor; because there are fewer noises, or counter vibrations, to interfere with the transmission of sound; and because there are no elevated objects to impede the progress of the vibrations.

734. _Why do sea-shells give a murmuring noise when held to the ear?_

Because what may be called _expended vibrations_ always exist in air where various sounds are occurring. These _tremblings_ of the air are received upon the thin covering of the shell, and thus being _collected into a focus_, are _transmitted to the ear_.

735. _Why can people in the arctic regions converse when more than a mile apart?_

Because there the air, being _cold and dense_, is a very good conductor; and the _smooth surface of the ice_ also favours the _transmission of sound_.

736. _Why do savages lay their heads upon the earth to hear the sounds of wild beasts, &c.?_

Because the earth is a good conductor of sound. For this reason, also, persons _working under ground in mines_ can hear each other digging at considerable distances.

737. _Why can church clocks be heard striking much more clearly at some times than at others?_

Because the density of dry air improves the _sound-conducting power_ of the atmosphere. The transmission of sounds is also assisted by the direction of the winds.

738. _Why may the scratching of a pin at one extremity of a long pole be heard by applying the ear to the opposite extremity?_

Because wood is a good conductor of sound, and its atoms are _susceptible of considerable vibration_. It is, therefore, chosen in numerous instances for the construction of _musical instruments_.

[Verse: "The morning is come unto thee, O thou that dwellest in the land: the time is come, the day of trouble is near, and not the sounding again of the mountains."--EZEKIEL VII.]

Deaf persons have been known to derive pleasure from music by placing their hands upon the wood-work of musical instruments while being played upon.

739. _Why is the hearing of deaf persons assisted by ear-trumpets?_

Because ear-trumpets _collect the vibrations of the air_ into a focus, and make the sounds produced thereby more intense.

740. _Why are sounding-hoards used to improve the hearing of congregations?_

Because, being suspended over, and a little behind, the speaker, they _collect the vibrations_ of the air, and _reflect_ them towards the congregation.

741. _What are echoes?_

Echoes are sounds _reflected_ by the objects on which they strike.

742. _Why do some echoes occur immediately after a sound?_

Because the reflecting surface is _very near_; therefore the sound returns immediately.

743. _Why do some echoes occur a considerable time after a sound?_

Because they are at a considerable distance, and the sound takes time to travel to it, and an equal time to return.

744. _Why do some echoes change the tone and quality of sound?_

Because the reflecting surface, having vibratory qualities of its own, _mingles its own vibrations with that of the sound_.

745. _Why are there sometimes several echoes to one sound?_

Because there are various _reflecting surfaces_, at different distances, each of which returns an echo.

[Verse: "And God said, Let the waters under the heaven be gathered together onto one place, and let the dry land appear: and it was so."--GEN. I.]

746. _Are sounds reflected only by distant objects?_

Sounds are doubtless reflected by _walls and ceilings_ around us. But we do not perceive the echoes, because they are so near that they occur at the same moment with the sound. In lofty buildings, however, there is frequently a _double sound_, making the utterance of a speaker indistinct. This arises from the echo following very closely upon the sound.

747. _Why, when we are walking under an arch-way or a tunnel, do our voices appear louder?_

Because the sounds of our voices are _immediately reflected_. And as a _gas reflector increases the intensity of light_, so _a sound reflector_ will _increase the apparent strength of our voices_.

There are many places where remarkable echoes occur. On the banks of the Rhine, at Lurley, if the weather be favourable, the report of a rifle, or the sound of a trumpet, will be repeated at different periods, and with various degrees of strength, from crag to crag, on opposite sides of the river alternately. A similar effect is heard in the neighbourhood of some of the Lochs in Scotland. There is a place at Woodstock, in Gloucestershire, which is said to echo a sound fifty times. Near Rosneath, a few miles from Glasgow, there is a spot where, if a person plays a bar of music upon a bugle, the notes will be repeated by an echo, but a third lower; after a short pause, another echo is heard, again in a lower tone; then follows another pause, and a third repetition follows in a still lower key. The effect is very enchanting. The whispering galleries of St. Paul's, of the cathedral church of Gloucester, and of the Observatory of Paris, owe their curious effects to those laws of the reflection of sound, by which echoes are produced; but in these cases the effect is assisted by the elliptical form of the edifice, each person being in the focus of an ellipse.

CHAPTER XXXV.

748. _What is water?_

Water is a fluid composed of _two_ volumes of _hydrogen_ to _one_ of _oxygen_, or _eight_ parts by weight of _oxygen_ to _one_ of _hydrogen_. It is nearly colourless and transparent.

749. _Why, if a saucer of water be exposed to the air, will it gradually disappear?_

Because water is highly expansive, and _rises in thin vapour_, when in contact with warm and dry air.

[Verse: "Behold there ariseth a little cloud from the sea, of the bigness of a man's hand. And it came to pass in the meantime, that the heaven was black with clouds and wind, and there was a great rain."--1 KINGS XVIII.]

750. _Why does steam issue from the spout of a kettle?_

Because the heat of the fire passes into the water, and _drives_ its atoms apart, making those of them that rise quickly to the surface _lighter than the air_, upon which they consequently rise.

751. _Why does water become solid when it freezes?_

Because the _latent heat of the water_ passes away from between its atoms into the air; the atoms, therefore, draw closer together.

752. _Why, if the atoms of water draw closer together when freezing, does ice expand, and occupy greater space than water?_

Because, _when the atoms of water are congealing_, they do not form a _compact mass_, but arrange themselves _in groups of crystal points_, which occupy greater space. Water _contracts_ when freezing until it sinks to 40 deg., and then it _expands_ as ice is formed.

32 deg. is said to be the _freezing_ point, but it should be called the _frozen_ point.

753. _Why does water boil?_

Because heat, _entering into the lower portions_ of the water, _expands_ it; the heated portions are then _specifically lighter_ than those that are cooler; the hot water therefore _rises upward, and forces the cooler water down_.

754. _What proportion of the earth's surface is covered with water?_

There are about one hundred and forty seven millions of square miles of _water_, to forty-nine and a half millions of square miles of _land_.

755. _What is the amount of water pressure?_

The pressure of the sea, at the depth of 1,100 yards, is equal to 15,000 lbs. _to the square inch_.

[Verse: "But the land, whither ye go to possess it, is a land of hills and valleys, and drinketh water of the rain of heaven."--DEUT. XI.]

756. _What element is the most abundant in nature?_

_Oxygen_, which forms so large a part of _water_. Of animal substances, _oxygen_ forms _three-fourths_; of _vegetable substances_ it forms _four-fifths_; of _mineral substances_ it forms _one-half_; it forms _eight-ninths_ of the _waters_ and _one-fifth_ of the _atmosphere_; and aggregating the whole creation, from _one-half to two-thirds_ consists of _oxygen_.

757. _In what ways does man use oxygen?_

Man _eats_, _drinks_, _breathes_, and _burns_ it, in various proportions and combinations. It is estimated that _the human race_ consume in those various ways 1,000,000,000 lbs. daily; that the _lower animals_ consume double that amount; and that, in the varied works of nature, no less than 8,000,000,000 lbs. of _oxygen_ are used _daily_.

758. _Why does water dissolve various substances?_

Because the _atoms of water_ are very minute; they therefore _permeate the pores_, or spaces, between the atoms of those bodies, and _overcoming their attraction for each other_, cause them to separate.

759. _Why does hot water dissolve substances more readily than cold?_

Because the _heat assists to repel the particles_ of the substance undergoing solution, and _gives the water a freer passage_ between the atoms.

760. _Why is pump water sometimes hard?_

Because, in passing through the earth, it has become impregnated with mineral matters, usually the _sulphate_ and _carbonate of lime_.

761. _Why is rain water soft?_

Because it is derived from vapours which, in ascending to the clouds, _could not bear up the mineral waters with them_. It therefore became purified or distilled.

762. _Why do kettles become encrusted with stony deposits?_

Because that portion of the water which is driven off in steam _leaves the mineral matters behind_; they therefore form a crust around the sides of the kettle.

[Verse: "He gathereth the waters of the sea together as an heap; he layeth up the depth in storehouses."--PSALM XXXIII.]

It is said that if a child's marble be placed in a kettle, it will attract the earthy particles, and prevent the encrusting of the sides of the vessel.

763. _Why is it difficult to wash in hard water?_

Because the soap unites with the mineral matters in the water, and being _neutralised_ thereby, cannot _dissolve the dirt_ which we desire to cleanse away.

764. _Why is the sea salt?_

Because salt is a mineral which prevails largely in the earth, and which, _being very soluble in water_, is taken up by the ocean.

Lakes and rivers, also, even those that are considered fresh, hold in solution _some degree of saline matters_, which they contribute to the ocean.

As, in the evaporations from the sea, the salt remains in it, while the vapours fall as rain, and again wash the earth and carry some of its mineral properties to the ocean, the _greater saltness of the sea_, as compared with rivers, is accounted for.

By some persons the opinion is entertained that the sea has been _gradually getting salter_ ever since the creation of the world. This, they say, arises from the evaporation of water free from salt, and the returns of the water to the sea, taking with it salt from the land.

765. _What is the estimated amount of salt in the sea?_

The amount of common salt in the various oceans is estimated at 3,051,342 _cubic geographical miles_, or about five times more than the mass of the mountains of the Alps.

766. _What is the depth of the sea?_

The extreme depth has not, probably, been ascertained. But Sir James Ross took soundings about 900 miles west of St. Helena, whence he found the sea to be nearly _six miles in depth_. Now, if we take the height of the highest mountain to be five miles, the distance from that extreme rise of the earth, to the known depth of the sea, will be no less than _eleven miles_.

767. _Why are the waters of some springs impregnated with mineral matters?_

Because the water passes through beds of soda, lime, magnesia, carbonic acid, oxides of iron, sulphate of iron, &c., &c., and _takes up in some slight degree the particles of those minerals_, according to the proportions in which they abound.

[Verse: "Who hath measured the waters in the hollow of his hand, and meted out heaven with the span, and comprehended the dust of the earth in a measure and weighed the mountains in scales, and the hills in a balance?"--ISAIAH XL.]

768. _Why does iron rust rapidly when wetted?_

Because the water contains a large proportion of oxygen, some of which combines with the iron and forms _an oxide of iron_, which is _rust_.

769. _Why does stagnant water become putrid?_

Because the _large amount of oxygen_ which it contains accelerates the decomposition of dead _animal and vegetable substances_ that accumulate in it.

770. _Is there danger in drinking water on account of the living animalcules which it contains?_

No danger arises from the _living creatures_ in water; but _putrefactive_ matters may produce serious diseases.

771. _What is the best method of guarding against impurities?_

By obtaining water from the purest sources, and by filtering it before drinking, by which nearly all extraneous matters would be _separated from it_.

CHAPTER XXXVI.

772. _What is attraction?_

Attraction is the tendency of bodies to _draw near to each other_. It is called _attraction_, from two Latin words signifying _drawing towards_.

773. _How many kinds of attraction are there?_

There are five principal kinds of _attraction_:--

1. The attraction of _gravitation_. 2. The attraction of _cohesion_. 3. The attraction of _chemical affinity_. 4. The attraction of _electricity_. 5. And _capillary attraction_.

[Verse: "Behold, the nations are as a drop of a bucket, and are counted as the small dust of the balance: behold, he taketh up the isles as a very little thing."--ISAIAH XL.]

774. _Why do all bodies heavier than the air fall to the earth?_

Because they are influenced by the _attraction of gravitation_, by which all bodies are drawn towards _the centre of the earth_.

775. _Why do bodies lighter than the air ascend?_

Because the air, being a denser body, _obeys the law of attraction_, and in doing so _displaces lighter bodies_ that interfere with its gravitation.

776. _Why do fragments of tea, and bubbles floating upon the surface of tea, draw towards each other, and attach themselves to the sides of the cup?_

Because they are influenced by the _attraction of cohesion_.

_Cohesion._--The act of sticking together.

777. _Why will a drop of water upon the blade of a knife leave a dark spot?_

Because the _iron of the knife attracts the oxygen of the water_, by _chemical affinity_; and the two substances form a thin coating of _oxide of iron_.

_Affinity._--Attraction between dissimilar particles through which they form new compounds.

778. _Why do clouds sometimes move towards each other from opposite directions? and_

779. _Why do light particles of matter attach themselves to sealing wax, excited by friction?_

Because they are moved by the _attraction of electricity_.

780. _Why will a towel, the corner of which is dipped in water, become wet far above the water?_

Because the water is conveyed up through the towel, by _capillary attraction_. The atoms of the water are attracted by the _threads of the towel_, and drawn up into the _small spaces between the threads_.

_Capillary._--Resembling a hair, small in diameter.

[Verse: "He stretcheth out the north over the empty place, and hangeth the earth upon nothing."--JOB XXVI.]

781. _Why do small bodies floating upon water move towards larger ones?_

Because the attractive power of a _large body_ is greater than that of _a small one_. As each atom of matter has inherent power of attraction, it follows that a _large aggregation of particles_ must attract in proportion to the number of those particles.

782. _Why do clouds gather around mountain tops?_

Because they are _attracted by the mountains_.

783. _Why would a piece of lead tied to a string, and let down from a church steeple, incline a little from the perpendicular towards the church?_

Because the _masses of stone_ of which the church is built would _attract the lead_.

784. _How can man weigh the earth?_

By observing what is called the _deflection_ of small bodies _when brought within given distances of larger bodies_, the degree of attraction _exercised by the large body upon the smaller one_ becomes known. This attraction of the _large body_ exercised over the _smaller body_ is an opposing influence, _acting against the earth's attraction of the small body_, which is drawn out of its course: it constitutes a _natural balance between the influence of the earth and another body, acting in opposition to it_. Founded upon these, and some other data, man can weigh the earth, and give a morally certain result!

_Deflection_.--The act of turning aside.

785. _How can man weigh the planets?_

The planets exercise as certain an influence upon each other _as do two pieces of wood floating upon a basin of water_. As the planetary bodies fly through their prescribed orbits, _and approach nearer to, or travel further from, each other_, they are observed to _deviate_ from that course which they must have pursued _but for the increase or the decrease of some influence of attraction_. By making observations _at various times_, and by comparing _a number of results_, it is possible _to weigh any planetary body, however vast, or however distant_.

[Verse: "Is not God in the height of the heaven? and behold the height of the stars, how high they are?"--JOB XII.]

786. _How can man measure the distances of the planets?_

By making observations at _different seasons of the year_, when the earth is in _opposite positions in her orbit_; and by recording, by _instruments constructed with the greatest nicety_, the _angle of sight_, at which the planetary body is viewed; by noticing, also, _the various eclipses_, and estimating _how long the first light after an eclipse has ceased_ reaches the earth, it is possible to estimate the _distances_ of heavenly bodies, _no matter how far in the depths of the universe those orbs may be_.

787. _What are the opinions founded upon estimates respecting the magnitude of the sun?_

The _diameter_ of the _sun_ is 770,800 geographical miles, or 112 times greater than the diameter of the earth; its _volume_ is 1,407,124 times that of the earth, and 600 times greater than _all the planets together_; its _mass_ is 359,551 times greater than the earth; and 738 times greater than that of _all the planets_. A _single spot_ seen upon its surface has been estimated to extend over 77,000 miles in diameter, and a _cluster of spots_ have been estimated to include an area of 3,780,000 miles.

788. _What is the weight of the earth?_

The earth has a _circumference_ of 25,000 miles, and is estimated to _weigh_ 1,256,195,670,000,000,000,000,000 tons.

789. _What is the specific gravity of a body?_

It is its weight estimated _relatively to the weights of other bodies_.

790. _What determines the force with which bodies fall to the earth?_

Generally speaking, their _specific gravity_, which is proportionate to the density, or _compactness of the atoms_ of which they are composed.

791. _Why does a feather fall to the earth more gradually than a shilling?_

Because the _specific gravity_ of the feather and of the shilling is _relative to that of the air_, the medium through which the feather and the shilling pass. If there were _no air_, a shilling and a feather dropped at the same time from a height of forty miles, _would reach the earth at the same moment_.

CHAPTER XXXVII.

[Verse: "Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding."]

792. _What is repulsion?_

Repulsion is that property in matter by which it _repels_ or _recedes from_, those bodies for which it has _no attraction or affinity_.

793. _Why does dew form into round drops upon the leaves of plants?_

Because it _repels the air_, and the _substances of the leaves_ upon which it rests. Because, also, its own particles _cohere_.

794. _Why do drops of water roll over dusty surfaces?_

Because they _repel_ the particles of dust; and also because their own particles have _a stronger attraction for each other_ than for the particles of dust.

795. _Why does a needle float when carefully laid upon the surface of water?_

Because the needle and the water _mutually repel each other_.

796. _Why does water, when dropped upon hot iron, move about in agitated globules?_

Because the _caloric_ repels the particles of the water.

797. _Why does oil float upon the surface of water?_

Because, besides being specially lighter than water, the particles of the oil and the water _mutually repel each other_.

798. _What is carbonic acid?_

Carbonic acid is a mixture of _carbon_ and _oxygen_, in the proportion of 3 lbs. of carbon to 8 lbs. of oxygen.

[Verse: "Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it?"]

799. _Where does carbonic acid chiefly exist?_

It exists in various natural bodies in which carbon and oxygen are combined; it is evolved by the decomposition of numerous bodies called carbonates, in which carbon is united with a particular base, such as the carbonate of lime, the carbonate of iron, the carbonate of copper, &c. It is also evolved by the processes of _fermentation_, by the _breathing of animals_, the _combustion of fuel_, and the _functions of plants_. Carbonic acid also _exists in various waters_.

Carbonic acid is _found most largely in solid combinations with other bodies_: it forms 44-100ths of all limestones and marbles, and it exists in smaller quantity, combined with other earths, and with metallic oxides.

800. _What are the states in which pure carbonic acid exists?_

Pure carbonic acid may exist in the _solid_, the _liquid_, or the _æriform_ state. In the _solid state_ it is produced only by artificial means, and it is then a white crystallised body, in appearance _like snow_; in the _liquid state_ it is a _heavy colourless fluid_; in the _æriform state_ it is a _pungent_, _heavy_, _colourless gas_, and is known as _carbonic acid gas_.

801. _Why does bottled porter produce large volumes of froth, much more than the bottle could contain?_

Because, by the fermentive process, _carbonic acid_ has been developed in the porter, and is held in _liquid solution_; but it always has a _strong tendency to escape_, and directly the pressure is removed, it _evolves into gas_, by which it occupies much greater space, and forces the porter in millions of small bubbles out of the bottle.

802. _Why does soda-water effervesce?_

Because _carbonic acid gas_ is forced into the water _by pressure_. Pressure _alters the gas into a liquid_, and directly the pressure ceases, the liquid again _evolves into gas_.

803. _Why does spring water taste fresh and invigorating?_

Because it contains _carbonic acid_.

[Verse: "Whereupon are the foundations thereof fastened? or who laid the cornerstone thereof."--JOB XXXVIII.]

804. _Why does boiled water taste flat and insipid?_

Because the _carbonic acid_ has been _driven off_ by boiling.

805. _Why does beer which has been standing in a glass taste flat?_

Because its _carbonic acid_ has escaped as _carbonic acid gas_.

806. _Why, when we look into a glass of champagne, do we see bubbles spontaneously appear at the bottom, and then rise to the top?_

Because, in the places where the bubbles are formed, the _liquid carbonic acid_ is evolving into _carbonic acid gas_.

807. _Why do the bubbles arise from two or three points in columns, rapidly succeeding each other?_

Because, when the formation of gas once begins, and bubbles ascend, there is _less pressure_ in the line of the _column of bubbles_; the carbonic acid, therefore, draws towards those points as the _easiest channel of escape_.

These explanations equally apply to the "working" of beer, by which yeast is produced; to the effervescence of various waters, acidulated drinks, ginger beer, &c., and also to the "sponging" of bread, &c.

808. _Why does gunpowder explode?_

Gunpowder is made of a very intimate _mechanical mixture_ of _nitrate of potash_, _charcoal_, and _sulphur_. When these substances are heated to a certain degree, the nitrate of potash is decomposed, and its _oxygen_ combines with the _charcoal_ and _sulphur_, instantaneously forming _large volumes of carbonic acid gas_ and _nitrogen_, which, seeking an escape, produce an explosion.

809. _Why does charcoal act as a powerful disinfectant?_

Because the _carbon_ readily absorbs, and combines with _various gases_, neutralising their _offensive odours_, and destroying their _unhealthy properties_.

Let us now pause for a few moments to consider the importance of those two great divisions of nature, Air and Water, and to reflect upon the wisdom of some of those laws which are connected with the phenomena thereof, and which have not yet been sufficiently explained.

[Verse: "Thus saith the Lord, Let not the wise man glory in his wisdom, neither let the mighty man glory in his might, let not the rich man glory in his riches."--JEREMIAH IX.]

We have seen that the air is a thin elastic body surrounding the globe; that it consists of certain gases essential to the life of animals, and to the growth of plants; and that it takes part in most of those chemical changes, which mark the transformations of the inorganic creation. Whether it be the burning of a piece of wood, the evaporation of a drop of water, the breathing of an animal, the respiration of a plant, or the fermentation of bodies, the air in almost every instance gives or receives--and in most of the operations in which it engages, it does both.

But there is one point of view, which we must add to those which have already been considered: the order of nature consists of generation, life, and death. Every beat of the watch signals the birth of millions of living things, and the same beat proclaims that as many living organisms have yielded up their vital spark, and that forthwith the elements of which they are composed must be dissolved, and restored to the great laboratory of nature.

The air is the vast receptacle of those organic matters which are undergoing dissolution. The body of the shipwrecked mariner, cast upon the shore of a desolate island, blackens in the sun, and the full round form gradually dwindles to skin and bone, until at last the few atoms that remain crumble into dust, and are scattered to the wind. The same process occurs, with some modifications, whether bodies are buried in the earth, or dissolve upon its surface. The leaves of forests fall and accumulate in heaps, where they ferment and dissolve, leaving only their more earthy particles behind.

The amount of matter which day by day passes from the state of the living to that of the dead, must be enormous; but from the difficulties of acquiring data, beyond the possibility of calculation. Such statistics as we have, however, enable us to form conclusions as to the mighty agencies in which the air is constantly engaged. There are on the earth 1,000,000,000 inhabitants of whom nearly 35,000,000 die every year, 91,824 every day, 3,730 every hour, and 60 every minute. But _even the living die daily_, and undergo an invisible change of substance, as we shall hereafter explain.

The bodies of those many millions are dissolved in the air, in vapours and gases which, before the dissolution of each corporeal organism is complete, begin to live again in the various forms of vegetable and animal life.

Of the number of animals living and dying upon the face of the earth, we can form no adequate estimate. Of mammals there are about 2,000 ascertained species; of birds 8,000 species; of reptiles 2,000 species; of fishes some 8,000 or 10,000 species; of molluscs some 15,000 species; of shell fish 8,000 species; of insects 70,000 species. And, including others not specified here, the total number of _species_ of animals probably amounts to no less than 250,000,--each species consisting of _many millions_ of living creatures.

In the area of London alone, no less than 200,000 tons of fuel are annually cast into the air in the form of smoke. And if we take into account the vast operations of nature in evaporation, fermentation, and putrefactive decomposition, we may be enabled to form a conception of the mighty part which that _thin air_, of which we think so little, plays in the grand alchemy of nature.

[Verse: "I will praise thee; for I am fearfully and wonderfully made; marvellous are thy works; and that my soul knoweth right well."--PSALM CXXXIX.]

In addition, also, to the facts already communicated, respecting the sound-bearing and light-refracting properties of air, it must be remarked, that but for the atmosphere, and the general refraction of light by its particles--each atom as it were catching a fairy taper, and dancing with it before our view--the condition of vision would be widely opposite to that which exists, and totally unsuited to our wants. The various objects upon which the illuminating rays of the sun fell, would be lighted up with an intense glare, but all around would be darkness, just as when a single ray of light is passed into a dark chamber, and directed upon a solitary object. The air, without becoming itself visible, _diffuses luminous rays_, in modified intensity, in every direction. If the air reflected so much light as to render _itself visible_, it would appear like the glittering surface of the water reflecting the solar rays, and we should then be unable to see the various objects which surround us.

Of the importance of Water in the scheme of creation, man generally entertains an imperfect conception. It is simply supposed to afford moisture to plants, drink to animals, and to promote salubrity by its cleansing properties. Let us, however, contemplate man as he stands before us, noble in form, erect in position, full of strength, joy, ambition. How much of that noble form is composed of water? Suppose that it could all be instantaneously withdrawn--not the oxygen and the hydrogen, which might combine to form water--but the fluid that exists in his body as _water_, unchanged--except by mechanical admixture with the secretions of the body--Why then that beautiful temple would collapse and become a mere shred, so thin, that it would seem but a shadow of the body as it existed before, and the beholder might doubt whether life ever inhabited a frame whose structure was so frail. It is said that _three-fourths_ by weight of the human body consist of _water_. Thus, if man weighs 120lbs., 90lbs. consist of water, and this subtracted, only 30lbs. of solid matter remain. This statement is rather under than over the fact.

The assertion is startling, but so true that it can be verified by simple experiment. A piece of lean flesh--say of beef--cut an inch thick, and placed in a slow oven, and allowed to remain until all its water was driven off in vapour, would become as thin as a wafer, and as light as a cork. With a more scientific arrangement, it would be possible to collect the water, and the weights of the condensed vapour, and of the solid residue, would together make up the weight of the beef: if the piece weighed sixteen ounces, the weight of the water would be about 14 ounces, and the _solid matter_ about _two ounces_.

Water holds a similar proportion in the bodies of all animals, and of vegetables. It is evident, therefore, that it occupies a more important place in the scale of creation than is generally accorded to it by the unobservant mind. We are indebted to it for those atmospheric changes which constitute the peculiar feature of our varying climate. Rising in invisible vapours, it builds palaces of glory in the skies, and often presents to the view of man the imagery of heaven. Persons who have ascended above the altitude of the clouds, have described the scene upon looking down towards them as the most celestial that the mind can conceive. Fields of fleecy radiance, majestically rolling like a sea of gold, occupied the whole range of vision, and seemed to embellish an eternity of space. Those golden clouds that at one time are decked in the richest splendour, and occupy the upper chambers of the Court of Nature, become grave councillors when the earth grows thirsty, and the plant droops with languor. They roll their heavy brows together, as in consultation upon some grave necessity: down come the refreshing showers, the mighty tongue of thunder rocks the air, the earth is drenched, and becomes fresh with the salubrity of her toilette; obnoxious substances, with their offensive exhalations, are swept away: living things rejoice, and beautiful flowers throw their incense in thanksgiving into the air; the broad blue heavens for a time look down and smile upon the blessed work; and then the clouds again gather in a golden train, and one by one fill the high arches of the atmosphere, until the earth once more grows thirsty, and the flower supplicates for drink.

[Verse: "How mighty are his wonders! his kingdom is an everlasting kingdom, and his dominion is from generation to generation."--DANIEL IV.]

With reference to Light, its wonders, and the curious but imperfect theories respecting it, we have little to add, except with regard to its physiological action upon the eyes of man and of animals, which will be given in another place. But of its sister, Darkness--for it would not do now to call darkness the antagonist of light, since it will be seen that they work harmoniously for good--we have to say, that recent discoveries indicate that darkness is as necessary to the health of nature as light. Not only is it necessary to compose man and animals to sleep, to give rest to the over-wrought nerves of the industrious--but light is the quickening power of vegetation, and although plants grow by night, they grow, as man does, when stretched upon his bed--but some of their functions, which are actively excited in the presence of light, are at rest in darkness. Nor is this all: there is not an atom upon the face of the earth which is not affected by the rays of the sun, their light, their heat, their actinism. Colours change: some are bleached, others are darkened. All bodies are expanded. The hardest rock sustains _an effect_ from the sun's rays; and an unceasing sun, shining upon the hardest granite, would in time produce such a disturbance of its atomic condition, that adamant would crumble away to dust.

The going down of the sun, therefore, marks the period when not only does the bird fly to her resting-place, and man turn to his couch; but when _every atom of a vast hemisphere_ subsides into a state of quietude, and when homogeneous particles of matter return to their mutual rest.

In a few succeeding lessons, we intend to point out some of the scientific truths that are _illustrated in the use of toys_. We think we shall be able to show to our young readers, that even the hours of play may be made the periods of delightful instruction; and that there is _no_ "reason why" the acquirement of knowledge should not sweetly accord with the occasional pursuit of those pastimes by which health of body and vigour of mind are induced.

But before we commence the discharge of that pleasant duty, let us say a few words respecting Carbon, that important agent in the world's history. It is, doubtless, perplexing to the minds of many persons, to understand how the _diamond_ can be _pure carbon_; how _charcoal_ can be _carbon_ a _little less pure_ than the diamond; and how _coal_ and _sugar_ can also be carbon, _less pure_ than the charcoal. The statement that in the diamond carbon exists in a different atomic condition, is almost as instructive to the inquiring mind, as to say, "It is so, _because it is_."

Diamonds are expensive things, and so difficult to experiment upon, even if they were not expensive, that the doors of inquiry seem locked. To turn diamonds into charcoal, or into carbonic acid gas, is a very costly formula of experiment. Charcoal fires, thus sustained, would soon burn a man out of his house; and soda water, impregnated with carbonic acid gas, produced from diamonds, would be a very expensive beverage. If we could only turn charcoal into diamonds, and carbonic acid gas into brilliants, that would be quite another affair. A new Eldorado would be discovered, and there would be so many experimenters that, when they all succeeded, they would find that diamonds had lost their value. However, as a fact for the encouragement of those who would like to be early in the race, we may state that the atoms of charcoal which are repulsed from the charcoal points, during the electric agitation which produces the electric light, acquire a hardness and a sharpness almost equal to that of the diamond--only there is still the awkward obstacle in the way, that _they happen to be black_.

[Verse: "He delivereth and rescueth, and he worketh signs and wonders in heaven and in earth."--DANIEL VII.]

We must see, therefore, whether there is anything in nature that we can experiment upon, theoretically or practically, to give us a clearer conception of this difficult matter. There is a large _dew-drop_ resting upon a luxuriant cabbage leaf--one of those great leaves that have flourished in defiance of the snail, and now spreads out like the gigantic frond of the _Victoria Regina_. That dew-drop is one of the beautiful diamonds which Nature sprinkles about on cloudless nights, as if to show the stars, in answer to their twinkling, that we have something that will glisten and twinkle too.

The dew-drop is a very good imitation of a diamond, and to the lover of God's works, quite as precious as the stone set in gold. It does not consist of carbon--it probably may have a mite of carbonic acid in its embrace--but that is not necessary to our purpose: all we want to know is, _the different atomic conditions_ of which bodies are susceptible, and the very dissimilar appearances they exhibit under the variations of atomic states. It doesn't glisten so much as the diamond, _because it is round_--if we could cut it into a number of _facets_, it would refract light almost as perfectly as the diamond. It is not _solid_--but we can freeze it, and we shall at once exhibit two different atomic conditions, that will represent nearly enough the diamond, and the liquid carbonic acid. Then, if we evaporate the dew-drop, we shall produce a volume of vapour nearly _two thousand times as large as the dew-drop_. The steam will be white; but we have only to imagine it black, and then we get an analogy of the differences of the atomic conditions that prevail in _the diamond_, _carbonic acid_, and _charcoal_, _tinder_, _lamp-black_, or any light form of carbon. Of course we have been illustrating _atomic conditions only_, and not chemical composition.

There are a few other facts connected with carbon that merit consideration. Carbonic acid gas, _entering the lungs_, is a _deadly poison_; but _entering the stomach_, which lies close under the lungs, and is over-lapped by them, it is a _refreshing beverage_. Although charcoal, when burnt, gives off the most poisonous gas, it seems to be very jealous of other gaseous poisons; for if it be powdered, and set about in pans where there is a poisonous atmosphere, it will seize hold of poisonous gases, and, by absorbing, imprison them. Even in a drop of toast and water, the charred bread seizes hold of whatever impurities exist in the water; and water passed through beds of charcoal, becomes filtered, and made beautifully pure, being compelled to give up to the charcoal whatever is obnoxious. If a piece of meat that has already commenced putrifying, be sprinkled with charcoal, it will not only object to the meat putrifying any further, but it will _sweeten that which has already undergone putrefaction_. Although, in the form of gas, it will poison the blood, and cause speedy stupefaction and death; if it be powdered, and stitched into a piece of silk, and worn before the mouth as a respirator, it will say to all poisonous gases that come to the mouth with the air, "I have taken this post to defend the lungs, and I arrest you, on a charge of murderous intention." Such are the various facts connected with carbon; and they forcibly indicate that those who understand Nature's works, are likely to receive her best protection.

[Verse: "The father of the righteous shall greatly rejoice; and he that begetteth a wise child shall have joy of him."--PROVERBS XXIII.]

CHAPTER XXXVIII.

810. _Why does a humming-top make a humming noise?_

Because the hollow wood of the top vibrates, and the edges of the hole in its sides _strike against the air as it spins_; the air is thereby set in vibration.

811. _Why does a peg-top hum less than a humming-top?_

Because, _being a solid body of wood_, and having no _hole in its sides_, its particles are _not so easily thrown into vibration_; consequently it does not so readily impart vibrations to the air.

812. _Why does a peg-top sometimes hum, and at other times not?_

Because, if it is spun with _great force_, and its peg is _struck sharply_ against the pavement, _the wood is set in vibration_, and the surface of the top, repelling the air by its rapid motion, causes _vibratory waves_. But if it be spun with insufficient force, _the wood is not set in vibration_.

[Illustration: Fig. 23.--HUMMING-TOP BEFORE SPINNING.]

[Illustration: Fig. 24.--HUMMING-TOP SPINNING.]

813. _Why do we see the figures painted upon the humming-top, before it spins, but not while it is spinning?_

Because the rapid whirling of the top brings the images of its different parts so quickly in succession upon _the retina of the eye_, that they _deface each other_, and _impart an impression of coloured rings, instead of definite objects_.

[Verse: "Train up a child in the way he should go; and when he is old, he will not depart from it."--PROVERBS XXII.]

814. _Why does a top stand erect when it spins, but fall when it stops?_

Because the top is under the influence of, and is balanced between _opposing forces_. The rapid rotation of the top gives to all its particles a tendency to _fly from the centre_. If the atoms of the wood were not held together by the _attraction of cohesion_, they would fly away in a circle outward from the top, _just as drops of water fly off from a mop, while it is being twirled_. If you take a spoonful of sand, salt, or dust, and drop it upon the top, it will be scattered in a circle, just as the atoms of the top would be, _if they were free to separate_, but not with the same force, because the atoms of the salt, &c., not being in an active state of rotation, would only be influenced _by momentary contact with the rotating body_. This tendency of the particles of a rotating body to fly outward from the centre, is called _the centrifugal force_.

_Centrifugal._--From two Latin words meaning receding from the centre.

The other force influencing the top is _the attraction of gravitation_: the attraction which, were the top not spinning, would draw it towards the earth. The "spill" projecting from the bottom of the top _stands in the line in which the top is drawn towards the earth_ and keeps it from obeying the law of gravitation. Therefore the rotatory motion given to the top, by the rapid unwinding of the string, and the tendency of its atoms to fly outward, _balance the top_ upon the line in which it is drawn to the earth, and which is occupied by the spill, which prevents it falling to the ground.

815. _Why does a top first reel around upon the spill, then become upright, and "sleep," and then reel again, and fall?_

[Illustration: Fig. 25.--PEG-TOP "REELING."]

Because, in being thrown from the hand, the top is delivered a little out of the perpendicular, but the spill _is rounded off at the point_, and when the top is rotating rapidly, the gravitative force which attracts the top to the ground continually acting upon it, _draws the weight of the top on to the extreme centre of the round point_. When the rotation subsides, and the centrifugal force is weakened, then the top _is no longer balanced upon the extreme point of the spill_, but falls upon its sides, until the force of gravitation is exerted _beyond the line of the spill_, upon the body of the top, and then it falls to the ground.

[Verse: "Even a child is known by his doings, whether his work be pure, and whether it be right."--PROVERBS XX.]

816. _Why does a top "sleep?"_

Because at that period of its spinning, which is called "sleeping," the _centrifugal_ and the _gravitative forces_ acting upon the top, are _nearly balanced_; and the top, obeying chiefly the _rotatory force_, appears to be in a state of comparative rest.

817. _Why does the top cease to spin?_

Because _the friction of the air against its sides_, and the _friction of the spill against the ground_, act in opposition to the _rotatory force_, which is a temporary impulse applied by external means--the hand of the person who spins it--and as soon as this _applied force_ is expended, the top yields to the law of gravitation, which is _a permanent and ever-prevailing force_.

818. _Why does a marble revolve, as it is propelled along the ground?_

Because, in propelling the marble, _the thumb impels the upper surface forward, and the finger draws the under surface backward_. This gives a tendency to the upper and lower hemispheres of the marble _to separate_, which they would do, but for the _cohesion of the atoms_ of the marble. The upper part of the marble, therefore, rolls forward, _drawing after it the under part_, which acquires a forward motion by the force with which it is drawn upward, and in this way the opposite portions of the marble act upon each other in the successive revolutions.

When the marble strikes upon the earth, a new influence is exerted upon it, which is _the friction of the earth_ upon the surface that comes in contact with it; but the upper part of the marble, being free, _overcomes the friction acting upon the lower part_, and thus the marble continues to progress, until _the applied force which projected it is expended_.

[Verse: "Better is a poor and a wise child, than an old and foolish king who will no more be admonished."--ECCLESIASTES IV.]

819. _Why does a striped marble appear to have a greater number of stripes when rolling, than when at rest?_

Because the stripes are presented in _rapid succession_ to the eye; and as the eye receives _fresh impressions of stripes before the previous impressions have passed away_, the stripes appear multiplied.

[Illustration: Fig. 26.--MARBLE AT REST.]

[Illustration: Fig. 27.--MARBLE ROLLING.]

820. _Why does a marble rebound when dropped upon the pavement?_

Because the force of its fall to the earth _compresses the atoms_ of which the marble is composed; and the atoms then exert the force of _elasticity to restore themselves to their former condition_; and by the exercise of this force the marble is _repelled_, or _thrown upward from the pavement_. Although a marble may be made of very hard stone, yet that stone may be _elastic_, and possess, though in a much less degree, _the same kind of elasticity which causes the India-rubber ball to rebound from the earth_.

821. _Why does a marble, assuming it to be impelled with equal force, roll further on ice than on pavement, and further on pavement than on a pebble walk?_

Because the _friction_ is greater upon pavement than upon ice, and greater upon a pebble walk than upon pavement.

822. _How many forces contribute to stay the progress of a rolling marble?_

The friction of the _air_, the friction of the _earth_, and the _attraction of gravitation_, which tends to bring all bodies to a state of rest.

[Verse: "He shall turn the heart of the fathers towards the children, and the heart of the children to their fathers."--MALACHI IV.]

823. _Why do the stripes upon a marble disappear when it is spun with great velocity?_

Because, as in the case of the humming-top, the different parts of the surface are _brought so rapidly in succession to the sight_, that they _deface or confuse_ the impressions upon the retina.

[Illustration: Fig. 28--MARBLE SPINNING RAPIDLY.]

824. _Why are rings most perceptible at the opposite points, or poles, of the marble?_

Because the point, or pole, _upon which the marble spins_, and that which _corresponds to it_, on the upper surface, travel _less rapidly_ than the central portions, which being of a larger circumference, pass through a greater amount of space, in the same period of time. The stripes at the _poles_ of the marble, are, therefore visible, while those at its _equator_ are imperceptible. (_See_ 522.)

CHAPTER XXXIX.

825. _Why are soap-bubbles round?_

Because they are _equally pressed upon all parts of their surface_ by the atmosphere.

826. _Why are bubbles elongated when being blown?_

Because the _unequal pressure of the current of breath_ by which they are being filled, alters the _relative pressure_ upon the outer surfaces.

827. _Why does the bubble close, and become a perfect sphere, when shaken from the pipe?_

Because the _attraction of cohesion_ draws the particles of soap together, directly the bubble is set free from the bowl.

[Verse: "Children's children are the crown of old men; and the glory of children are their fathers."--PROVERBS XVII.]

[Illustration: Fig. 29.--BLOWING SOAP BUBBLES.]

828. _Why do bubbles, blown in the sunshine, change their colours?_

Because the films of the bubbles constantly change in thickness, through the atoms from the upper part descending towards the bottom, and therefore the varying thickness of film _refracts, in different degrees, the rays of light_.

829. _Why do bubbles burst?_

Because the atoms that compose their films _fall towards the earth by gravitation_; the upper portion of the bubbles then _becomes very thin_, and as the denser air of the atmosphere _presses towards the warm breath within the bubble, it bursts the film_.

_See_ 236, 237, _etc._, 501, _etc._

830. _Why do balloons ascend in air?_

Because the air or gas which they contain is _specifically lighter than the atmosphere_; _the atmosphere, therefore, forces itself underneath the balloon_, by its own tendency towards the earth, and the balloon is thereby raised upwards. _A balloon is but a larger kind of bubble, made of stronger materials._

831. _Why does an air-balloon become inflated when the spirit set upon the sponge is lit?_

Because the _heat_ of the flame, and the _burning of the spirit_, A, create a volume of _rarefied_, or _thin air_, which inflates the balloon, and makes it _specifically lighter_ than the surrounding medium.

[Verse: "A wise son heareth his father's instruction."--PROVERBS XIII.]

832. _Why do balloons sometimes burst when they ascend very high?_

Because, as they get into the _thinner air_, which exists at _high altitudes_, the gas within them expands, and the coating of the balloon is burst asunder.

[Illustration: Fig. 30.--AIR-BALLOON.]

833. _Why does the gas of balloons expand in thin air?_

Because the air exerts a _less amount of pressure_ upon the air or gas contained in the balloons.

834. _Why do parachutes fall very gradually to the ground?_

Because the _air_, coming in contact with the _under surface_ of the expanded head of the parachute resists its downward progress.

[Illustration: Fig. 31.--PAPER PARACHUTE.]

835. _Why does a shuttlecock travel slowly through the air?_

Because the air acts upon the feathers of the shuttlecock, in the same manner as it does upon the parachute--it strikes against their expanded surface, and resists their progress through the air.

836. _Why does the shuttlecock spin in the air?_

Because the surfaces of the feathers fall upon the air _obliquely_, or slantingly, and therefore, as the shuttlecock descends, it turns in the air.

[Verse: "Come ye children, hearken unto me, I will teach you the fear of the Lord."--PSALM XXXV.]

[Illustration: Fig. 32.--BATTLEDORE AND SHUTTLECOCK.]

837. _Why do we hear a noise when we strike the shuttlecock with the battledore?_

Because the _percussion_ of the shuttlecock upon the parchment of the battledore causes it to vibrate, and the vibrations are imparted to the air.

838. _Why is the sound a dull and short one?_

Because the vibrations of the parchment are _not very rapid_, therefore there is _little intensity_ in the vibrations of the air.

839. _Why does the exercise, afforded by playing battledore and shuttlecock, make us feel warm?_

Because it makes us breathe _more freely_, and causes the _blood to flow faster_; we, therefore, inhale more _oxygen_, which produces heat by combining with the _carbon_ of our _blood_.

840. _Why does a kite rise in the air?_

A kite rises in the air by the force of the wind, which _strikes obliquely_ upon its _under surface_. The string is attached to the "belly-band" in such a manner that it is nearer the _top_ than the _bottom_ of the band: this causes the bottom of the kite, when its surface is met by the wind, to recede in the direction of the wind: the top is accordingly _thrown forward_, and the kite is made to _lie obliquely_ upon the current of air moving against it. The kite then being _drawn by the string in one direction_, and _pressed by the air in another direction_, moves in a line which _describes a medium between the two forces acting upon it_.

[Verse: "Be ye therefore followers of God, as dear children; and walk in love, as Christ also hath loved us."--EPHESIANS V.]

[Illustration: Fig. 33.--DIAGRAM EXPLAINING THE FLIGHT OF A KITE.]

841. _Why does the kite-string feel hot when running through the hand?_

Because the _rapid friction_ sets free the _latent heat_ of the _string_, attracts the heat of the _hand_ to the spot where the friction occurs, and sets free the latent heat of the _air_, which follows the _string_ through the hand, and is compressed by the friction.

842. _Why does running with the kite cause it to rise higher?_

Because it _increases the force_ with which the wind strikes upon the surface of the kite. If a person were to _run with a kite at the rate of five miles an hour, through a still air_, the effect would be _equal to a wind flying at the rate of five miles an hour_ against a kite held by a _stationary string_.

843. _Why does the flying-top rise in the air?_

Because its wings _meet the air obliquely_, just as the surface of the kite does. And the _twirling of the top_, causing the oblique surfaces of its wings to strike the air, produces _the equivalent effect of a wind from the earth blowing the top upwards_.

[Verse: "Children obey your parents in the Lord: for this is right."]

844. _Why does the flying-top return to the earth when its rotations are expended?_

Because the _reaction_ produced by its wings striking upon the air, is insufficient to counteract the _attraction of gravitation_.

[Illustration: Fig. 34.--FLYING-TOP.]

[Illustration: Fig. 35.--PEA AND PIPE.]

845. _Why does a pea, into which a pin has been stuck, dance in suspension upon a jet of air blown through a pipe?_

Because the jet of air, being _slightly compressed_ under the _convex_ form of the pea, by the weight of the pin, forms a _concave cup of air_, in which the pea rests.

In the case put, it is supposed that the pin is _passed through the pea_ until its head comes in contact with it. The pin is dropped into the hole of the pipe, and the breath is then applied, the pipe being held upright. The pea will rise in the air, and be suspended upon the jet, while the point of the pin will rotate around the stem of the pipe. There are other methods of fixing the pin which alter the result, and require a different explanation to that given above.

LESSON XL.

846. _Why does a mouse, painted upon one side of a card, and a trap upon the other, represent to the eye a mouse in a trap when the card is rapidly twirled upon a string?_

Because the image of the mouse is brought to the retina of the eye before the image of the trap has passed away. The two impressions, therefore, _unite upon the retina_, and produce the image of a mouse in a trap.

[Verse: "Honour thy father and thy mother * * That it may be well with thee, and thou mayest be long on the earth."--EPHESIANS VI.]

[Illustration: Fig. 36.--CARD WITH MOUSE-TRAP.]

[Illustration: Fig. 37.--REVERSE OF CARD WITH MOUSE.]

847. _Why will a bow stretched out of its natural position, propel an arrow through the air?_

Because its substance, being _highly elastic_, the particles thereof seek to restore themselves to their former state, as soon as the resisting power is withdrawn. The _force_ derived from this elasticity, is communicated to the arrow by the string against which it is placed.

848. _Why is the arrow propelled forward?_

Because the elasticity of the bow, _acting equally upon its two ends_, to which the string is fastened, produce a line of force in a _diagonal direction_. It thus illustrates the law, that _when a body is acted upon by two forces at the same time, whose directions are inclined to each other, it will not follow either of them, but will describe a line between the two_.

849. _What forces tend to arrest the flight of the arrow?_

The _friction of the air_, and the _attraction of gravitation_.

[Verse: "My son, give, I pray thee, glory to the Lord God of Israel, and make confession unto him."--JOSHUA VII.]

850. _Why are feathers usually fastened to the ends of arrows?_

Because the _greater friction_ of air acting upon them, opposes the progress of that part of the arrow in a greater degree than it does the other portion. The effect is, _to keep the point of the arrow forward_, and in a straight line with its opposite extremity. If the arrow were shot the reverse way from the bow, it would _turn round_, in the course of its flight, in consequence of the friction of the air, offering greater resistance to the progress of the feathered end.

[Illustration: Fig. 38.--BOW AND ARROW.]

[Illustration: Fig. 39.--JEW'S HARP.]

851. _Why does a Jew's harp give musical sounds?_

Because the _vibrations of the metal tongue_ are communicated to the ear.

852. _Why will not the Jew's harp produce loud sounds unless it is applied to the mouth?_

Because the vibrations are not very intense, but when it is blown upon by the breath, the air is pressed upon it, and the vibrations are thereby rendered more powerful.

853. _Why does the alteration of the arrangement of the mouth, affect the formation of the sounds?_

Because it sends the air to the tongue of the harp in _a greater or lesser degree of compression_.

[Verse: "Hear, ye children, the instruction of a father, and attend to know understanding."--PROVERBS IV.]

854. _Why does the pressure applied to the handle of an air pistol propel the cork?_

Because, between the cork A and the air-tight piston C, there is a _closed chamber of air_ B. When the handle D, which moves the piston C, is rapidly pushed in, it _compresses the air_ until it is so much condensed, that it forces out the cork A.

[Illustration: Fig. 40.--AIR PISTOL, OR "POP-GUN."]

855. _Why must the handle be drawn out, before the cork is placed in?_

Because otherwise a partial _vacuum_ would be formed between A and C, and there would not be sufficient air to force out the cork by the return of the piston C D.

856. _Why does water rise in a syringe when the handle is drawn out?_

Because the pressure of the air on the water outside of the syringe, forces it into the space vacated by the drawing up of the handle, and where, otherwise, a _vacuum_ would be formed.

[Illustration: Fig. 41.--SYRINGE, WITH JET OF WATER.]

857. _Why does not the water run out when the syringe is raised?_

Because the pressure of the air upon the small orifice resists the weight of the water.

858. _Why does the water leak out, but not run?_

Because water has a tendency always to _move to the lowest point_, but as the air does not enter freely the water cannot escape. It therefore _drops_, as small portions of the air enter.

[Verse: "Remember now thy creator in the days of thy youth."--ECCLESIASTES XI.]

859. _Why cannot the handle be pressed in, if the finger is applied to the orifice?_

Because water is not _compressible_, like air; it must therefore escape before the handle can be pressed in. Air may be forced into a much smaller compass than is natural to it; but it is impossible to _compress water_ in any great degree.

[Illustration: Fig. 42.--"SUCKER."]

[Illustration: Fig. 43.--HOOP.]

860. _Why does a "sucker" raise a stone?_

Because underneath the sucker _a vacuum_ is formed and the external air, pressing on all sides _against the vacuum_, lifts the stone. The term "sucker" is founded upon the mistaken notion that the leather "sucks," or "draws" the stone. That such is not the case is evident: if, when the stone is suspended, a pin's point be passed under the leather, so as to open a small passage for the air, the stone will _drop instantly_.

861. _Why does a hoop roll, without falling to the ground?_

Because the _centrifugal force_ gives it a motion which is called the _tangent to a circle_--that is, a tendency in all its parts _to fly off in a straight line_. When a piece of clay adhering to the hoop flies off, it leaves the hoop in a line which is straight with the part of the surface from which it was propelled; this line is _the tangent to the circle of the hoop_; and the tendency of all the parts of the hoop to fly off in this manner, counteracts the attraction of the earth, so long as the hoop is kept in motion.

[Verse: "Children obey your parents in all things: for this is well-pleasing unto the Lord."--COLOSSIANS III.]

862. _Why does the hoop, in falling, make several side revolutions?_

Because its onward movement, not being quite expended, influences the _centre of gravity of the hoop_, and changes its line of direction. The hoop is also elastic, and when its sides strike the earth, they spring up again, and continue turning until the opposing forces are overcome by the _attraction of gravitation_.

863. _Why will a little boy balance a large boy on a see-saw?_

Because the "see-saw" may be placed so that its ends are at _unequal distances from the centre_. This gives the little boy the power of _leverage_, by which is meant the increase of power, or weight, by _mechanical means_.

[Illustration: Fig. 44.--BOYS AND "SEE-SAW."]

864. _Why does the little boy sink to the ground when the larger boy slightly kicks the earth?_

Because the larger boy, by kicking against the earth, opposes by mechanical force the _attraction of gravitation_ acting upon him, and he becomes _temporarily_ less attracted to the earth than the little boy.

865. _Why can the little boy, if he choose, keep the big boy up, when once he is up?_

Because, as the big boy is then on _an inclined plane_ with the _fulcrum_, or centre upon which the see-saw moves, the arm of _the lever_, upon which the big boy sits, is _relatively shortened_, and he has then _less mechanical power_. Also, a portion of the weight of the larger boy is transmitted along the lever _to the arm upon which the little boy sits_.

[Verse: "Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous."--1 JOHN III.]

[Illustration: Fig. 45.--TRAP AND BALL.]

866. _Why is the ball propelled upward, in the game of trap and ball, when the trigger is struck?_

Because, when the trigger is struck at A, it is forced downwards, turning upon the fulcrum B, the opposite end, forming the spoon, is thereby forced upwards, describing a small _arc_, or curved line; but directly the ball is set free from the spoon, it rises in a _right line_ with the _direction it was taking, at the moment it was set free_.

[Illustration: Fig. 46.--BAT AND BALL.]

867. _What principles of natural philosophy are illustrated by the results of bat and ball?_

_Percussion_, when the bat strikes the ball; _rotatory motion_, when the ball is sent whirling away; _momentum_, which it acquires by velocity; _elasticity_, when it rebounds from an object against which it strikes; _reflected motion_, when it is turned by a body upon which it impinges; _friction_, as it rolls along the ground; the _communication of force_, when it sets another body in motion against which it strikes; _gravitation_, when it falls to the earth; and _inertia_, when it lies in a state of rest.

[Verse: "A wise son makes a glad father: but a foolish son is the heaviness of his mother."--PROVERBS X.]

868. _Why do pith-tumblers always pitch upon one end?_

Because the _lead_ B is _specifically heavier_ than the _pith_ to which it is attached; it therefore always falls undermost; and as the lead is rounded off, just like the spill of a top, after the head has oscillated a little, and expended the force of the momentum of its fall, it will settle upon its _centre of gravity_, or the point through which it is _attracted to the earth_.

[Illustration: Fig. 47. PITH-TUMBLER.]

869. _Why do the figures upon the "Thaumatrope" appear to dance, when they are made to revolve before a mirror?_

Because the eye, in looking through the holes in the card, towards the reflections in the mirror, receives a _rapid succession of impressions_. As the figures upon the card are represented in a graduated series of positions--the _first_ one standing upright, the _second_ with his knees a little bent, the _third_ a little more bent, as in the act of springing, and so on, the _figure_ being in each case _the same_, but the position _slightly altered_, imparts an impression to the mind, through the eye, that _one figure_ is passing through a _series of motions_.

_Thaumatrope._--From two Greek words, meaning _wonder_ and _to turn_.

We have said enough, we hope, to show that even the play-hours of children may be made instructive to them; and that the simplest toys may be used to illustrate some of the grandest laws of nature. Nor may this kind of instruction be confined to children alone. Grown-up people, whether participators in the sports of youth, or simple observers of their games, may gain instruction for themselves, and be the better teachers of their children, by taking an interest in their enjoyments, and giving to their minds, through the attractiveness of pastime, a taste for observing and estimating the varied phenomena which present themselves.

[Verse: "Jesus said, Suffer little children, and forbid them not, to come unto me; for of such is the kingdom of heaven."--MATTHEW XIX.]

Moreover, we think that parental government acquires a greater power when it leans towards the natural desires of childhood, and wins those desires into a proper direction. Love existing between parent and child is the best tie to home, and the strongest incentive to duty. There is also something in the gentleness of childish nature which may influence for good the sterner mould of man, too often warped and clouded by the cares of life.

[Illustration: Fig. 48.--THAUMATROPE, OR "WONDER-TURNER."]

In Kay's "Life of Sir John Malcolm," we find an admirable and apt passage. Sir John says:--"I have been employed these last few hours with John Elliot, and other boys, in trying how long we could keep up two cricket-balls. Lord Minto caught us. He says he must send me on a commission to some very young monarch, for that I shall never have the gravity of an ambassador for a prince turned of twelve. He, however, added the well-known and admirable story of Henry IV. of France, who, when caught on all fours carrying one of his children, by the Spanish envoy, looked up and said, 'Is your excellency married?' 'I am, and have a family,' was the reply. 'Well, then,' said the monarch, 'I am satisfied, and shall take another turn round the room,' and off he galloped, with his son on his back flogging and spurring him. I have sometimes thought of breaking myself of what are termed boyish habits; but reflection has satisfied me that it would be very foolish, and that I should esteem it a blessing that I can find amusement in everything, from tossing a cricket-ball, to negotiating a treaty with the Emperor of China. Men who will give themselves entirely to business, and despise (which is the term) trifles, are very able, in their general conception of the great outlines of a plan, but they feel a want of knowledge, which is only to be gained by mixing with all classes in the world, when they come to those lesser points upon which its successful execution may depend."

[Verse: "Whether therefore ye eat, or drink, or whatsoever ye do, do all to the glory of God."--CORINTH. X.]

CHAPTER XLI.

869. _Why do we eat food?_

Because the atoms of which our bodies are composed are _continually changing_. Those atoms that have fulfilled the purposes of nature are removed from the system, and, therefore, new matter must be introduced to supply their place.

870. _Why do we eat animal and vegetable food?_

Because their substances are composed of _oxygen_, _hydrogen_, _carbon_, and _nitrogen_--the four chemical elements of which the human system is formed. They are, therefore, capable of nourishing the body, after undergoing digestion.

871. _Why do we masticate our food?_

Because mastication is _the first process towards the digestion of food_. Before animal or vegetable substances can nourish us, their condition must be entirely changed, their _organic_ states must be dissolved, and they must become simple matter, in a homogeneous mass, consisting of the four chemical elements necessary to nutrition, and they must again be restored to an organic condition.

872. _Why does saliva enter the mouth when we are eating?_

Because, in addition to the _mechanical_ grinding of the food by the action of the teeth, it is necessary that it should undergo certain chemical modifications to adapt it to our use. There are placed, therefore, in various parts of the body, _glands_, which secrete peculiar fluids, that have a chemical influence upon the food.

The first of these glands are the _salivary glands of the mouth_, which pour out a clear watery fluid upon the food we eat, and which fluid has been found to possess a property which contributes to the digestion of food.

The moisture afforded by the salivary secretion is also necessary to enable us to swallow the food.

[Verse: "And the Lord said unto him, Who hath made man's mouth? or who maketh the dumb, or the seeing, or the blind? have not I the Lord?"--EXODUS IV.]

873. _Why does the salivary juice enter the mouth just at the moment that we are eating?_

Because the glands, which are buried in the muscles of the mouth, and which in their form are much like bunches of currants, are always full of salivary secretion. There are nerves which are distributed from the brain to these glands, and when other nerves which belong to the senses of taste, of sight, or of feeling, are excited by the presence of food, _a stimulus_ is imparted to the salivary glands, through the nerves that surround them, their cells collapse, and the juice which they contain is poured out through their stems, or ducts, into the mouth.

874. _How do we know that impressions imparted to one set of nerves, may be imparted to another set, so as to put any particular organ in action._

Because very frequently _the mere sight_ of rich fruit, or acid substances, _will cause the saliva to flow freely_. In this case it is evident that the salivary glands _could not see or know_ that such substances were present. An impression must, therefore, be made upon the brain, _through the organ of vision_, and the desire to taste the substances being awakened, a nervous stimulus is _imparted to the glands of the mouth_, and they at once commence their action, _as if food were present_.

875. _Why does food descend into the stomach?_

Because, after the teeth, the tongue, and the muscles of the mouth generally, have rolled the food into a soft bolus, it is conveyed to the back of the mouth, where it is set upon the opening of the throat (_oesophagus_). It does not then descend through the throat by its own gravity, because the throat is generally in a compressed or collapsed state, like an empty tube; and we know that persons can eat or drink when with their heads downwards. The oesophagus is formed of a number of muscular threads, or rings, and _each little thread is like a hand ready to grasp at the morsel that is coming_. As soon as the bolus is presented at the top of the throat, these little muscular hands lay hold of it, and transmit it downward, passing it from one to another, until it is conveyed through the long passage, to the door of the stomach, which it enters.

[Verse: "Remove far from me poverty and lies; give me neither poverty nor riches; feed me with food convenient for me."--PROVERBS XXX.]

[Illustration: Fig. 49.--SECTION OF THE STOMACH, &c.]

A. The inner coat of the _stomach_. (The stomach is here represented cut through its length, so that we can see its inside.)

B. The lower extremity of the throat, or _oesophagus_, through which food enters the stomach.

C. The passage out of the stomach, called the _pylorus_, where a muscular contraction prevents the escape of undigested food.

D. The _duodenum_, and the ducts through which the _bile_ and _pancreatic_ juices enter and mingle with our food.

876. _Why do we not feel the food being transmitted through the throat?_

Because the nerves of the body differ in their powers: some are nerves of _feeling_, some of _motion_, and others are nerves of the _senses_. The nerves of feeling are most abundantly distributed to those parts _where feeling is most useful and necessary to us_. But the faculty of feeling our food undergoing digestion would be no service to us whatever; therefore the nerves of _motion_ are plentifully distributed to the throat and stomach, but very few of the nerves of _feeling_--just as many as will tell us when we eat anything _too hot_, or _too cold_, or that the stomach is _out of order_.

877. _Why do we feel uneasy after eating to excess?_

Because the stomach is _distended_, and presses upon the other organs by which it is surrounded.

[Verse: "Who satisfieth thy mouth with good things; so that thy youth is renewed like the eagles."--PSALM CIII.]

878. _Why do we feel drowsy after eating heartily?_

Because, while the stomach is in action, _a great proportion of the blood of the body is drawn towards it_, and as the blood is withdrawn from the other parts of the body, they fall into a state of languor.

879. _Why does blood flow more freely to the stomach during digestion?_

Because the energy of an organ is _increased by the flow of blood_, which supplies the _material_ of which our organs are composed, and in which the _vital essence_, supporting life, resides.

880. _Why does excess in eating bring on indigestion?_

Because the power of the stomach to digest food is _governed by the amount of food required by the system_. It seems to be an instinct of the stomach to hold back food which is in excess, and by indications of pain and disturbance to warn its master that _excess has been committed_.

881. _Why is food digested in the stomach?_

Because it enters the stomach in the form of a paste, produced by the action of the mouth; and directly food enters, the _gastric juice_, which is formed by glands embedded in the coats of the stomach, trickles down its sides. This is a more _powerful solvent_ than the salivary juice--it is like the same kind of fluid, only much stronger, and it soon turns the food from a rough and crude _paste_ into a _greyish cream_ (chyme). The heat of the stomach assists the operation, and the muscular threads of the coats move the cream along, in the same manner that the muscles of the oesophagus brought down the food.

The cream is passed towards the door which leads outward from the stomach (_pylorus_); but if, in the midst of the cream, there are any undissolved particles of food, it closes upon them, and they return again to the stomach to be further changed.

[Verse: "When thou hast eaten and art full, then thou shalt bless the Lord thy God for the good land which he hath given thee."--DEUT. VIII.]

882. _Why does indigestion bring on bilious attacks?_

Because the _liver_ secretes a fluid to assist in the digestion of food. The liver is a gland--a similar organ to the glands of the mouth--and it forms _bile_ in the same manner that they form the salivary juice. Only the liver is a _much larger gland_, and a much greater quantity of blood passes through it. The liver pours its secretion into the biliary duct (Fig. 49) to mix with the grey cream as it passes onward, and to further dissolve it. But when the stomach is excited by food which it cannot dissolve, and when the owner of the stomach, disregarding its remonstrances, will persist in over-eating, or in eating things that disagree with the system, then _the liver and the stomach sympathise_, and the muscular threads, or hands, that prevail all through the alimentary organs, instead of moving _onward_, move backward, and _throw some bile into the stomach_ to assist to dissolve and remove the excessive or improper food.

CHAPTER XLII.

883. _Why does some portion of the food we eat nourish the system, while other portions are useless?_

Because most food contains some particles that are indigestible, or that, if digested, are innutritious, and not necessary for the system. The _liver_ is the organ by whose secretion the _useful is separated from the useless_; for when the bile enters through the duct (Fig. 49) and mixes with the grey cream coming from the stomach, it remains no longer a grey cream, but turns into a mass coloured by bile, having upon its surface _little globules of milk_, small, but very white. Those minute globules of milk (_chyle_) are the nutritious particles derived from the food; the other portion, coloured with bile, is the useless residue, or rather the _bulk from which the nutrition has been extracted_.

[Verse: "God hath made of one blood all nations of men for to dwell on all the face of the earth."--ACTS XVII.]

884. _Why does the milky, or nutritious matter, separate from the innutritious, upon admixture with bile?_

Because the bile contains an oily matter which _repels_ the watery _milk of nutrition_.

The _pancreatic juice_ also enters through the same duct with the bile. But its precise use is not understood. It is a fluid much like the salivary secretion of the glands of the mouth.

[Illustration: Fig. 50.--GREAT VESSELS OF THE CIRCULATION, AND THE DUCT WHICH CONVEYS NUTRITIVE MATTER TO THE BLOOD.]

A B. _Jugular veins_ which return blood from the head to the heart.

C. The _superior venæ cava_, or trunk vein, which pours the blood returned from the upper part of the system into the heart. There is a similar large vessel which meets this one and brings back blood from the lower part of the body, and they both pour the blood into the right side of the heart.

D E. The branches of the _venous system_ which bring back the blood from the arms.

F F. The _great aorta_, the blood vessel which conveys arterial blood from the heart, and gives off branches that supply every part of the body.

G. Another large vein which returns the blood from the muscles of the chest, &c.

H H. The _thoracic duct_, which receives the newly dissolved food from the small absorbents, that collect it from the intestines. It conveys this nutrition (called chyle) upward along the back, until it reaches where the duct turns into the junction of two veins, and pours its contents into the veins bringing blood back to the heart. The nutrition, therefore, is at this moment mixed with the venous blood, and is sent to the lungs to be oxygenised.

[Verse: "But now hath God set the members in the body, every one as it pleased him."--1 CORINTHIANS XII.]

885. _How is the nutrition taken away from the bilious residue?_

The muscular threads (or hands, as we figuratively call them) continue to push forward the digested matter through a long tube, called _the alimentary canal_, or bowels. This canal is some thirty feet in length, and is folded in various layers across the abdomen, and tied to the edge of a sort of apron, which is gathered up and fastened to the back-bone. All along this alimentary canal those muscular hands are pushing the digested mass along. But upon the coat or surface of the canal there are millions of little vessels called _lacteals_, which look out for the minute globules of milk as they pass, and _absorb_ them, which means that they pick them up, and carry them away. There is an immense number of these little vessels, all busily at work picking up food for the system.

Then there is a large vessel, called the _thoracic duct_, which comes down and communicates with those little vessels (it is a sort of overseer, having a large number of workmen,) and collects the produce of their toil, and carries it upwards to the part where it passes _from the organs of digestion_ into the _vessels of circulation_.

886. _What becomes of the nutrition, when it has entered the vessels of the circulation?_

It is sent through a large vein into _the heart_, entering that organ on the right side, from which the heart propels it into the lungs, mixed with _venous blood_; and the venous, or blue blood, is sent into the lungs, _taking with it the milk_, the formation of which we have traced.

887. _Why are the venous blood and the chyle sent to the lungs?_

Because the venous blood, in its circulation through the body, has parted with its _oxygen_, and taken up _carbon_, and it requires _to get rid of the carbon, and take up more oxygen_. The chyle, also, now combined with the blood, requires _oxygen_, and having obtained it, is converted into _bright red blood_, and the blue blood of the veins, having got rid of its carbon, which formed the carbonic acid of the breath, has again become _bright red blood_. We must therefore, in pursuing our description, cease to speak of blue, or _venous blood_, and of white milk, or _chyle_, for the two have now combined, and, with the oxygen of the air, have formed _arterial blood_.

[Verse: "My flesh and my heart fainteth; but God is the strength of my heart, and my portion for ever."--PSALM LXXIII.]

888. _What becomes of the arterial blood thus formed?_

It is sent back from the lungs to the right side of the heart, from which it is sent into the _great trunk of the aorta_, and from thence it passes into smaller blood-vessels, until it finds its way to _every part of the system_.

[Illustration: Fig. 51.--THE ORGANS OF RESPIRATION.]

A. The _heart_.

B B. The _lungs_.

C. The _aorta_, and on either side of the aorta the vessels which convey the venous blood to the lungs to be _oxygenised_, and the corresponding vessels which return it to the heart, after it has undergone that operation. (For _aorta_ _see_ Fig. 50.)

D. The _trachea_, or large air passage, through which the air passes into the spongy texture of the lungs, when we breathe.

E E. _Arteries_ and _veins_, being the trunks of the vessels that supply the head, &c.

889. _Why does the chest expand when we breathe?_

Because the lungs consist of _millions of hollow tubes_, and _cells_, which, having been emptied by throwing off _carbonic acid gas_ and _nitrogen_, become compressed, and the atmospheric air flowing into these millions of spaces, and filling the lungs, just as water fills and swells a sponge, causes them to expand, and occupy greater room.

[Verse: "All the while my breath is in me, and the spirit of God is in my nostrils. My lips shall not speak wickedness, nor my tongue utter deceit."--JOB XXVII.]

890. _How does the blood communicate with the air in the lungs?_

Through the _sides of very minute vessels_, of which, perhaps, a _fine hair_ gives us the best conception. But these vessels are _twisted and wound round each other_ in such a curious manner, that they form _millions of cells_, and by being twisted and wound, a much _greater surface of air and blood_ are brought to act upon each other, than could otherwise be accomplished.

891. _Why does the blood which is thus formed, impart vitality to the parts to which it is sent?_

Because the blood is itself _vitalised_--is, in fact, _alive_, and capable of diffusing life and vitality to the organisation of which it forms a part.

This is a very wonderful fact, but no less true than wonderful, that dead matter which, but a little while ago, was being ground by the teeth, softened by the saliva, and solved by the gastric juice and bile, has now acquired _life_. Nobody can tell the precise stage or moment when it began to live. But somewhere between the stomach and the lungs, melted by the gastric juice, softened by the secretion of the pancreas, separated by the bile of the liver, macerated by the muscular fibres of the bowels, taken up by the absorbents, warmed by the heat of the body, and ærated in the lungs, it has by one, or by all of these processes combined, been changed from the dead to the living state, and now forms part of the _vital fluid of the system_.

CHAPTER XLIII.

892. _Why do we know that the blood has become endowed with vital powers?_

Because, in the course of its formation, it has not only undergone change of condition and colour; but, if examined now by the microscope, it will be found to consist of millions of minute cells, or discs, which float in a watery fluid. The paste produced by mastication consisted of a crude admixture of the atoms of food; the cream (_chyme_) formed from this in the stomach, presents to the microscope a heterogeneous mass of matter, exhibiting no appearance whatever of a new organic arrangement; the milk (_chyle_) which is formed in the intestines is found to contain a great number of very small molecules, which probably consist of some fatty matter; as the chyle progresses towards the _thoracic duct_ (Fig. 50), it appears to contain more of these, and slight indications present themselves of the approach towards a new organic condition.

But wherever _vitalisation begins_, no human power can say with confidence. Yet there can be no doubt that the blood is both _organised_ and _vitalised_, and that it consists of corpuscles, or little cells, enclosing matters essential to life.

[Verse: "But they that wait upon the Lord shall renew their strength; they shall mount up with wings as eagles; they shall run and not be weary; and they shall walk and not faint."--ISAIAH XL.]

893. _Why does the blood circulate?_

Because all the bones, muscles, blood-vessels, nerves, glands, cartilages, &c., of which the body is composed, are constantly undergoing a _change of substance_. It is a condition of their life, health, and strength, that they shall be "_renewed_," and the blood is the great source of the _materials_ by which the living temple is kept in repair.

894. _How is the body renewed by the blood?_

Every drop of blood is made up of a large number of corpuscles, each of which contains some of the elements essential to the wants of the system.

Let us, to simplify the subject, consider the blood vessels of the body to be so many _canals_, on the banks of which a number of inhabitants live, and require constant sustenance. The corpuscles of the blood are the _boats_ which are laden with that sustenance, and when the heart beats, it is a signal for them to start on their journey. Away they go through the arch of the great _aorta_, and some of the earliest branches which it sends off convey blood to the arms. We will now for a moment dismiss the word _artery_, and keep up the figure of a system of canals, with a number of towns upon their banks.

Well, away go a fleet of boats through the _aorta_ canal, until they reach a point which approaches Shoulder-town; some of the boats pass into the _axillary_ canal and Shoulder-town is supplied; the other boats proceed along the _humeral_ canal until they approach Elbow-town, when another division of the boats pass into other branch canals and supply the wants of the neighbourhood; the others have passed into the _ulnar_ canals and the _radial_ canals until they have approached Wrist-town and Hand-town, which are respectively supplied; and then the two canals have formed a junction across the palm and supplied Palm-town, where they have given off branches and boats to supply the four Finger-towns, and Thumb-town.

[Verse: "Though hand join in hand, the wicked shall not be unpunished; but the seed of the righteous shall be delivered."--PROVERBS XXI.]

[Illustration: Fig. 52.--ILLUSTRATION OF THE SYSTEM OF CANALS THAT SUPPLY THE FORE-ARM WITH BLOOD.]

Between A and B the _brachial canal_, which gives off branches to supply Elbow-town, &c., and then divides into two main courses, diverging to the opposite sides of the arm, and sending a smaller canal down the centre.

D D. The point where the _ulnar canal_ and the _radial canal_, after having passed and supplied Wrist-town, form a junction, running through Palm-town, and in their course giving off branches to supply the four Finger-towns and Thumb-town.

_For further explanations of the engraving, see 57._

895. _How does the blood return to the lungs, after it has reached the extremities?_

The _veins_ constitute a system of vessels corresponding to the arteries. We may say that the arteries form _the down canal_, and the veins _the up canal_. The arteries, commencing in the great trunk of the _aorta_, branch off into large and then into smaller tubes, until they form _capillary_ or hair-like vessels, penetrating everywhere.

[Verse: "As for man his days are as grass; as a flower of the field so he flourisheth."--PSALM CIII.]

_The capillary extremities of the arteries, unite with the capillary extremities of the veins,_ and the blood passes from the one set of vessels into the other. As the _arteries become smaller_ from the point where they receive the blood, so the _veins grow larger_; the venous capillaries, pour their contents into small vessels, and these again into larger ones, until the great _venous_ trunks are reached, and the blood is passed again into the heart as at first described. (Fig. 50.)

896. _Why do we see blue marks upon our arms and hands?_

Because large veins lie underneath the skin, through which the blood of the fingers and hand is _conveyed back to the heart_.

897. _Why are the veins more perceptible than the arteries?_

Because the arteries are buried _deeper in the flesh, for protection_. It would be _more dangerous to life to sever by accident an artery than a vein_. A person might bleed longer from a vein than from an artery, without endangering life; because the arteries supply the _life sustaining blood_. The Almighty, therefore, has buried the arteries for safety.

898. _Why when we prick the flesh with a needle does it bleed?_

Because the capillary arteries and veins are so fine, and are so thickly distributed all over the body, that not even the point of a needle can enter the flesh without penetrating the coats of several of these small vessels.

[Verse: "Let every thing that hath breath praise the Lord. Praise ye the Lord."--PSALM CL.]

899. _What occurs during the circulation of the blood?_

Not only do the various parts to which the boats are sent take from them whatever they require, but _the boats collect all those matters for which those parts have no further use_. The bones, the nerves, the muscles, &c., all renew themselves as the boats pass along; and all give something to the boats to bring back. One of the chief exchanges is that of _oxygen_ for _carbon_, by which a gentle _heat_ is diffused throughout the system. It is for this purpose that _fresh air_ is so _constantly necessary_.

But other exchanges take place. The blood, in addition to oxygen and carbon, contains _hydrogen_ and _nitrogen_. But it contains its four elements in _various forms of combination_, producing the following _materials_ for the use of the body: of 1,000 parts of blood, _about_ 779 are _water_; 141 are _red globules_; 69 are _albumen_; 3 are _fibrin_; 2 are _fatty matter_; 6 are various _salts_.

Albumen and fibrin are a kind of flesh imperfectly formed, and probably are chiefly used in repairing the muscles. The red corpuscles contain the oxygen which goes to combine with the superabundant carbon, and develop heat; the fatty matters probably repair the fatty tissues, and glands that are of a fatty nature; and the various salts contribute to the bones, and to the chemical properties of those secretions which are formed by the glands, &c., while the great proportion of water is employed in cleansing, softening, and cooling the whole, or the living edifice, and it is the medium through which all the nutrition of the body is distributed.

900. _Why do we feel the pulse beat?_

Because every time that the heart contracts it send a fresh supply of blood to the blood-vessels, and the motion thus imparted creates _a general pulsation throughout the system_: but it is more distinctly perceived at the pulse, because there a rather _large artery lies near to the surface_.

[Verse: "Thy hands have made me and fashioned me: give me understanding, that I may learn thy commandments."--PSALM CXIX.]

901. _What becomes of the matter collected by the blood in the course of its circulation?_

We have already explained that carbon is thrown off from the lungs in the form of carbonic acid gas. But there are many other matters to be separated from the venous blood, and its purification is assisted by the action of the liver, which is supplied with a large vein, called the _portal vein_, which conveys into the substance of the liver, a large proportion of the venous blood, from which that organ draws off those matters which form the bile, and other matters which are transmitted with the bile to the bowels. The _liver_ and the _lungs_, therefore, are the great purifiers of the venous blood. But there are also smaller organs that assist in the same work.

[Illustration: Fig. 53.--SHOWING THE DISTRIBUTION OF BLOOD THROUGH BRANCHES OF THE AORTA.]

A. The _aorta_.

B. Branches given off for the _aorta_ to supply one portion of the intestines.

C. Branches given off by the aorta to supply other portions of the intestines. A complete communication may be traced between these vessels from the origin of one to that of the other.

D. The _pancreas_, or sweetbread, a large gland that forms the pancreatic juice, which it pours in through the duct. (_See_ Fig. 50.)

E E E. The _large intestines_, forming the termination of the alimentary canal.

CHAPTER XLIV.

902. _Why when we cut our flesh does it heal?_

Because the blood coagulates over the cut, and throws out a kind of _lymph_, which forms an incipient flesh, and excludes the air while the blood-vessels are engaged in _repairing the part_.

[Verse: "And God said, Let us make man in our own image, after our likeness; and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing."--GEN. I.]

903. _Why, since all the substance of the body undergoes change, do we preserve the same features throughout our lives?_

Because our substance changes in the _minutest atoms_; and each separate atom has a life of itself, the maintenance of which preserves the _unity and permanence of the whole_.

904. _Why do moles upon the skin continue permanent, while bruises and wounds disappear?_

Because moles are themselves _organised formations_, and repair themselves just as any other part of the body does. But bruises and wounds are the result of _accidental disturbances_, which in course of time become removed.

905. _Why do the marks of deep cuts sometimes remain?_

If the cut is so deep and serious as to destroy the _system of vessels_ which supply and repair the part, then it is evident that they cannot work so perfectly as when in their sound condition. Their functions are, therefore, interfered with, and instead of having flesh uniform with the other parts of the system; there results a _scar_, or a wound _imperfectly repaired_.

906. _Why when we hold our hands against a candle-light do we perceive a beautiful crimson colour?_

Because the fluids and vessels of the body are in some degree transparent, and the thin textures of the sides of the fingers allows the light to pass, and shows the beautiful crimson colour of the blood.

If the web of a frog's foot be brought in the field of a good microscope, and set against a strong light, the blood may be seen in circulation, with the most wonderful effect. Each vessel, and every globule of blood, can be seen most distinctly, and the junction of the arteries and veins can be clearly traced. The little boats of nutrition may be seen chasing each other in rapid succession, and when the animal exerts itself to escape, the flow of the blood increases; and not unfrequently, under these circumstances of agitation, have we seen two or three blood discs struggling together to enter a vessel that was too small for them. Again and again they have endeavoured to find a passage, until one of them happening to slip forward, got away, followed by the others!

[Verse: "Know ye that the Lord he is God: it is he that hath made us, and not we ourselves: we are his people, and the sheep of his pasture."--PSALM C.]

907. _Why does the flesh underneath the nails look red?_

Because the transparent texture of the nails enables us to see the colour of the _vascular structure_ that lies underneath the skin.

_Vascular_.--Full of vessels. In this instance, full of capillary blood-vessels.

908. _Why have we nails at our fingers' ends?_

Because they give _firmness to the touch_, and enable us to apply the extremities of the fingers to many useful purposes for which they would otherwise be unfitted. They enable us to _press the tips of the fingers_, where the highest degree of sensitiveness prevails, so as to bring _the largest amount of nervous perception_ into the sense of touch.

909. _Why do white spots occur upon the nails?_

Because the vascular surface underneath is attached to the horny texture of the nail; but by knocks and other causes, the nail sometimes _separates in small patches from the membrane_ below, and becomes _dry and opaque_.

910. _Why is there a circular line of whitish colour at the root of the nail?_

Because there the nail is _newly formed_ by the vascular substance out of which it grows, and has not yet assumed its proper horny and transparent nature.

911. _Why is the eyeball white?_

Because the blood-vessels that supply its surface are so very fine that they do not admit the _red corpuscles_ of the blood.

912. _Why does the eyeball sometimes become blood-shot?_

Because, under exciting causes of inflammation, the _blood-vessels become distended_, and the red corpuscles enter, producing a net-work of red blood-vessels across the white surface of the eye.

913. _Why are the lips red?_

Because the lips are formed of the _mucous membrane_ that lines the body internally, and covers the surface of most of the internal parts. This membrane contains a great number of minute red vessels, which give softness and moisture to the surface. A very beautiful illustration of the softness, moisture, and delicate colour of the mucous membrane is afforded by turning up and examining the under surface of the upper eyelid.

[Verse: "Hast thou not known, hast thou not heard, that the everlasting God, the Lord, the Creator of the ends of the earth, fainteth not, neither is weary? there is no searching of his understanding"--ISAIAH XL.]

914. _Why do delicate persons look pale and languid?_

Because, generally from the want of exercise and fresh air, their blood is deficient of the healthy proportion of _red corpuscles_.

915. _Why does exercise and fresh air impart to healthy persons a red and fresh appearance?_

Because the redness of the blood is due to the _amount of oxygen_ which it contains, and air and exercise _oxygenise_ the blood, and diffuse it throughout the system.

916. _How is the blood propelled through the arteries?_

By the very powerful contraction (and alternate dilation) of the thick _muscles of the heart_, assisted also by the _muscular cords of the blood-vessels_ themselves, and in many instances by the _compression of the muscles_ in which the arteries lie embedded.

917. _Why are the capillary arteries capable of receiving the great quantity of blood sent out through the larger vessels?_

Because the capillary vessels are _so numerous_, that though they are infinitely smaller, they are capable of receiving in their minute tubes _the whole of the quantity of blood_ transmitted to them through the larger vessels.

918. _Why, when we sit with our legs crossed, do we see the foot that is raised move at regular intervals?_

Because the pressure upon the muscles of the leg retards the progress of the blood until it forces _itself through the compressed vessels_, and thereby imparts a pulsation which moves the leg and foot.

919. _Why are capillary blood-vessels found in every part of the system?_

Because it is _through these small vessels alone_ that the substances of the body are renewed and changed. Even the larger blood-vessels _do not sustain themselves upon the blood which they contain_, but receive into their coats numerous capillary vessels by which they are nourished.

[Verse: "All my bones shall say, Lord, who is like unto thee, which deliverest the poor from him that is too strong for him, yea, the poor and the needy from him that spoileth him?"--PSALM XXXV.]

920. _How much blood does the human body contain?_

From _twenty-five_ to _thirty-five_ pounds. (_See_ 623.)

921. _How does the blood ascend in the veins, in opposition to gravitation?_

In addition to the muscular coats of the veins, and the influence of muscular action upon them, there are in the veins numerous semi-circular valves, which are not found in the arteries. These valves extend from the sides of the veins in such a manner that they allow the free passage of the blood upwards, but a backward motion of the blood would expand the cup-like valves and stop the passage; so that the blood can only move in one direction, and that _towards the heart_.

922. _How frequently does the total amount of blood circulate through the system?_

The blood circulates once through the body in about _two minutes_. If, therefore, we estimate the amount of blood at twenty-four pounds, it follows that no less than _twelve pounds of blood pass through the heart every minute_; and it is estimated that if the blood moved with equal force in a straight line it would pass through _one hundred and fifty feet in a minute_.

CHAPTER XLV.

923. _How many bones are there in the human body?_

There are _two hundred and forty-six_, and they are apportioned to the various parts of the body in the following numbers:--

Head 8 Ears 6 Face 14 Teeth 32 Back-bone and its base 26 Chest, &c. 26 Arms and Hands 64 Legs and Feet 62 Small moveable bones 8

[Verse: "Our bones are scattered at the grave's mouth, as when one cutteth and cleaveth wood upon the earth."--PSALM CXLI.]

924. _Of what substances are the bones composed?_

One hundred parts of bone consist of

Cartilage 32·17 parts Blood-vessels 1·13 " Carbonate of lime 11·30 " Phosphate of lime 51·04 " Fluate of lime 2·00 " Phosphate of Magnesia 1·16 " Soda, chloride of sodium 1·20 " ------ 100·00

925. _What are the uses of the bones?_

They _protect_ soft and delicate organs; they form a framework to which the organs are attached, and by which they are _kept in their places_; and they supply a _mechanism_, by which the _motions of the body are produced_, in combination with the muscles.

926. _Why is the brain placed within the skull?_

Because that delicate and vital organ, being the _centre and the root of the nervous system_, requires a position of the _greatest safety_.

927. _Why are the bones that constitute the vertebræ (back-bone) hollowed out, so as to form a continuous groove?_

Because through that groove the _spinal cord_ passes out from the brain. Being in the centre of that column of bones, the spinal cord receives from them a similar protection _to that which the brain obtains from the skull_.

928. _Why is the head set upon the neck?_

Because in that position it obtains the _freest motion_, can turn in _any direction_, and is placed relatively to the other parts of the body, in that situation where it acquires _the greatest possible advantage_.

929. _Why are the eyes placed in the sockets of the skull?_

Because the bones of the skull _afford protection_ to the delicate and complicated structure of the eyes, and supply points of attachment, and grooves, by which the muscles are enabled to _turn the eyes freely_, and thereby _extend the field of vision_.

[Verse: "Thus saith the Lord God unto these bones, Behold I will cause breath to enter into you, and ye shall live:"]

930. _Why are the bones of the skull arched?_

Because in that form they acquire _greater strength_, and hence the utmost degree of safety is combined with extreme _lightness of material_.

[Illustration: Fig. 54.--VIEW OF THE BONES OF THE THORAX, OR CHEST, SHOWING THE PROTECTION AFFORDED TO THE ORGANS OF CIRCULATION AND RESPIRATION.]

A. The sternum, or breast-bone.

B B. The _ribs_, which rise a little from behind, and fall as they come forward, by which they acquire a greater flexibility.

C C. The _cartilaginous points_ of the short ribs, by which their expansive and compressive powers are much increased.

D E. Part of the vertebral column, or back-bone.

931. _Why are the bones of the skull divided by sutures (seams), with points which fit into each other like small teeth?_

Because, by that arrangement, _concussions of the skull_, which might be fatal to the brain, are _deadened_, and injuries from accident _greatly modified_.

[Verse: "And I will lay the sinews upon you, and will bring up flesh upon you, and cover you with skin, and put breath in you, and ye shall live; and ye shall know that I am the Lord."--EZEKIEL XXXVII.]

932. _Why are the heart, lungs, &c., placed within the chest?_

Because the functions of those organs require _considerable space_, while their importance in the system of life, renders it essential that they should be _securely protected_ from the probabilities of accident.

933. _Why are the heart and lungs enclosed for protection in a series of ribs, and not in a close case, like the brain?_

Because, by the inflation and contraction of the lungs, their _capacity is constantly changing_. When man takes a moderate inspiration, he inhales about _thirty cubic inches of air_, and the lungs increase in size _one-eighteenth of their whole capacity_. Consequently, were they enclosed in a frame of _fixed dimensions_, it must needs be, to that extent at least, larger than is necessary, when the frame is made to dilate and contract with the capacity of the lungs.

So perfect is the Almighty contrivance, that not only are the ribs made to _protect_ the lungs, but, by their elasticity, and the contractions and dilations of the muscles which lie between them, they _assist the lungs in their labours_, and work with them in perfect harmony.

934. _Why are the bones of the arms, legs, &c., made hollow?_

Because _lightness_ is thereby combined with _strength_. There is a provision by which, in the extremities of bones, where an enlarged surface is required, _lightness_ is still combined with the necessary degree of strength.

The bones are made up of a _cellular formation_; and this generally occurs in parts which are much called into action, in the various movements of the body.

[Illustration: Fig. 55.--SECTION OF THE KNEE JOINT, SHOWING THE CELLULAR STRUCTURE OF BONE, BY WHICH LIGHTNESS AND STRENGTH ARE OBTAINED.]

A. Lower part of the bone of the thigh.

B. Head of the bone of the leg.

C. The knee cap, showing its relation to the other bones, and the manner in which it is enclosed by the tendons seen at Fig. 58.

D. A pad of fat, lessening the friction of the bones, and modifying the shocks produced by jumping, &c.

[Verse: "Again he said unto me, Prophesy upon these bones, and say unto them, O ye dry bones, hear the word of the Lord."--EZEKIEL XXXVII.]

935. _Why are the bones of the arms and legs formed in long shafts?_

Because a considerable _leverage_ is gained, by which the advantages of _quickness of motion_, and _increase of mechanical power_, are secured.

936. _Why are the bones of the hands and feet numerous and small?_

Because the motions of the hands and feet are very _varied and complicated_. There are no less than _twenty-eight bones_ in one hand and wrist; and about _as many_ in a foot and ankle. To these are fastened a great number of _ligaments and muscles_, by which their varied compound movements are controlled. But for the complexity of the mechanism of our hands and feet, our motions would be extremely awkward, and many of the valuable mechanical inventions which now benefit mankind, could never have been introduced. The bones of the hands and feet are in number equal to _one-half of the whole of the bones of the body_.

CHAPTER XLVI.

937. _What are ligaments?_

Ligaments consist of bands and cords of a _tough_, _fibrous_, _and smooth substance_, by which the bones are bound together and held in their places, allowing them freedom to move, and supplying smooth surfaces over which they glide.

938. _Why are the joints bound with ligaments?_

Because the bones would otherwise be constantly liable to _slip from their places_.

[Verse: "That which is born of the flesh is flesh; and that which is born of the Spirit is spirit."--JOHN III.]

939. _What are tendons?_

Tendons are _long cords_, of a substance similar in its nature to _cartilage_, by which _the muscles are attached to the bones_.

[Illustration: Fig. 56.--SHOWING A BALL AND SOCKET JOINT, AND THE MANNER IN WHICH LIGAMENTS ARE EMPLOYED TO HOLD BONES IN THEIR POSITIONS.]

A. The _ball_, or _head_ of the thigh bone.

B. The _socket_, showing the ligament in the socket, which holds the head of the bone in its place, but allows it free motion.

C. _Ligaments_ tied from bone to bone, giving firmness to the parts.

940. _Why are tendons used to attach the muscles to the bones?_

Because, by this arrangement, the large muscles by which the extremities are moved, _may be placed at some distance_ from the bones upon which they act, and thus the extremities, instead of being large and clumsy, are _small_ and _neat_.

941. _How many muscles are there in the human body?_

There are about _four hundred and forty-six muscles_ that have been dissected and described, and the actions of which are perfectly understood. But there is probably a much larger number of muscles, and of compound actions of muscles, than the skill of man has been able to recognise.

[Verse: "All flesh is not the same flesh: but there is one kind of flesh of men, another flesh of beasts, another of fishes, and another of birds."--CORINTHIANS XVI.]

[Illustration: Fig. 57.--ILLUSTRATION OF THE RELATION OF MUSCLES, TENDONS, AND BONES.]

942. _What is the constitution of a muscle?_

Every muscle is made up of a number of _parallel fleshy fibres_, or threads, which are bound together by a smooth and soft tissue, forming a sheath or case to the muscle, and enabling it to _glide freely_ over the surfaces upon which it moves.

A. Lower extremity of the muscle which draws the fore-arm towards the upper-arm, bends the elbow, raises the hand to the head, and is powerfully exerted in pulling, lifting, &c.

C. A muscle which gives off four long _tendons_, which pass under the _ligaments_ of the wrist, one to each finger, and by which the fingers are bent upon the palm of the hand, as in grasping, &c.

F. _Tendon_ of a muscle which draws the little finger and the thumb towards each other.

The _ligaments_ may be seen enfolding the finger-joints, and also crossing the wrist, underneath the _tendons_.

The muscles are compressed into _tendinous cords_ at their ends, by which they are _united to the bones_.

They are arranged in _pairs_, having reciprocal actions--each muscle having _a companion muscle_ by which the part which it moves is restored to its original position, when the influence of the first muscle is withdrawn, and the stimulus given to bring back the part.

943. _Why can we raise our fingers?_

Because muscles which lie _on the fore-arm_, and have their tendons fastened at the ends of the fingers, _contract_, and by becoming shorter, _draw the fingers upward_, and towards the arm.

[Verse: "Thou hast clothed me with skin and flesh, and hast fenced me with bones and sinews."--JOB XI.]

944. _Why can we throw back the fingers after they have been raised?_

Because the muscles at the back of the arm, _whose tendons are attached to the back of the fingers_, contract and restore them to their former position.

945. _What degree of strength do the muscles possess?_

The degree of strength of a muscle depends upon the _healthy condition_ of the muscle, the _amount of stimulus_ which it receives at the time of exertion, and the manner in which _its powers are applied_.

The great muscle of the calf of the leg has been found, when removed from a dead body, to be capable of sustaining a weight equal to _seven times the weight of the entire body_.

_But the contractile power of the living muscles_ is very great: the thigh bone has frequently been broken by muscular contractions in fits of epilepsy. And in cases where there has been a dislocation of the thigh, the head of the thigh-bone being thrown out of its socket, (Fig. 56) it has been found necessary to employ strong ropes, attached to a wheel turned by several hands, in order to _overcome the contraction of the excited muscles, and to enable the operator to restore the bone to its place_.

946. _What is the stimulus which sets the muscles in action?_

The muscles are excited to action by _the nerves_, which they receive from the _spinal cord_.

947. _Why does it require the influence of the will to set the arms in motion?_

Because the muscles which form their mechanism are _voluntary_ muscles--that is, they are subject to the _will of man_, and influenced by impulses directed to them through the nervous system _by the mind_, which is the governing power.

[Verse: "And he took him by the right hand, and lifted him up; and immediately his feet and ankle bones received strength."--ACTS III.]

948. _Why does the heart beat without any effort of the will?_

Because the muscles of the heart are _involuntary muscles_--that is, they are _independent of the will_, and receive _a continuous nervous stimulus_ which is not _under the controul of the mind._

[Illustration: Fig. 58.--MUSCLES AND VESSELS OF THE LEG AND FOOT.]

A. A large _ligament_, which covers the knee pan, or moveable bone of the knee, by which the ends of the bones of the thigh and leg are kept from slipping over each other.

B. A muscle which passes underneath the cartilages of the ankle, and gives off four _tendons_, which are distributed to the toes, and by which they are extended in elongating the foot, walking, &c.

C. Part of the muscle which forms the fleshy bulb of the calf of the leg, and which terminates in the large _tendon_ attached to the heel, called the _tendon of Achilles._

D. One of the ligaments which bind the tendons and the bones of the ankle.

E. _Arteries_ proceeding from the large vessel descending the leg, by which the toes are supplied.

949. _Why are the muscles of the arms, &c., made subject to the controul of the will?_

Because, as they supply the mechanism through which we adapt ourselves to our varying wants and circumstances, it was necessary that they should be placed under the controul of the mental power, and be moved _only in accordance with man's necessities._

[Verse: "If thou sayest, Behold, we knew it not; doth not he that pondereth the heart consider it? and he that keepeth thy soul, doth not he know it? and shall not he render to every man according to his works?"--PROVERBS XXIV.]

950. _Why are the motions of the heart, &c., made independent of the will?_

Because, as the necessity for the heart's motion is _fixed and unalterable_, the constant motion of the heart could be best secured by giving it a _fixed nervous influence_, by which it might be unfailingly prompted to fulfil its functions.

If the movements of man's heart were _subject to his will_, he would be constantly required to regard the operations of that organ; and so large an amount of mental care and physical exertion would have to be employed in that direction, that man's sole work would be to keep himself alive. Hence we see the goodness of the Creator in _giving_ life to man, and in _keeping the vital impulses under his divine care_.

CHAPTER XLVII.

951. _What are nerves?_

The nerves are branches of the _brain_ and the _spinal cord_; they are distributed in great numbers to all the active and sensitive parts of the body.

952. _What is the spinal cord?_

The spinal cord is a long and large cord of nervous matter, which extends from the brain through a continuous tube formed by corresponding hollows in the bones of the back. It serves as a nervous trunk _for the distribution of nerves_, just as the aorta distributes branches of blood-vessels.

953. _Why is the spinal cord placed in the grooves formed by the back-bone?_

Being a very vital part of the system, and from the delicacy of its structure liable to injuries, it is set in the back-bone for _protection_; and so great is its security that it is only by force of an unusual kind that it can be injured.

[Verse: "A sound heart is the life of the flesh: but envy is the rottenness of the bones."--PROVERBS XIV.]

954. _How can branches proceed from it, if it is so securely encased in bone?_

Because in the bones, on each side of the spinal cord, there are _smaller grooves_ for the transmission of the nervous branches.

955. _Of what does the nervous system consist?_

Of the _brain_, the _spinal cord_, and the branches which are called _nerves_.

[Illustration: Fig. 59.--SHOWING THE DISTRIBUTION OF NERVES AND VEINS, AND ILLUSTRATING THE MANNER IN WHICH THEY PASS THROUGH THE FLESH TO REACH THE PARTS TO WHICH THEIR FUNCTIONS BELONG.]

A. B. _Veins_ of the fore-arm.

B. Canal formed in the muscle, through which a _trunk-vein_ emerges.

C. Canal formed in the muscle, through which a large _nerve_ emerges.

D. Canal through which a _vein_ enters to communicate with the deep muscles of the arm.

956. _What is the constitution of a nerve?_

It consists of a thin membrane, or sheath, surrounding _a greyish oily matter_, which forms the nervous marrow. In the centre of this marrow is usually found _a small fibre_, which is supposed to be the essential part of the nerve; and most nerves consist of a number of these sheaths enclosing fibres running in parallel directions.

957. _What is the nervous fluid?_

The term _nervous fluid_ is used to express our ideas of the mode by which the brain and spinal cord influence the remote parts: just as we say the _electric fluid_, without knowing that such a fluid exists. It is the most convenient form of expression.

958. _How many classes of nerves are there?_

There are:--

1. The nerves of _motion_.

2. The nerves of _sensation_.

3. The nerves of _special sense_.

4. The nerves of _sympathy_.

[Verse: "Having many things to write unto you, I would not write with paper and ink; but I trust to come unto you, and speak face to face, that our joy may be full."--II JOHN.]

959. _What are the nerves of motion?_

The _nerves of motion_ are those which, in obedience to the will, _stimulate the muscles to act_, and apportion the amount of stimulation they convey to the degree of exertion required.

[Illustration: Fig. 60.--MUSCLES OF THE HEAD AND FACE, WITH NERVES DISTRIBUTED THERETO.]

A A A. The _facial nerve_ emerging from underneath the ear, and distributing branches to the cheeks, temple, forehead, &c. This nerve excites the muscles of the face, and is chiefly instrumental in producing the expressions of the countenance under the changing emotions of the mind.

B B B. _Muscles_ by which various motions are imparted to the head, face, mouth, &c., under the _stimulus of the nerves_.

960. _What are the nerves of sensation?_

The _nerves of sensation_ are those which _impart a consciousness to the brain_ that its commands to the nerves of motion have been obeyed, and how far they have been fulfilled.

[Verse: "Oh that men would praise the Lord for his goodness, and for his wonderful works to the children of men."--PSALM CVII.]

Let us perform a simple experiment, which will more clearly illustrate the phenomena of _motion_ and of _sensation_, which we are now describing, than a great deal of writing upon the subject. You hold in your hand this book: close it, and set it upon the table; lay your hands passively upon your lap, and then _will_ your hand, to take up the book, which is the same as to say, _command_ your hand to take up the book. What occurs? The hand, immediately obeying your desire, stretches forward to the book, and takes hold of it. How do you know that you have hold of it? You _see_ that you have: but were your eyes closed, you would be equally aware that the hand had reached the book, and fulfilled your wishes. It is by the nerves of _sensation_ that you are made aware that the hand has fulfilled your instructions.

Consider what took place in the simple action. In the first instance, a desire arose in your mind to take up the book. The _brain_ is the organ of the mind; and having branches either proceeding from itself, or from the spinal cord, to every part of the body--branches that traverse like telegraphic wires throughout every part of the system,--it transmitted instructions along the nerves that proceed to the muscles of the arm and hand, directing them to take up the book. This was done instantly; and as soon as it was done you became conscious that your will had been obeyed--because _the nerves sent back a sensation to the brain acquainting it that the book had been taken up_, and that at the moment of the dispatch it was in the firm hold of the hand.

In all the varied motions of the body this double action of the nerves takes place. It is obvious that without an _outward_ impulse from the brain, upon which the desire of the mind first made an impression, no motion of the muscles of the arm and the hand could have taken place; and it is also obvious that without an _inward_ impulse from the nerves to the brain you would not have known that the muscles had fulfilled your instructions. The hand might have dropped by the side of the book, or have gone too far, or not far enough, and you would not have been aware of the result, but for an inward communication through the nerves.

We are not now speaking of the nerves which endow us with the sense of _feeling_, because they are regarded as separate and distinct from those nerves that produce in us consciousness of muscular response. When we walk, rise, or sit, we are made conscious, without any special feeling being exerted, that the muscles have placed the limb, or the body, in the desired position, that it is set down safely and firmly, and that we may repose upon it securely without further attention. We refer the impressions made by the book upon the nerves of the hand, and which enable us to tell whether it feels hot or cold, whether its surface is rough or smooth, and so on, to the special sense of _feeling_. The consciousness of muscular action is a separate and distinct function; and it is generally believed that the same nerves that convey the command of the will outward, bringing back the intimation that the will has been obeyed, but that _different fibres_ of the nerves convey the _outward_ and the _inward_ impulses. A single nerve may therefore be likened to a _double wire_ connected with the electric telegraph: one transmitting despatches in one direction, and the other in the opposite direction.

961. _What are the nerves of special sense?_

The nerves of special sense are those through which we _hear_, _see_, _feel_, _smell_, and _taste_.

[Verse: "For the Lord seeth not as man seeth; for man looketh on the outward appearance, but the Lord looketh on the heart."--SAMUEL XVI.]

962. _What are the nerves of sympathy?_

The nerves of sympathy, or the system of _sympathetic nerves_, are those which are distributed to the _internal organs_, and which are independent of the will. They regulate the motions of the heart, the lungs, the stomach, &c., and stimulate the organs of secretion, so that those organs _work in harmony with each other_.

As the internal organs are all more or less dependent upon each other, and unite their functions for similar ends, it is obvious that there should prevail among them a _mutual consciousness_ of their state. Otherwise, when the stomach had formed chyme, the liver might have no bile ready to fulfil its office; the absorbents might be in a state of rest at the moment when nutrition was set before them; and the heart might beat slowly, while the lungs were in active exertion to obtain additional blood to support an active exercise. The sympathetic system of nerves therefore _regulates and harmonises these internal functions_.

CHAPTER XLVIII.

963. _Why do we see objects?_

Because the light which is reflected from them enters our eyes and produces images of their forms upon a membrane of nerves called the _retina_, just as images are produced upon a mirror.

964. _Why does this enable us to see?_

Because the membrane which receives the images of objects is connected with the _optic nerve_ which transmits to the brain impressions made by the reflections of light, just as other nerves convey the effects of feeling, hearing, tasting, &c.

965. _Why are we enabled to move our eyes?_

Because various muscles are so placed in relation to the eyeball, that their contraction draws the eye in the direction required. We are thus enabled to adjust the direction of the eye to the position of the objects we desire to see, in other words to _set the mirror in such a position that it will receive the reflection_. (See 517.)

[Verse: "Truly the light is sweet, and a pleasant thing it is for the eyes to behold the sun."--ECCLESIASTES XI.]

966. _Why are we enabled to see large objects upon so small a surface?_

Because the lenses and humours of the eye _collect the rays of light_ coming from every direction, and, _bringing them into a focus_, transmit them to the retina, where each ray impresses upon the nervous surface the qualities it received from the object which reflected it.

[Illustration: Fig. 61.--THE EYEBALL AND ITS MUSCLES.]

A. Portion of _bone_ through which the optic nerve passes in its communication between the brain and the eye.

B. The _optic nerve_, from before which an _external muscle_ has been cut away, leaving its two attachments.

C. The _globe_ of the eye.

D. The muscle which turns the eye _outward_, and which is counteracted by a muscle on the other side.

E. The muscle which passes through a loop, or staple of cartilage I, and _turns the eye obliquely_. It is counteracted by a muscle situated underneath.

F. The muscle situated underneath, which turns _the eyeball upwards_, and is counteracted by

G. The muscle which _turns the eyeball downwards_.

H. The muscle attached to a bone which _turns the eyeball upwards_.

I. The _cartilaginous loop_ through which a muscle passes.

J. The front chamber of the eye filled with a clear fluid.

K. Fragment of the bone by which one of the muscles is fastened.

967. _Why do some persons squint?_

Because it sometimes happens that a muscle of the eye _acts too powerfully_ for its companion muscle, and draws the eye too much on one side.

968. _Why does the pupil of the eye look black?_

Because the pupil is an _opening_ through which the rays of light pass into the chamber of the eye. There is, therefore, nothing in the pupil, of the eye to reflect light.

[Verse: "Keep me as the apple of thine eye; hide me under the shadow of thy wings."--PSALM XVII.]

969. _Why is the pupil of the eye larger sometimes than at others?_

Because the _iris_, a ring of extremely fine muscles which surround the pupil, contracts when too much light falls upon the retina, and dilates when the light is feeble. It therefore enlarges or diminishes the size of the pupil to _regulate the admission of light_.

[Illustration: Fig. 62.--SECTION OF THE EYE SEEN FROM BEHIND.]

A. The _pupil_ of the eye through which the light enters.

B. The _iris_, which dilates or contracts, and thereby increases or lessens the size of the _pupil_.

C. The three coats of the eye, called the _sclerotic_, _choroid_, and _retina_.

D. The _ciliary processes_, or hair-like muscles, which have a slight vibratory motion which they impart to the fluids of the eye.

E. The dark coat of the _choroid_, the coat forming the _retina_ removed.

970. _Why have we two eyes?_

Because the field of vision is thereby _much extended_; the _intensity_ of sight is also increased, the impressions upon the brain being clearer and better defined, just as in a _stereoscope_ the effect of vision is heightened by a double picture; the sense of sight being more _constantly_ exercised than any other sense during our waking moments, _one eye is frequently called upon to give rest to the other_; and the important faculty of vision, being endangered by the necessary exposure of some parts of the eye, and the equally necessary delicacy of an organ formed to receive impressions from so ethereal an element as light, is rendered the more secure to us, since though one eye may become enfeebled, diseased, or wholly lost, _the other eye will retain the blessing of sight_.

[Verse: "The eyes of the Lord are upon the righteous, and his ears are open unto their cry."--PSALM XXXIV.]

971. _Why, having two eyes, and each eye receiving a reflection upon its retina, does the brain experience only one impression of an object?_

Because, besides those optical laws which bring upon the two retinas the exactly corresponding images of the same objects, the optic nerves _meet_ before they reach the brain, _and blend the impulses which they convey_.

972. _Why are the eyes provided with eyelids?_

Because the eyes require to be _defended_ from floating particles in the air, and to be kept _moist and clean_. The eyelids form the shutters of the eye, defending it when waking, by closing upon its surface whenever danger is apprehended, moistening its surface when it becomes dry, and covering it securely during the hours of sleep.

973. _Why are the eyelids fringed with eyelashes?_

Because the eyelashes assist to modify the light, and to protect the eye, without actually closing the eye-ids. When the eyelids are partially closed, as in very sunny or dusty weather, the eyelashes cross each other, forming a kind of shady lattice-work, from the interspaces of which the eye looks out with advantage, and sees sufficiently for the guidance of the body.

974. _Why are we able to see at long or short distances?_

Because the _crystalline lens_ of the eye is a moveable body, and is pushed forward, or drawn back by fine muscular fibres, according to the distances of the objects upon which we look. By these means its _focus_ becomes adjusted.

975. _Why do we wink?_

Because, by the repeated action of winking, _the eye is kept moist and clean_, and the watery fluid secreted by little glands in the eyelids, and at the sides of the eye, is spread equally over the surface, instead of being allowed to accumulate. But the action of winking, or brightening the eye, is so instantaneous that it does not impede the sight.

[Verse: "And the eye cannot say unto the hand, I have no need of thee; nor again the head to the feet, I have no need of you."--CORINTH. XII.]

976. _Whence are the humours and secretions of the eye derived?_

From the blood, which flows abundantly to the eyes, and is circulated in capillary vessels that are spread out upon the membranous coats of the eye-balls.

[Illustration: Fig. 63.--SECTION OF THE EYE.]

A and B. The _sclerotic_, _choroid_, and _retina_, the three layers or coats which form the walls of the globe of the eye, and enclose its humours.

C C. The _iris_.

D. The front chamber of the eye, filled with watery humour.

E. The _pupil_, through which the rays of light pass to

F. The _crystalline lens_.

G G. The _vitreous humour_ enclosed in cells formed by the _hyaloid membrane_.

H. An _artery_ which supplies blood to the _crystalline lens_, and which passes through the centre of the _optic nerve_.

G. The _optic nerve_, showing the sheath in which the nerve is enclosed.

977. _Why do tears form in the eyes?_

Because, under the emotions of the mind, the circulation of blood in the brain, and in its nearest branches, becomes considerably quickened. The eyes receive a larger amount of blood, and the secretion of the lachrymal glands being increased, the fluid overflows, and tears are formed. The use of tears is probably _to keep the eyes cool during the excitement of the brain_. They are formed also during _laughing_, but less frequently.

[Verse: "If the whole body were an eye, where were hearing? if the whole were hearing, where were smelling?"--CORINTHIANS XII.]

978. _Why do we feel inconvenienced by sudden light?_

Because an excess of light enters the eye before the _iris_ has had _time to adjust the pupil_ to the amount of light to be received.

979. _Why if we look upon a very bright light, and then turn away, are we unable to see?_

Because the _iris_ has so reduced the pupil while we were looking at the bright light, that immediately upon turning to a darker object, _the pupil is too small_ to admit sufficient rays to enable us to see.

[Illustration: Fig. 64.--CAPILLARY BLOOD-VESSELS OF THE EYE.]

A A. Capillary veins distributed over the _sclerotic coat_.

B. One of the trunks of the _optic nerve_.

C. A _nerve_ communicating with the _ciliary processes_.

D. A _vein_ running parallel with the nerve to the _ciliary processes_.

E. Side view of the _iris_.

980. _Why do we see better after a short time?_

Because the _iris_ has relaxed and enlarged the pupil, therefore _we receive more rays of light_ from the comparatively dark object, and are enabled to see it more clearly.

981. _Why do cats, bats, owls, &c., see in the dark?_

Because their eyes are made highly sensitive to _small quantities of light_. It is also believed that there are certain properties of light which affect their eyes, but do not affect ours. In other words, that there are some rays which are luminous to them which are not luminous to us. Hence they find _light_ in what we call _darkness_.

[Verse: "He that hath ears to hear, let him hear."--MATTHEW XI.]

982. _Why does the pupil of a cat's eye appear nearly closed by day?_

Because the cat's eye is so sensitive to light that the iris _closes the pupil almost entirely_ to shut out the too powerful light.

CHAPTER XLIX.

983. _Why do we hear?_

Because the _tympanum_ of the ear _receives impressions from sounds_, and transmits those impressions to the brain in a similar manner to that in which the retina of the eye transmits the impressions made upon it by light.

984. _Why is one part of the ear spread out externally?_

The external ear is a _natural ear-trumpet_, and serves to collect the vibrations of sound, and to conduct them towards the internal ear.

985. _Why is the ear allowed to project, whilst the eye is carefully enclosed?_

Because the external ear, being formed of tough cartilaginous substance, and being very simple in its organisation, is but little liable to injury.

986. _Why do hairs grow across the entrance of the ears?_

Because they prevent the intrusion of insects, and of particles of dust, by which otherwise the faculty of hearing would be impaired.

The insect called the _earwig_ is popularly supposed to be so named from its tendency to get into the human ear, and cause pain and madness by penetrating to the brain. An earwig, however, is no more likely to get into the ear than any other insect whose habit it is to penetrate the corollas of flowers; and should an insect enter the ear, it could get no further than the _membrane of the tympanum_, which spreads all over the auditory passage, just as the parchment of a drum spreads over the entire circumference of that instrument. The fact is, that the wing of the insect, when spread, _resembles the external ear_ in shape. It is similar to the wing of the stag beetle (_see_ illustration), and this fancied resemblance of the wing of the insect to the ear of man may have given rise to the name of _ear-wing_, which became corrupted to _earwig_.

[Verse: "Doth not the ear try words? and the mouth taste his meat."--JOB XII.]

987. _Why is wax secreted at the entrance of the ear?_

Because, by the peculiar resinous property which it possesses, _it improves the sound-conducting power_ of the auditory canal through which it prevails.

[Illustration: Fig. 65.--THE STRUCTURE OF THE EAR.]

A A. Glands which secrete wax in the walls of the tube of the ear.

B. The membrane of the _tympanum_, or drum of the ear, formed in the shape of a funnel.

C C. Bones which act as a sort of sounding-board to the ear, giving strength to the vibrations.

D. The Eustachian tube, which opens into the root of the mouth, and which serves to preserve an equilibrium in the density of the air occupying the tubes of the ear.

E and F. The _labrynth_ of the ear, consisting of folds of membraneous tubes, filled with fluid, which serves to undulate with the vibrations of the _tympanum_, and thus gives clearness and precision to the sounds.

The _auditory nerves_ are distributed in the tubes above described (the _vestibule_ and the _cochlea_ E F), and the nerves receive their impressions from the undulations of the fluid.

988. _Why do we sometimes hear singing noises in the ear?_

Because the ear is liable to inflammation from various causes, and when the blood flows unduly through the vessels of the ear it _produces a slight sound_.

[Verse: "Apply thine heart unto instruction, and thine ears to the words of knowledge."--PROVERBS XXIII.]

989. _Why do people become deaf?_

Because the ear may be injured in various ways: the tympanum may be impaired, the fluid of the ear dried up, or the nerves be pressed upon by swellings in the surrounding parts. When, therefore, the _mechanism of hearing_ is impaired, the sense of hearing becomes weakened, or altogether lost.

990. _Why do persons accustomed to loud noises feel no inconvenience from them?_

Because the _sensitiveness_ of the nerves of the ear becomes deadened. They do not convey to the brain such intense impulses when they are frequently acted upon by loud sounds.

991. _Why do persons engaged in battle often lose their hearing?_

Because the vibrations caused by the sounds of artillery are so violent that they overpower the mechanism of the ear, and frequently _rupture the connection of the fine nervous filaments_ with the textures through which they spread.

The violent concussions of the air produced by volleys of cannon, or by loud peals of thunder, have an overpowering effect upon persons nervously constituted, and upon the organ of hearing, which is more especially affected. As persons have been struck blind by intense light, so others have been deafened by intense sounds. In 1697 a butcher's dog was killed by the noise of the firing to celebrate the proclamation of peace. Two troops of horse were dismounted, and drawn up in a line to fire volleys. At the moment of the first volley a large and courageous mastiff, belonging to a butcher, was lying asleep before the fire. At the noise of the first volley the dog started up, and ran into another room, where it hid itself behind a bed; on the firing of the second volley, it ran several times bout the room, trembling violently; and when the third volley was fired it ran around once or twice with great violence, and then dropped down dead, with blood flowing from its mouth and nose. Persons who are painfully affected by loud noises should put a little wool in their ears when such noises are occurring; they will thereby save themselves from temporary inconvenience, and probably preserve the sense of hearing from permanent injury.

992. _Why do we smell?_

Because minute particles of matter, diffused in the air, come in contact with the filaments of the _olfactory nerve_, which are spread out upon the walls of the nostrils, and those nerves transmit impressions to the brain, constituting what we call the _odour of substances_.

[Verse: "And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul."--GENESIS II.]

[Illustration: Fig. 66.--SHOWING THE DISTRIBUTION OF THE NERVOUS FILAMENTS UPON SENSITIVE MEMBRANES.]

A. The _olfactory nerve_, distributed in minute branches upon the membrane of the nostril.

B. The _bulb_ of the _olfactory_ nerve.

C. The _roots_ from which the olfactory nerve originates.

D E. _Nerves of the palate_, showing the manner in which they are passed through the bones of the roof of the mouth.

993. _Why do hairs grow across the passages of the nostrils?_

Because they form a _defence_ against the admission of dust and insects, which would otherwise frequently irritate the nervous structure of the nose.

994. _Why are the nostrils directed downwards?_

Because, as odours and effluvia _ascend_, the nose is directed towards them, and thereby receives the readiest intimation of those bodies floating in the air which may be pleasurable to the sense, or offensive to the smell, and injurious to life.

[Verse: "Can that which is unsavoury be eaten without salt? or is there any taste in the white of an egg?"--JOB VI.]

995. _Why is the nose placed over and near the mouth?_

Because, as one of the chief duties of that organ is to _exercise a watchfulness_ over the purity of the substances we eat and drink, it is placed in that position which enables it to discharge that duty with the greatest readiness.

CHAPTER L.

996. _Why do we taste?_

Because the tongue is endowed with _gustatory_ nerves, having the function of _taste_ as their _special sense_, just as the _optic_, the _auditory_, and the _olfactory_ nerves, have their special duties in the eyes, ears, and nose.

997. _Why do some substances taste sweet, others sour, others salt, &c.?_

It is believed that the impressions of taste arise from the various _forms of the atoms of matter_ presented to the nerves of the tongue.

998. _Why do we taste substances most satisfactorily after they have remained a little while in the mouth?_

Because the nerves of taste are most abundantly distributed to the under surface of the tongue; and when solid substances have been in the mouth a little while, they impregnate the saliva of the mouth with their particles _and come in contact in a fluid solution with the gustatory nerves_.

999. _Why if we put a nub of sugar to the tip of the tongue has it no taste?_

Because the gustatory nerves are _not distributed to that part of the tongue_.

[Verse: "Wine is a mocker, strong drink is raging; and whosoever is deceived thereby is not wise."--PROVERBS XX.]

1000. _Why, when we draw the tongue in, do we recognise the sweetness of the sugar?_

Because the dissolved particles of sugar are _brought in contact_ with the nerves of taste.

1001. _Through what nerves are we made sensible of the contact of sugar with the tip of the tongue?_

Through the nerves of _feeling_, which are abundantly distributed to the tongue to guide it in its controul over the mastication of food.

1002. _Why do connoisseurs of wines close their mouths and distend their chins for a few seconds, when tasting wines?_

Because they thereby bring the wine in contact with the under surface of the tongue, _in which the gustatory nerves chiefly reside_.

1003. _Why do they also pass the fumes of the wines through their nostrils?_

Because _flavour_, in its fullest sense, comprehends not only the _taste_, but the _odour_ of a substance; and, therefore, persons of experience attend to both requisites.

The various conditions of taste are defined to be:--

1. Where sensations of _touch_ are alone produced, as by glass, ice, pebbles, &c.

2. Where, in addition to being _felt_ upon the tongue, the substance excites sensation in the _olfactory nerves_, as by lead, tin, copper, &c.

3. Where, besides being _felt_, there are peculiar sensations of _taste_, expressive of the properties of bodies, as salt, sugar, tartaric acid, &c.

4. Where, besides being _felt_ and _tasted_, there is an _odour_ characteristic of the substance, and essential to the full development of its flavours, as in cloves, lemon-peel, caraway-seed, and aromatic substances generally.

1004. _Why do we feel?_

Because there are distributed to various parts of the body fine nervous filaments, which have for their special duty the transmission to the brain of impressions made upon them _by contact_ with substances.

[Verse: "The works of the Lord are great, sought out of all them that have pleasure therein."--PSALM CXI.]

1005. _In what parts of the body does the sense of touch more especially reside?_

In the points of the fingers and in the tongue. By laying a piece of paper upon a table, and upon the paper a piece of cloth, on the piece of cloth a bit of silk, and on the bit of silk a piece of leather, so that the edge of each would be exposed to the extent of half-an-inch, it would be possible by the touch to tell when the finger passed successively over the leather, silk, cloth, or paper, and arrived on the table.

Those impressions of touch must have been communicated, with their extremely nice distinctions, to the sensitive nerves that lie underneath the skin, and must have been transmitted all the way through the arm to the brain, although the touch itself was so light as scarcely to be appreciable with regard to the force applied.

A hair lying on the tongue will be plainly perceptible to the touch of the tongue; and the surface of a broken tooth will often cause the tongue great annoyance, by the acute perception it imparts of the roughness of its surface.

The toes are also highly sensitive, though their powers of touch are seldom fully developed. Persons who have lost their arms, however, have brought their feet to be almost as sensitive as fingers. Blind persons increase, by constant exercise, their powers of touch to such a degree that they are able to read freely by passing their fingers over embossed printing; and they have been known to distinguish _colours_ by differences in their grain, quite unappreciable by other persons.

1006. _Why is feeling impaired when the hands are cold?_

Because, as the blood flows slowly to the nerves, they are less capable of that perception of touch which is their _special sense_. The skin contracts upon the nervous filaments, and _impairs the contact_ between them and the bodies which they touch.

1007. _Why do the fingers prick and sting when they again become warm?_

Because, as the warmth expands the cuticle, and the blood begins to flow more freely through the vessels, _the nerves are made conscious of the movements of the blood_, and continue to be so until the circulation is equally restored to all the parts.

[Verse: "In the sweat of thy face shalt thou eat bread, till thou return to the ground; for out of it thou wast taken: for dust thou art, and unto dust shalt thou return."--GENESIS III.]

1008. _Why do persons whose legs and arms have been amputated fancy they feel the toes or fingers of the amputated limb?_

Because the nervous trunk which formerly conveyed impressions from those extremities remains in the part of the limb attached to the body. _The mind has been accustomed to refer the impulses received through that nervous trunk to the extremity where the sensations arose._ And now that the nerve has been cut, the painful sensation caused thereby is referred to the extremity which the nerve supplied, and the sufferers for a time appear to _continue to feel the part which they have lost_.

CHAPTER LI.

1009. _Why do we perspire?_

Because the skin is filled with very _minute pores_, which act as outlets for a portion of the water of the blood, that serves to _moisten and cool_ the surface of the body, and to carry away some of the matter _no longer needed in the system_.

1010. _How is the perspiration formed?_

By very small _glands_, which lie embedded in the skin. It is estimated that there are about 2,700,000 perspiratory glands distributed over the surface of the body, and that these glands find outlets for their secretion through no less than _seven millions of pores_.

1011. _What is insensible perspiration?_

Insensible perspiration is that _transmission of watery particles through the skin_ which is constantly going on, but which takes place so gently that it cannot be perceived. It is, however, very important in its results, as no less than _from twenty to thirty-three ounces of water may pass imperceptibly through the skin in twenty-four hours_.

1012. _What is sensible perspiration?_

Sensible perspiration is that moisture which exudes upon the skin _in drops large enough to be perceptible_, when the body is heated by exercise or other means.

[Verse: "And Elisha sent a message unto him, saying, Go and wash in Jordan seven times, and thy flesh shall come again to thee, and thou shalt be clean."--II KINGS V.]

1013. _Why does a sudden change from heat to cold bring on illness?_

Because the effect of cold _arrests the action of the vessels of the skin_, and suddenly throws upon the internal organs the excretory labour which the skin should have sustained.

1014. _Why does a chill upon the skin frequently produce inflammation of the lungs?_

Because the lungs and the skin together discharge the chief proportion of the watery fluid of the body. _When the skin's action is checked, the lungs have to throw off a much greater amount of fluid._ The lungs, therefore, become _over worked_, and inflammatory action sets in.

1015. _Why does cleanliness promote health?_

Because every atom of dirt which lodges upon the surface of the body serves to clog and check the working of those _minute pores_, by which much of the fluid of the body is changed and purified.

In the internal parts of the system, the Creator has made ample provision for cleanliness. Every organ is so constituted that it cleanses and lubricates itself. Every surface of the inner body is perfectly clean, and as soft as silk.

Nature leaves to man the care of those surfaces which are under his immediate observation and controul; and he who, from idleness, or indifference to nature's laws, is guilty of personal neglect, _opposes the evident intentions of the Creator_, and must sooner or later pay the penalty of disobedience.

1016. _Why does exercise promote health?_

Because it _assists all the functions upon which life depend_. It quickens the circulation, and thereby nourishes every part of the body, causing the bones to become firm, and the muscles to become full and healthy. It promotes breathing, by which oxygen is taken into the system, and carbon thrown off, and thereby it produces a higher degree of organic life and strength than would otherwise exist. It promotes perspiration, by which, through the millions of pores of the skin, much of the fluid of the body is changed and purified. And it induces that genial and diffused warmth, which is one of the chief conditions of a high degree of vitality.

[Verse: "Love not sleep lest thou come to poverty: open thine eyes, and thou shalt be satisfied with bread."--PROV. XX.]

1017. _Why do we feel fatigue?_

Because those organs which stimulate the mechanism of the body to act, _themselves require rest and repair_. When the brain and nerves arrive at that state, they make their condition known to the system generally, by indications which we denominate _fatigue_.

1018. _Why, after rest, do we return invigorated to our labours?_

Because the nervous system has accumulated, during the hours of rest, a fresh amount of that _vital force_ which we call the nervous fluid, and by which the various organs of the body are excited to perform the duties assigned to them.

1019. _What is sleep?_

Sleep is understood to be that state of the body in which _the relation of the brain to some parts of the body is temporarily suspended_.

There are some parts of the body that _never sleep_: such are the heart, the lungs, the organs of circulation, and those parts of the nervous system that direct their operations.

But when sleep overtakes the system, it seems as if the _relations_ of those parts under the controul of the will were temporarily suspended; as if, for instance, those nerves which move the arms, the legs, the eyes, the tongue, &c., were all at once unfastened, just as the strings of an instrument are relaxed by the turning of a key, or the throwing down of a bridge over which they were stretched.

What is meant by the temporary suspension of the relation of the brain to some parts of the body, may be thus explained. Notice a man when he sits dosing in a chair: at first his head is held up, the brain controlling the muscles of the neck, and keeping the head erect. But drowsiness comes on, the brain begins to withdraw its influence, and the muscles of the neck becoming as it were "unstrung," the head drops down upon the breast. But the sleep is unsound, and disturbed by surrounding noises. The brain is therefore frequently excited to return its influence to the muscles, and draw up the head of the sleeper. He gives a sudden start, every muscle is tightened in an instant, up goes the head, the eyes open, the ears listen, until a feeling of security and composure returns; the sleep again deepens, _the nervous connection is again withdrawn_, and then down drops the head as before.

[Verse: "Yet a little sleep, a little slumber, a little folding of the hands to sleep: So shall thy poverty come as one that travelleth; and thy want as an armed man."--PROVERBS XXV.]

1020. _Why do we dream?_

Dreams appear to arise _from the excitement of the brain during those hours when its connection with the other parts of the living organism is suspended_. For instance: a man dreams that he is pursued by a furious animal, and the mind passes through all the excitement of flying from danger; but the _connection_ between the moving power, and the machinery of motion being suspended, no motion takes place. The same impressions upon the brain, when the nerves were "strung" to the muscles, would have caused a rapid flight, and a vigorous effort to escape from the apprehended danger.

1021. _Why do suppers, when indigestible substances are eaten, produce dreaming?_

Probably because, as the digestive organs are oppressed, and those parts of the nervous system which stimulate the organs of digestion are _excited by excessive action_, those portions of the brain which are not immediately employed by the digestive process are disturbed by that _sympathy_ which is observed to prevail between the relative parts and functions of the body.

1022. _Why do we yawn?_

Because, as we become weary, the nervous impulses which direct the respiratory movements are enfeebled. It has been said that those movements are involuntary, and that the parts engaged in producing them are not subject to fatigue. But the operation of breathing is, _to some extent, voluntary_, though when we cease to direct it voluntarily, it is involuntarily continued by organs which know no fatigue.

When, therefore, we feel weary--still controuling our breathing in our efforts to move or to speak--there frequently arrives a period when, for a few seconds, the respiratory process is suspended. It seems to be the point at which the voluntary nerves of respiration are about to deliver their office over to the involuntary nerves; but the pause in the respiration has caused a momentary deficiency of breath, and the involuntary nerves of respiration, coming suddenly to the aid of the lungs, cause a spasmodic action of the parts involved, and _a yawn_, attended by a _deep inspiration to compensate for the cessation of breathing_, are the result.

[Verse: "And it shall be, when they say unto thee, Wherefore sighed thou that thou shalt answer, For the tidings, because it cometh; and every heart shall melt, and all hands shall be feeble."--EZEKIEL XXI.]

1023. _Why do we cough?_

Because the respiratory organs are excited by the presence of some body foreign or unnatural to them. A cough is an effort on the part of the air tubes to free themselves from some source of irritation. And so important are the organs of breathing to the welfare of the body, that the muscles of the chest, back, and abdomen, _unite in the endeavour to get rid of the exciting substance_.

1024. _Why do we sneeze?_

Because particles of matter enter the nostrils and excite the nerves of feeling and of smell. In sneezing, as in coughing, the effort is to free the parts affected from the intrusion of some matters of an objectionable nature. And in this case, as in the former one, there is a very general sympathy of other organs with the part affected, and an energetic effort to get rid of the evil.

1025. _Why do we sigh?_

The action of sighing arises from very similar causes to those of yawning. But in sighing, the nervous depression is caused by _grief_; while in yawning, it is the result of _fatigue_. In sighing, the effect is generally erased by an _expiration_--in yawning by an _inspiration_. The mind, wearied and weakened by sorrow, omits for a few seconds to continue the respiratory process; and then suddenly there comes an involuntary expiration of the breath, causing a faint sound as it passes the organs of the voice.

1026. _Why do we laugh?_

Laughing is caused by the very opposite influences that produce sighing. The nervous system is highly excited by some external cause. The impression is so intense, and the mind so fixed upon it, that the respiratory process is irregular, and uncontrolled. Persons excited to a fit of laughter generally hold their breath until they can hold it no longer, and then suddenly there is a quick expiration causing eccentric sounds, the mind being too intently fixed upon the cause of excitement, _either to moderate the sounds, or to controul the breathing_.

[Verse: "Except ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air."--CORINTH. XIV.]

1027. _Why do we hiccough?_

Hiccough is caused by a spasmodic twitching of the diaphragm, a thin muscular membrane which divides the chest from the abdomen. It generally arises from sympathy with the stomach; and it is highly probable that the muscular twitches and jerks are so many efforts on the part of the diaphragm to _assist the stomach to get rid of some undigested matter_.

1028. _Why do we snore?_

Snoring is caused by air sweeping through the passages that lead from the mouth through the nostrils, and which, in our waking moments, are capable of certain muscular modifications to adapt them to our breathing. But as in sleeping the nervous controul over them is withdrawn, they are left to the action of the air which, in sweeping by them, _sets them in vibration_.

We have endeavoured, by the employment of the simplest language, and by reference to some of the most familiar phenomena of nature, to impart to the reader a clear conception of those sublime laws which control our being, and afford evidence of the goodness and power of that Almighty God to whom we are indebted for the life that we enjoy, and the varied and beautiful existences which, to the rightly constituted mind, make the earth a vast aggregation of interesting objects. We will now, before we pass on to the final section of our work, review some of the more important facts that have been communicated, and devote a few pages to meditations upon the formation of the human body--that wonderful temple of which each of us is a tenant.

We have described man's organisation. What is that organisation for? _It is to make use of the elements upon which man exists._ The lungs make use of the air; the eye makes use of the light; the stomach, and the system generally, make use of water; every part of the body uses heat; and all parts of the system demand food. The hand feeds as constantly as the mouth. The mouth is the receptacle of food, by which the body is to be fed; the stomach is the kitchen in which food is prepared for the use of the body; and the blood-vessels are the canals through which the food is sent to those members of the body that are in need of it. When we speak of man's "organs" or "members," we speak of those parts of the living machinery by which the elements are used up, or employed, for man's benefit. And this view of the subject, bearing in mind that the body is held together as the temple of a living Spirit, superior to mere flesh and blood, gives us a higher and clearer perception of the distinction between the body and the soul than that which we might otherwise entertain. The body is a machine, working for the spirit, which is its owner. While the machine works, the spirit directs and influences its actions. But when the machine stops, the spirit resigns its power over a ruined temple, quits it, and flies to a region where, as a spirit, it becomes subject to a new order of existence consistent with its severance from earthly things and laws, and there it enters upon its eternal destiny, according to the judgments and appointments of God. It is no longer dependent upon a relation between spiritual and material laws.

[Verse: "Not unto us, O Lord, not unto us, but unto thy name give glory, for thy mercy, and for thy truth's sake."--PSALM CXV.]

Suppose that the air which man breathes, instead of returning from his lungs clear and imperceptible to sight, were tinged with colour; we should see, that every time a man breathed, the air would rush in a stream into his mouth, and then return again; and the air which returned would, being warm, be lighter than the outer air, and would rise upward over the man's head, where, cooling and mingling with the outer air, it would descend again. We do, in fact, see this action evidenced; when in winter time the cold condenses the vapour of the breath, we see the little cloud constantly rising before the breather's face, and dispersing in the surrounding air.

Is it not a wonderful thing that that clear and elastic substance, which you cannot feel, though it touches every part of your body, and which you cannot see, is composed of two distinct bodies, having very different properties; and that the two bodies can easily be separated from each other?

Air is of the first importance to life. Hence it is provided for us everywhere. We require air every second, water every few hours, and food at intervals considerably apart. Air is therefore provided for us everywhere. Whether we stand or sit; whether we dwell in a valley or upon a mountain; whether we go into the cellar under our house, or into the garret at the top of it, air is there provided for us. God, who made it a law that man should breathe to live, also sent him air abundantly, that he might comply with that law. And all that is required from man in this respect is, that he will not shut out God's bounty, but receive it freely.

As we have employed the idea that if the air were coloured we should have the opportunity of marking the process of breathing, let us enlarge upon this, and suppose that every time the air were returned from the lungs it became of a darker colour, the darkness denoting increasing impurity. If we placed a man in a room full of pure air, we should see the air enter his lungs, and sent back slightly tinged; but this would disperse itself with the other air of the room and scarcely be perceptible. As the man continued to breathe, however, each measure of air returning from the lungs would serve to pollute that abiding in the room, until at last the whole mass would become cloudy and discoloured, and we should see such a change as occurs when water is turned from a pure and clear state into a muddy condition. The air does become polluted with each respiration, and although it is colourless, it is as impure as if with every breath given off from the lungs it became of a dark colour in proportion to its impurity.

Thus we see how important it is that we should provide ourselves with pure air; and that, in seeking warmth and comfort in our houses, we should provide an adequate supply of fresh atmosphere--because it is more vital to life than either water or food.

Indeed, so constant is our requirement of air, that _if we had to fetch it, for purposes of breathing, or simply to raise it to our mouths, as we do water when we drink, it would be the sole occupation of our lives_--_we could do nothing else_. For this reason, God has sent the air to us, and not required us to go to the air. And the great error of man is, that in too many instances, he shuts off the supply from himself, and brings on disease and pain by inhaling a poisonous compound, instead of air of a healthful kind, which bears an adaptation to the wants of life.

[Verse: "There is a natural body, and there is a spiritual body."--I CORINTHIANS XV.]

Whilst the rooms of our house are filled with air, it is otherwise with water, which we require in less degree than air. If we have not the artificial means by which water is brought to our houses, through the pipes of a water company, there is a spring or a pump in the garden; or in the absence of these, a good sound cask, standing at the end of our house, forming a receptacle to the water-pipes that surround it, provides us with a supply of water distilled from the clouds. If we were to drink a good draught of water once a day, that would be sufficient for all the purposes of life, as far as regards the alimentary uses of water. Man is, therefore, allowed to go to the stream for his drink, and is required to raise it to his lips at those moments when he uses it.

Although, in breathing, man separates the _oxygen_ of the air from the _nitrogen_ thereof, he does not separate the _oxygen_ of the water from the _hydrogen_. Water, in fact, undergoes no change in the body, excepting that of admixture with the substances of the body. And its uses are, to moisten, to cool, to cleanse, and also to nourish the parts with which it comes in contact. But it affords no nourishment of itself; it mixes with the blood, of which it forms a material part, and is the means of conveying _the nourishment of the blood_ to every part of the system. After it has filled this office, and taken up impurities that are required to be removed, it is cast out of the system again, without undergoing any chemical change.

Man's body is to his Soul, in many respects, what a house is to its occupant. But how superior is the dwelling which God erected, to that which man has built. Reader, come out of yourself, and in imagination realise the abstraction of the Soul from the body. Make an effort of thought, and do not relinquish that effort, until you fancy that you see your image seated on a chair before you. And now proceed to ask yourself certain questions respecting your bodily tenement--questions which, perchance, have never occurred to you before; but which will impress themselves the more forcibly upon you, in proportion as you realise for a moment the idea of your Soul examining the body which it inhabits. There sits before you a form of exquisite proportions, with reference to the mode of life it has to pursue--the wants of the Soul for which it has to care, and which it has to guard, under the direction of that Soul, its owner and master.

Over the brows that mark the intellectual front of that due form, there fall the auburn locks of youth, or the grey hair of venerable age. Each of those hairs is curiously organised. If you take a branch of a tree, and cut it across, you will find curious markings caused by vessels of various structure, all necessary to the existence of the plant. In the centre will be found either a hollow tube, or a space occupied by a soft substance called pith. Each hair of your head is as curiously formed as the branch of a tree, and in a manner not dissimilar, though its parts are so minute that the unaided eye cannot discern them. Every hair has a root, just as a tree has, and through this root it receives its nourishment. As the vessel which feed a plant are always proportionate to the size of the plant itself, how fine must be those vessels which form the roots of the hair, being in proportion to the size of the hair, which is in itself so small that the eye cannot see its structure? The hair is, in fact, an animal plant, growing upon the body in much the same manner that plants grow upon the surface of the earth. But how does this hair grow? Not alone by the addition of matter at its roots, pushing up and elongating its stem: nourishment passes up through its whole length, and is deposited upon its end, just as the nourishment of a tree is deposited upon its extreme branches. If, after having your hair cut, you were to examine its ends by the microscope, you would discover the abrupt termination left by the scissors. But allow the hair to grow, and then examine it, and you will discover that it grows from its point which, in comparison with its former state, is perfect and fine. The reason why the beard is so hard is, that the ends of the hair are continually being shaved off. The hair of the beard, if allowed to grow, would become almost as soft as the hair of the head.

[Verse: "The very hairs of your head are all numbered."--MATTHEW XI.]

But why is man's head thus covered with hair? For precisely the same reason that a house is thatched--to keep the inmates warm. We might add, also, to give beauty to the edifice. But as beauty is a conventional quality--and if men were without it they would consider themselves quite as handsome as they do now--we will not enlarge upon the argument. Our bald-headed friends, too, might have reason to complain of such a partial hypothesis. The brain is the great organ upon which the health, the welfare, and the happiness of the system depends. The skull, therefore, may be regarded as analogous to the "strong box," the iron chest in which the merchant keeps his treasure. There is no point at which the brain can be touched to its injury, without first doing violence to the skull. Even the spinal cord runs down the back through a tunnel or tube, formed in a number of strong bones, so closely and firmly jointed together, that they are commonly termed "the back-bone."

Look at the eyebrows. What purpose do they fulfil? Precisely that of a shed, or arch placed over a window to shelter it from rain. But for the eyebrows the perspiration would frequently run from the brow into the eyes, and obscure the sight; a man walking in a shower of rain would scarcely be able to see; and a mariner in a storm would find a double difficulty in braving the tempest.

Now we come to the eye, which is the window of the Soul's abode. And what a window! how curiously constructed! how wisely guarded! In the eyelashes, as well as the eyebrows, we see the hair fulfilling a useful purpose, differing from any already described. The eyelashes serve to keep cold winds, dust, and too bright sun, from injuring or entering the windows of the body. When we walk against the east wind, we bring the tips of our eyelashes together, and in that way exclude the cold air from the surface of the eye; and in the same manner we exclude the dust and modify the light. The eyelashes, therefore, are like so many sentries, constantly moving to and fro, protecting a most important organ from injury. The eyelids are the shutters by which the windows are opened and closed. But they also cleanse the eye, keeping it bright and moist. There are, moreover, in the lids of each eye or window, little glands, or springs, by which a clear fluid is formed and supplied for cleansing the eye. The eye is placed in a socket of the skull, in which it has free motion, turning right or left, up or down, to serve the purpose of the inhabitant of the dwelling. Of the structure of the eye itself we will not say much, for the engravings will afford a clearer understanding than a lengthy written description. But we would have you examine the formation of the iris of the living eye, the ring which surrounds the pupil. Hold a light to it, and you will find that the iris will contract and diminish the pupil; withdraw the light, and the iris will relax, and the pupil expand, thus regulating the amount of light. The images of external objects are formed upon the retina of the eye, a thin membrane, spread out upon the extremity of a large nerve, which proceeds immediately to the brain, and forms the telegraphic cord by which information is given to the mind, of everything visible going on within the range of sight.

[Verse: "Thou art of purer eyes than to behold evil, and canst not look on iniquity."--HABAKKUK I.]

Now, think for a few moments upon the wonderful structure of those windows of the body. Can you fancy, in the walls of your house, a window which protects itself, cleanses itself, and turns in any direction at the mere will of the tenant; and when that tenant is oppressed by excess of light, draws its own curtain, and gives him ease; and when he falls asleep, closes its own shutters, and protects itself from the cold and dust of night, and the instant he awakes in the morning, opens, cleanses itself with a fluid which it has prepared during the night, and kept in readiness; and repeats this routine of duty day after day for half a century, without becoming impaired? Such, nevertheless, is the wonderful structure of the window of the body--the eye.

In some scientific works that have recently been published, curious investigations have been made known. It has been shown that the eye is impressed momentarily, as a photographic plate is impressed by the rays of the sun. But the photography of the eye has this extraordinary quality--that one image passes away, and another takes its place immediately, without confusion or indistinctness. But the most wonderful assertion of all is, that under the excitement of memory these photographic images are restored; and that when, "in our mind's eye," we see the image of some dear departed friend, the retina really revives an image which once fell upon its sensitive surface, and which image has been stored up for many years in the sacred portfolio of its affections!

Another extraordinary assertion is one which comes supported by a degree of authenticity that entitles it to consideration. It is said that the eye of a dead man retains an impression of the last picture that fell upon the faithful retina. Dr. Sandford, of America, examined the eye of a man named Beardley, who had been murdered at Auburn, and he published in the _Boston Atlas_ the following statement:--"At first we suggested the saturation of the eye in a weak solution of atrophine, which evidently produced an enlarged state of the pupil. On observing this, we touched the end of the optic nerve with the extract, when the eye instantly became protuberant. We now applied a powerful lens, and discovered in the pupil, the rude, worn-away figure of a man, with a light coat, beside whom was a round stone, standing or suspended in the air, with a small handle, stuck in the earth. The remainder was debris, evidently lost from the destruction of the optic nerve, and its separation from the mother brain. Had we performed the operation when the eye was entire in the socket, with all its powerful connection with the brain, there is not the least doubt but that we should have detected the last idea and impression made on the mind and eye of the unfortunate man. The picture would evidently be entire; and perhaps we should have had the contour, or better still, the exact figure of the murderer. The last impression on the brain before death is always more terrible from fear than any other cause, and figures impressed on the pupil more distinct, which we attribute to the largeness of the optic nerve, and its free communication with the brain." Whether the supposition, which seems to be supported by the experiment above detailed, be correct or not, it is in no sense more wonderful than the facts which are already known respecting this curious and perfect organ.

[Verse: "Be not rash with thy mouth, and let not thine heart be hasty to utter anything before God: for God is in heaven, and thou upon earth; therefore let thy words be few."--ECCLESIASTES V.]

The nose is given us for two purposes--to enable us to respire and to smell. As odours arise from the surface of the earth, the cup or funnel of the nose is turned down to meet them. In the nostrils hair again serves a useful purpose. It not only warms the air which enters the nostrils, but it springs out from all sides, and forms an intersecting net, closing the nostrils against dust, and the intrusion of small insects. If by any means, as when taking a sharp sniff, foreign matters enter the nostrils, the nose is armed with a set of nerves which communicate the fact to certain muscles, and the organs of respiration unite with those muscles to expel the intruding substances. In this action, the diaphragm, or the muscle which divides the abdomen from the chest, is pressed down, the lungs are filled with air, the passage by which that air would otherwise escape through the mouth, is closed up, and then, all at once, with considerable force, the air is pressed through the nostrils, to free them from the annoying substance. So great is the force with which this action takes place, that the passage into the mouth is generally pushed open occasioning the person in whom the action takes place, to cry "'tsha!" and thus is formed what is termed a sneeze. As with the eye, so with the nose--innumerable nerves are distributed over the lining membrane, and these nerves are connected with larger nerves passing to the brain, through which everything relating to the sense of smell is communicated.

The nose acts like a custom-house officer to the system. It is highly sensitive to the odour of most poisonous substances. It readily detects hemlock, henbane, monk's hood, and the plants containing prussic acid. It recognises the foeted smell of drains, and warns us not to breathe the polluted air. The nose is so sensitive, that air containing a 200,000th part of bromine vapour will instantly be detected by it. It will recognise the 1,300,000th part of a grain of otto of roses, or the 13,000,000th part of a grain of musk! It tells us in the mornings that our bed-rooms are impure; it catches the first fragrance of the morning air, and conveys to us the invitation of the flowers to go forth into the fields, and inhale their sweet breath. To be "led by the nose," has hitherto been used as a phrase of reproach. But to have a good nose, and to follow its guidance, is one of the safest and shortest ways to the enjoyment of health.

The mouth answers the fourfold purpose of the organ of taste, of sound, of mastication, and of breathing. In all of these operations, except in breathing, the various parts of the mouth are engaged. In eating we use the lips, the tongue, and the teeth. The teeth serve the purpose of grinding the food, the tongue turns it during the process of grinding, and delivers it up to the throat for the purposes of the stomach, when sufficiently masticated. The lips serve to confine the food in the mouth, and assist in swallowing it, and there are glands underneath the tongue, and in the sides of the mouth, which pour in a fluid to moisten the food. And so watchful are those glands of their duty, that the mere imagination frequently causes them to act. Their fluid is required to modify the intensity of different flavours and condiments in which man, with his love of eating, will indulge. Thus, when we eat anything very acid, as a lemon, or anything very irritating, as Cayenne pepper, the effect thereof upon the sensitive nerves of the tongue is greatly modified by a free flow of saliva into the mouth. And if we merely fancy the taste of any such things, those glands are so watchful, that they will immediately pour out their fluid to mitigate the supposed effect.

[Verse: "I say unto you, Swear not at all; neither by heaven, for it is God's throne; Nor by the earth; for it is his footstool."--MATTHEW V.]

In speaking, we use the lips, the teeth, the tongue; and the chest supplies air, which, being controlled in its emission, by a delicate apparatus at the mouth of the wind-pipe, causes the various sounds which we have arranged into speech, and by which, under certain laws, we are enabled to understand each other's wants, participate in each other's emotions, express our loves, our hopes, our fears, and glean those facts, the accumulation of which constitutes knowledge, enhances the happiness of man, and elevates him, in its ultimate results above the lower creatures to which the blessing of speech is denied.

The curious structure of the tongue, and the organs of speech, would fill a very interesting volume. The tongue is unfortunately much abused, not only by those who utter foul words, and convert the blessing of speech, which should improve and refine, into a source of wicked and profane language; but it constantly remonstrates against the abuse of food, and the use of things which are not only unnecessary for the good of our bodies, but prejudicial to their health. When the body is sufficiently fed, the tongue ceases its relish, and derives no more satisfaction from eating: but man contrives a variety of inventions to whip the tongue up to an unnatural performance of its duty, and thus we not only over-eat, but eat things that have no more business in our stomachs, than have the stones that we walk upon. Can we wonder, then, that disease is so prevalent, and that death calls for many of us so soon.

That wonderful essence, the Soul of man, rises above all finite knowledge. Its wonders and powers will never, probably, be understood until when, in a future state of existence, the grandest of all mysteries shall be explained. When we talk of the brain, we speak of that which it is easy to comprehend as the organ, or the seat of the mind; when we speak of the mind, we have greater difficulty in comprehending the meaning of the term we employ; but when we speak of the Soul, we have reached a point which defies our understanding, because our knowledge is limited. The brain may be injured by a blow; the mind may be pained by a disagreeable sight, or offended by a harsh word; but the Soul can only be influenced secondarily through the mind, which is primarily affected by the organs of the material senses. Thus the happiness or the misery of the Soul depends to a very great extent upon the proper fulfilment of the duties of the senses, which are the servants of the Soul, over which the mind presides, as the steward who mediates between the employer and the employed.

The Ear, which is taught to delight in sweet sounds, and in pure language, is a better servant of the master Soul, than one which delights not in music, and which listens, with approbation or indifference, to the oaths of the profane. The Eye which rejoices in the beauties of nature, and in scenes of domestic happiness and love, is a more faithful servant than one that delights in witnessing scenes of revelry, dissipation, and strife. The Nose which esteems the sweet odour of flowers, or the life-giving freshness of the pure air, is more dutiful to his master than one that rejects not the polluted atmosphere of neglected dwellings. The Mouth which thirsts for morbid gratification of taste, is more worthless than one which is contented with wholesome viands, and ruled by the proper instincts of its duty. Who that can understand the wonderful structure of the tongue, and the complicated mechanism of the organs of speech and of hearing, could be found to take pleasure in the utterance of oaths, and of words of vulgar meaning? Were those beautiful cords that like threads of silk are woven into the muscular texture of the mouth, and along which the essence of life travels with the quickness of thought, to do the bidding of the will--were they given for no higher use than to sin against the God who gave them, and upon whose mercy their existence every moment depends?

[Verse: "Out of the same mouth proceedeth blessing and cursing. My brethren, these things ought not so to be."--JAMES III.]

The actions of the senses must necessarily affect the mind, which is the head steward of the Soul; and the Soul becomes rich in goodness, or poor in sin, in proportion as the stewardship, held by his many servants, is rightly or wrong-fully fulfilled. As in an establishment where the servants are not properly directed and ruled, they often gain the ascendancy, and the master has no power over them, so with man, when he gives himself up to sensual indulgences. The Soul becomes the slave of the senses--the master is controlled by the servants.

With regard to the mechanism of motion, let us take the case of a man who is walking a crowded thoroughfare, and we shall see how active are all the servants of the Soul, under the influence of the mind. He walks along in a given direction. But for the act of volition in the mind, not a muscle would stir. The eye is watching his footsteps. There is a stone in his path, the eye informs the mind, the mind communicates with the brain, and the nerves stimulate the muscles of the leg to lift the foot a little higher, or turn it on one side, and the stone is avoided. The eye alights on a familiar face, and the mind remembers that the eye has seen that face before. The man goes on thinking of the circumstance under which he saw that person, and partially forgets his walk, and the direction of his steps. But the nerves of volition and motion unite to keep the muscles up to their work, and he walks on without having occasion to think continually, "I must continue walking." He has not to make an effort to lift his leg along between each interval of meditation; he walks and meditates the while. Presently a danger approaches him from behind. The eye sees it not--knows no more, in fact, than if it were dead. But the ear sounds the alarm, tells the man, by the rumbling of a wheel, and the tramp of horses' feet, that he is in danger; and then the nerves, putting forth their utmost strength, whip the muscles up to the quick performance of their duty; the man steps out of the way of danger, and is saved. He draws near to a sewer, which is vomiting forth its poisonous exhalations. The eye is again unconscious--it cannot see the poison lurking in the air. The ear, too, is helpless; it cannot bear witness to the presence of that enemy to life. But the nose detects the noxious agent, and then the eye points out the direction of the sewer, and guides his footsteps to a path where he may escape the injurious consequences. A clock strikes, the ear informs him that it is the hour of an appointment; the nerves stimulate the muscles again, and he is hastened onward. He does not know the residence of his friend, but his tongue asks for him, and his ear makes known the reply. He reaches the spot--sits--rests. The action of the muscles is stayed; the nerves are for a time at rest. The blood which had flown freely to feed the muscles while they were working, goes more steadily through the arteries and veins, and the lungs, which had been purifying the blood in its course, partake of the temporary rest.

[Verse: "I am but a little child: I know not how to go out or come in."--I KINGS III.]

Let us remember that there are two sets of muscles, acting in unison with each other, to produce the various motions; they are known by the general terms of _flexors_ and _extensors_; the first enable us to bend the limbs, the other to bring the limbs back to their former position. The flexors enable us to close the hand, the extensors to open it again. The flexors enable us to raise the foot from the ground; the extensors set the foot down again in the place desired. Consider for a moment the nicety with which the powers of these muscles must be balanced, and the harmony which must subsist between them in their various operations. When we are closing the hand, if the extensor muscles did not gradually yield to the flexors--if they gave up their hold all at once, the hand, instead of closing with gentleness and ease, would be jerked together in a sudden and most uncomfortable manner. If, in such a case, you were to lay your hand with its back upon the table, and wish to close the hand, the fingers would fall down upon the palm suddenly, like the lid of a box. Again, consider how awkward it would be in such a case; our walk through the streets would become a series of jumps and jerks; when a man had raised his foot, after it had been jerked up, there it would stand fixed for a second before the opposite muscles could put on their power to draw it down again. This case is not at all suppositious: there is a derangement frequently observed in horses, in which one set of muscles becomes injured, and we may see horses suffering from this ailment, trotting along with one of their legs jerking up much higher than the others, and set down again with difficulty, just in the manner described.

It is also to be observed that very nice proportions must exist between the sizes of the muscles and the sizes of the bones. If this were not the case, our motions, instead of being firm and steady, would be all shaky and uncertain. In old persons the muscles become weak and relaxed; hence there is a tendency in the movements of the aged to fall, as it were, together; the head is no longer erect, the body bends, the knees totter, and the arms lean towards the body as for support.

In the child a somewhat similar state of things exists. The muscles have not been properly developed, nor have they been brought sufficiently under the controul of the nervous system. The child, therefore, totters and tumbles about, and it is not until it has stumbled and tumbled some hundreds of times in its little history, that the muscles have become strong enough to fulfil their office, or have been brought sufficiently under the controul of the nervous system, to perform well the various duties required from them.

In all these things, we recognise the perfection of the divine works. We are apt, too apt, to overlook this perfection, because it prevails in everything; but by speculating upon what inconveniences we might suffer, were not things ordained as they are, we obtain most convincing evidences of divine goodness and wisdom.

[Verse: "Watchman, what of the night? The watchman said, The morning cometh, and also the night; if ye will enquire, enquire ye; return, come."--ISAIAH XXI.]

Having taken this view of the muscular system of the external man, let us turn our attention to the muscles of the internal organs. The muscles of which we have been speaking are called the voluntary muscles, because we have them under our own controul--they are subject to the influences of our will. But there is the other set of muscles. What are they? We talk of the beating, or of the palpitation, of the heart. But, what is it that causes the heart to beat? You cannot, if you wish it, make your heart beat more quickly or more slowly. Place your finger upon your pulse, and notice the degree of rapidity with which its pulsations follow. Now think that you should like to double the frequency of those pulsations. Say to the heart, with your inner voice, that you wish it to beat 120 times in a minute, instead of 60. It does not obey you; it does not appreciate your command. Now place your finger on the table, and your watch by the side of your hand, and tell your finger to beat 60 times in the minute, or 100 times, or 150 times, or 200 times, and the finger will obey you--because it is _moved by muscles which are subject to the will_, while the heart is composed of muscles which are _not subject to the will_. Why should this be? Why should man have the power to regulate his finger, and not to regulate his heart?

For the sustentation of our bodies it is needful that the blood should ever be in circulation. If the heart were to cease beating only for three or four minutes (perhaps less) life would be extinct. In this short time the whole framework of man, beautiful in its proportions, perfect in its parts, would pass into the state of dead matter, and would simply wait the decay that follows death. The eye would become dull and glazed, the lips would turn blue, the skin would acquire the coldness of clay--love, hope, joy, would all cease. The sweetest, the fondest ties would be broken. Flowers might bloom, and yield their fragrance, but they would be neither seen nor smelt; the sun might rise in its brightest splendour, yet the eye would not be sensitive to its rays; the rosy-cheeked child might climb the paternal knee; but there, stiff, cold, without joy, or pain, or emotion of any kind, unconscious as a block of marble, would sit the man _whose heart for a few moments had ceased to beat_.

How wise, then, and how good of God, that he has not placed this vital organ under our own care! How sudden would be our bereavements--how frequent our deaths, how sleepless our nights, and how anxious our days, if we had to keep our own hearts at work, and death the penalty of neglect.

And yet, before we were born, until we reach life's latest moment--through days of toil, and nights of rest--even in the moments of our deepest sin against the God who at the time is sustaining us, our hearts beat on, never stopping, never wearying, never asking rest.

This brings us to another reflection. Our arms get weary, our legs falter from fatigue, the mind itself becomes overtaxed, and all our senses fall to sleep. The eye sees not, the ear is deaf to sound, the sentinels that surround the body, the nerves of touch, are all asleep--you may place your hand upon the brow of the sleeping man, and he feels it not. Yet, unseen, unheard, without perceptible motion, or the slightest jar to mar the rest of the sleeper, the heart beats on, and on, and on. As his sleep deepens, the heart slackens its speed, that his rest may be the more sound. He has slept for eight hours, and the time approaches for his awakening. But is the heart weary--that heart which has toiled through the long and sluggard night? No! The moment the waking sleeper moves his arm, the heart is aware that a motion has been made, that effort and exercise are about to begin. The nerves are all arousing to action; the eyes turn in their sockets, the head moves upon the neck; the sleeper leaves his couch, and the legs are once more called upon to bear the weight of the body. Blood is the food of the eye, the food of the ear, of the foot, the hand, and every member of the frame. While they labour they must be fed--that is the condition of their life, the source of their strength. The heart, therefore, so far from seeking rest, is all fresh and vigorous for the labours of the day, and proceeds to discharge its duty so willingly, that we do not even know of the movements that are going on within us.

[Verse: "Awake up, my glory; awake, psaltery and harp: I myself will awake early."--PSALM LVII.]

Thus we have seen the difference between the voluntary and the involuntary muscles, and we have perceived the goodness of our Creator in not entrusting to our keeping the controul of an organ so vital to life, as the heart.

But the heart is not the only organ which thus works unseen and unfelt. There are the lungs and the muscles of the chest, the stomach, and other parts occupying the abdomen, together with all those muscular filaments which enter into the structure of the coats and valves of the blood-vessels, and which assist to propel the blood through the system. All these are at work at every moment of man's life; and yet, so perfect is this complicated machinery, that we really do not know, except by theory, what is going on within us.

During the time that the sleeper has been at rest, the stomach has been at work digesting the food which was last eaten. Then the stomach has passed the macerated food into the alimentary canal, the liver has poured out its secretion, and produced certain changes in the condition of the dissolved food: and the lacteals, of which there may be many thousands, perhaps millions, have been busy sucking up those portions of the food which they knew to be useful to the system, whilst they have rejected all those useless and noxious matters upon which the liver, like an officer of health, had set his mark, as unfitting for the public use. This busy life has gone on uninterruptedly; every member of that body, every worker in that wonderful factory, has been unremitting in his duty, and yet the owner, the master, has been asleep, and wakes up finding every bodily want supplied!

Notwithstanding that much has already been said of the wonders that pertain to the eye, it has not yet been considered as the seat of _tears_, those mute but eloquent utterers of the sorrows of the heart. Beautiful Tear! whether lingering upon the brink of the eyelid, or darting down the furrows of the care-worn cheek--thou art sublime in thy simplicity--great, because of thy modesty--strong, from thy very weakness. Offspring of sorrow! who will not own thy claim to sympathy? who can resist thy eloquence? who can deny mercy when thou pleadest?

Every tear represents some in-dwelling sorrow preying upon the mind and destroying its peace. The tear comes forth to declare the inward struggle, and to plead a truce against further strife. How meet that the eye should be the seat of tears--where they cannot occur unobserved, but, blending with the beauty of the eye itself, must command attention and sympathy!

Whenever we behold a tear, let our kindliest sympathies awake--let it have a sacred claim upon all that we can do to succour and comfort under affliction. What rivers of tears have flown, excited by the cruel and perverse ways of man! War has spread its carnage and desolation, and the eyes of widows and orphans have been suffused with tears! Intemperance has blighted the homes of millions, and weeping and wailing have been incessant! A thousand other evils which we may conquer have given birth to tears enough to constitute a flood--a great tide of grief. Suppose we prize this little philosophy, _and each one determine never to excite a tear in another_. Watching the eye as the telegraph of the mind within, let us observe it with anxious regard; and whether we are moved to complaint by the existence of supposed or real wrongs, let the indication of the coming tear be held as a sacred truce to unkindly feeling, and our efforts be devoted to the substitution of smiles for tears!

[Verse: "Who is as the wise man? and who knoweth the interpretation of a thing? a man's wisdom maketh his face to shine, and the boldness of his face shall be changed."--ECCLESIASTES VIII.]

There is only one other matter to which we think it necessary to allude, before we pass to the concluding section of our work. It has been said (162), that snow which is _white_, keeps the earth warm; that _white_ as a colour is _cool_, and that _black_ absorbs _heat_ (230). These assertions may appear to be contradictory, and, taken in connection with the fact of the blackness of the skin of negroes in hot climates, may at a first glance be considered unsatisfactory. They are, however, perfectly reconcileable, and that too, without the slightest evasion of the real bearing of the asserted facts. White snow is warm _on account of its texture_, which, being woolly, forms a layer of non-conducting substance over the surface of the earth, and _keeps in its warmth_; white clothing, worn as a garment consisting of a thin material, is cool, because _the white colour_ turns back the rays of the sun that fall upon it. Swansdown, although white, being a non-conductor, would be warm, because, though it would reflect the light and heat, it would confine and accumulate the heat of the body. The black skin of the negro is a _living texture_, and is not subject to the same laws that govern dead matter. The skin of the negro is largely provided with cells which secrete a fatty matter that acts as a non-conductor of the _external_ heat, and also a much larger number of perspiratory glands than exist in the skins of Europeans. The perspiration cools the blood, and carries off the _internal_ heat, while the oily matter gives a shining surface to the skin, and reflects the heat, to which the fatty matter presents itself as a non-conductor. We see, therefore, that there are two express provisions for the cooling of the negroes' skin, independent of the colour. The skin of the Esquimaux who inhabits a cold country is _white_, though it might be supposed that a black skin would best conduce to the warmth of his body. But the Esquimaux has, underneath his skin, _a thick coating of fat_, by which the _internal heat_ of the body is prevented from escaping.

This _resume_ of the subjects embodied in the form of question and answer in the previous pages, will serve to impress the more important truths upon the mind of the reader, while it has enabled us to fill up many omissions necessitated by the arbitrary form of catechetical composition.

[Verse: "Ask now the beasts, and they shall teach thee; and the fowls of the air, and they shall tell thee."--JOB XII.]

CHAPTER LII.

1029. _Why are there so many bodily forms in the animal creation?_

Because the various creatures which God has created have different modes of life, and the forms of their bodies will be found to present _a perfect adaptation to the lives allotted to them_.

Because, also, the beauty of creation _depends upon the variety of objects of which it consists_. And the greatness of the Creator's power is shown _by the diversity of ends accomplished by different means_.

1030. _Why are birds covered with feathers?_

Because they require a high degree of _warmth_, on account of the activity of their muscles; but in providing that warmth it was necessary that their coats should be of the _lightest material_, so as not to impair their powers of flight; and feathers combine the _highest warming power, with the least amount of weight_.

1031. _Why have ostriches small wings?_

Because, having long legs, they do not require their wings for flight; they are merely used _to steady their bodies while running_.

1032. _Why are ostrich feathers soft and downy?_

Because, as the feathers are not employed for flight, the _strength of the feather as constructed for flying is unnecessary_, and the feathers therefore consist chiefly of a soft down.

1033. _Why have water-birds feathers of a close and smooth texture?_

Because such feathers keep the body of the bird warm and dry, by repelling the water from their surface. A bird could scarcely move through the water, with the downy feathers of the ostrich, because of _the amount of water the down would absorb_.

1034. _Why is man born without a covering?_

Because _man is the only animal that can clothe itself_. As in the various pursuits of life he wanders to every part of the globe, he can adapt himself _to all climates and to any season_.

[Verse: "Who teacheth us more than the beasts of the earth, and maketh us wiser than the fowls of heaven?"--JOB XXXV.]

1035. _Why do the furs of animals become thicker in the winter than in the summer?_

Because the creator has thus provided for the preservation of the warmth of the animals during the cold months of winter.

1036. _Why does a black down grow under the feathers of birds as winter approaches?_

Because the down is a non-conductor of heat, and black the warmest colour. It is therefore best adapted to _keep in_ their bodily warmth during the cold of winter.

1037. _Why has man no external appendage to his mouth?_

Because _his hands_ serve all the purposes of gathering food, and _conveying it to the mouth_. Man's mouth is simply an _opening_; in other animals it is a _projection_.

1038. _Why have dogs, and other carnivorous animals, long pointed teeth, projecting above the rest?_

Because as they have not hands to seize and controul their food, the projecting teeth enable them to _snap and hold_ the objects which they pursue for food.

1039. _Why is the under jaw of the hog, shorter and smaller than the upper one?_

Because the animal pierces the ground _with its long snout_, and then the small under jaw _works freely in the furrow_ that has been opened, in quest of food.

1040. _Why have birds hard beaks?_

Because, having no teeth, the beak enables them to _seize_, _hold_, and _divide their food_.

1041. _Why are the beaks of birds generally long and sharp?_

Because the greater number of birds live by _picking up small objects_, such as worms, insects, seeds, &c. The sharp beak, therefore, serves as a _fine pincers_, enabling them to take hold of their food conveniently.

[Verse: "As the fishes that are taken in an evil net, and as the birds that are caught in the snare; so are the sons of men snared in an evil time, when it falleth suddenly upon them."--ECCLESIASTES IX.]

1042. _Why have snipes and woodcocks long tapering bills?_

Because they live upon worms which they find in the soft mud of streams and marshy places; their long bills, therefore, enable them to _dig down into the mud after their prey_.

1043. _Why have woodcocks, snipes, &c., nerves running down to the extremities of their bills?_

Because, as they dig for their prey in the soft sand and mud, they cannot see the worms upon which they live. Nerves are, therefore, distributed to the very point of their bills (where, in other birds, nerves are entirely absent) _to enable them to prehend their food_.

[Illustration: Fig. 67.--SPOONBILL.]

1044. _Why have ducks and geese square-pointed bills?_

Because they not only feed by dabbling in soft and muddy soil, but they consume a considerable quantity of green food, and their square bills enable them to _crop off the blades of grass_.

[Verse: "Let the heaven and earth praise him, the seas, and everything that moveth therein."--PSALM LXIX.]

1045. _Why has the spoon-bill a long expanded bill, lined internally with sharp muscular points?_

Because the bird _lives by suction_, dipping its broad bill in search of aquatic worms, mollusks, insects and the roots of weeds. The bill forms _a natural spoon_, and the muscular points enable the bird to _filter the mud_, and to retain the nourishment which it finds.

1046. _Why has the spoon-bill long legs?_

Because it _wades in marshy places_ to find its food. Its legs are therefore long, for the purpose of keeping its body out of the water, and above the smaller aquatic plants, while it searches for its prey.

1047. _Why have the parrots, &c., crooked and hard bills?_

Because they live upon nuts, the stones of fruit, and hard seeds. The shape of the bill, therefore, enables them to _hold the nut or seed firmly_, and the sharp point enables them to _split or remove the husks_.

1048. _Why can a parrot move its upper as well as its lower bill?_

Because by that means it is enabled to bring the nut or seed nearer the fulcrum, or joint of the jaw. It, therefore, acquires greater power, just as with a pair of nut-crackers we obtain increased power by _setting the nut near to the joint_.

1049. _Why have animals with long necks large throats?_

Animals that graze, or feed from the ground, generally have a more powerful muscular formation of the throat than those which feed in other positions, because a greater effort is required to _force the food upward, than would be needed to convey it down_.

1050. _Why are the bones of birds hollow?_

Because they are thereby rendered _lighter_, and do not interfere with the flight of the bird _as they would do if they were solid_. Greater strength is also obtained by the _cylindrical form of the bone_, and a larger surface afforded for the _attachment of powerful muscles_.

[Verse: "And my hand hath found, as a nest, the riches of the people; and as one gathereth eggs that are left, have I gathered all the earth; and there was none that moved the whip, or opened the mouth, or peeped."--ISAIAH X.]

1051. _Why do all birds lay eggs?_

Because, to bear their young in any other manner, would _encumber the body_, and materially interfere with their powers of flight.

As soon as an egg becomes large and heavy enough to be cumbersome to the bird, it is removed from the body. A shell, impervious to air, protects the germ of life within, until from two to twenty eggs have accumulated, and then, although laid at different intervals, their incubation commences together, and the young birds are hatched at the same time.

CHAPTER LIII.

1052. _Why have birds with long legs short tails?_

Because the tails of birds are used to guide them through the air, by a _kind of steerage_. When birds with long legs take to flight, they throw their legs behind, and they then _serve the same purpose as a tail_.

[Illustration: Fig. 68.--PERCH.]

1053. _Why have fishes fins?_

The fins of fishes are to them, _what wings and tails are to birds_, enabling them to rise in the fluid in which they live by the _reaction of the motions of the fins upon its substance_.

[Verse: "Speak to the earth, and it shall teach thee; and the fishes of the sea shall declare unto thee. Who knoweth not in all these that the hand of the Lord hath wrought this."--JOB XII.]

1054. _Why are the fins of fishes proportionately so much smaller than the wings of birds?_

Because there is less difference between the _specific gravity_ of the body of a fish, and the water in which it moves, than between the body of a bird, and the air on which it flies. The fish, therefore _does not require such an expanded surface to elevate or guide it_.

1055. _Why have fishes scales?_

Because scales, while they afford protection to the bodies of fish, are conveniently adapted to their motions; and as the scales _present no surface to obstruct their passage through the water_, as hair or feathers would do, they evidently form the best covering for the aquatic animal.

1056. _Why do fishes float in streams (when they are not swimming) with their heads towards the stream?_

Because they _breathe_ by the transmission of water over the surface of their gills, the water entering at the mouth, and passing over the gills behind. When, therefore, they lie motionless with their heads to the stream, they are in _that position which naturally assists their breathing process_.

1057. _Why have fishes air-bladders?_

Because, as the density of water varies greatly at different depths, the enlargement or contraction of the bladder regulates the relation of _the specific gravity of the body of the fish to that of the water in which it moves_.

1058. _Why have whales a very large development of oily matter about their heads?_

Because their heads are thereby rendered the lighter part of their bodies, and a very slight exertion on the part of the animal will bring its head to the surface _to breathe air, which it constantly requires_.

1059. _Why have birds that swim upon water web-feet?_

Because the spreading out of the toes of the bird brings the membrane between the toes into the form of a fin, or _water-wing_, by striking which against the water, _the bird propels itself along_.

[Verse: "And Jesus saith unto him, The foxes have holes, and the birds of the air have nests; but the son of man hath not where to lay his head."--MATTHEW XIII.]

1060. _Why have birds that swim and dive short legs?_

Because long legs would greatly _impede their motions in the water_, by becoming repeatedly entangled in the weeds, and by striking against the bottom. _Waders_, however, require long legs because they have to move about through the _tall vegetation of marshy borders_.

[Illustration: Fig. 69.--STILT-PLOVER AND DUCK.]

1061. _Why have the feet of the heron, cormorant, &c., deep rough notches upon their under surface?_

Because, as those birds live by catching fish, they are enabled by the notches in their feet, to _hold the slippery creatures upon which they feed_.

1062. _Why have otters, seals, &c., web-feet?_

Because, while the feet enable them to _walk upon the land_, they are equally effective in their action upon the water, and hence they are _adapted to the amphibious nature of the animals to which they belong_.

1063. _Why do the external ears of animals of prey, such as cats, tigers, foxes, wolves, hyenas, &c., bend forward?_

Because they collect the sounds that occur _in the direction of the pursuit_, and enable the animal to _track its prey_ with greater certainty.

[Verse: "Doth the hawk fly by thy wisdom, and stretch her wings toward the south? "Doth the eagle mount up at thy command, and make her nest on high?"]

1064. _Why do the ears of animals of flight, such as hares, rabbits, deer, &c., turn backward?_

Because they thereby catch the sounds that give them _warning of the approach of danger_.

1065. _Why has the stomach of the camel a number of distinct bags, like so many separate stomachs?_

Because water is stored up in the separate chambers of the stomach, apart from the solid aliment, so that the animal can _feed_, without consuming all its drink. It is thereby _able to retain water to satisfy its thirst while travelling across hot deserts_, where no water could be obtained.

1066. _Why do woodpeckers "tap" at old trees?_

Because by boring through the decayed wood, with the sharp and hard bills with which they are provided, _they get at the haunts of the insects upon which they feed_.

1067. _Why are woodpeckers' tongues about three times longer than their bills?_

Because, if their bills were long, they would not bore the trees so efficiently; and when the trees are bored, and the insects alarmed, they endeavour to retreat into the hollows of the wood; _but the long thin tongue of the woodpecker fixes them on its sharp horny point_, and draws them into the mouth of the bird.

1068. _Why have the Indian hogs large horns growing from their nostrils and turning back towards their eyes?_

Because the horns _serve as a defence to the eyes_ while the animal forces its way through the thick underwood in which it lives.

1069. _Why have calves and lambs, and the young of horned cattle generally, no horns while they are young?_

Because the presence of horns would _interfere with the suckling of the young animal_. When, however, it is able to feed itself by browsing, _then the horns begin to grow_.

[Verse: "She dwelleth and abideth on the rock, upon the crag of the rock, and the strong place.

"From thence she seeketh the prey, and her eyes behold afar off. Her young ones also suck up blood: and where the slain are, there is she."--JOB XXXIX.]

1070. _Why have infants no teeth?_

Because the presence of teeth would interfere with their suckling, while the teeth would be of no service, until the child _could take food requiring mastication_.

1071. _Why cannot flesh-eating animals live upon vegetables?_

Because the gastric juice of a flesh-eating animal, being adapted to the duty which it has to perform, _will not dissolve vegetable matter_.

1072. _Why have birds gizzards?_

Because, having no teeth, the tough and fibrous gizzards are employed _to grind the food preparatory to digestion_.

1073. _Why are small particles of sand, stone, &c., found in the gizzards of birds?_

Because, by the presence of those rough particles, which become embedded in the substance of the gizzard, the food of the bird is more effectively ground.

When our fowls are abundantly supplied with meat, they soon fill their craw, but it does not immediately pass thence into the gizzard; it always enters in small quantities, in proportion to the progress of trituration, in like manner, as in a mill, a receiver is fixed above the two large stones which serve for grinding the corn, which receiver, although the corn be put into it by bushels, allows the grain to dribble only in small quantities into the central hole in the upper mill-stone.--_Paley._

CHAPTER LIV.

1074. _Why has the mole hard and flat feet, armed with sharp nails?_

Because the animal is thereby enabled to _burrow in the earth_, in search for worms. Its feet are so many _shovels_.

1075. _Why is the mole's fur exceedingly glossy and smooth?_

Because its smoothness enables it to work under ground _without the soil sticking to its coat_, by which its progress would be impeded. From soils of all kinds, the little worker emerges shining and clean.

[Verse: "I know all the fowls of the mountains, and the wild beasts are mine."--PSALM L.]

What I have always most admired in the mole is its _eyes_. This animal occasionally visiting the surface, and wanting, for its safety and direction, to be informed when it does so, or when it approaches it, a perception of light was necessary. I do not know that the clearness of sight depends at all upon the size of the organ. What is gained by the largeness or prominence of the globe of the eye, is width in the field of vision. Such a capacity would be of no use to an animal which was to seek its food in the dark. The mole did not want to look about it; nor would a large advanced eye have been easily defended from the annoyance to which the life of the animal must constantly expose it. How indeed was the mole, working its way under ground, to guard its eyes at all? In order to meet this difficulty, the eyes are made scarcely larger than the head of a corking-pin; and these minute globules are sunk so deeply in the skull, and lie so sheltered within the velvet of its covering, as that any contraction of what may be called the eyebrows, not only closes up the apertures which lead to the eyes, but presents a cushion, as it were, to any sharp or protruding substance which might push against them. This aperture, even in its ordinary state, is like a pin-hole in a piece of velvet, scarcely pervious to loose particles of earth.--_Paley._

[Illustration: Fig. 70.--ELEPHANTS DRINKING.]

1076. _Why has the elephant a short unbending neck?_

Because the elephant's head is so heavy, that it could not have been supported at the end of a long neck (or lever), without _a provision of immense muscular power_.

[Verse: "Be not afraid, ye beasts of the field: for the pastures of the wilderness do spring, for the tree beareth her fruit, the fig-tree and the vine do yield their strength."--JOEL II.]

1077. _Why has the elephant a trunk?_

The trunk of an elephant _serves as a substitute for a neck_, enabling the animal to crop the branches of trees, or to raise water from the stream.

1078. _Why do the hind legs of elephants bend forward?_

Because the weight of the animal is so great, that when it lay down it would _rise with great difficulty_, if its legs bent outward, as do the legs of other animals. Being bent _under the body_, they have a greater power of pushing directly upward, when the powerful muscles of the thighs straighten them.

According to Cuvier, the number of muscles, in an elephant's trunk, amounts to _forty thousand_, all of which are under the will, and it is to these that the proboscis of this animal owes its flexibility. It can be protruded or contracted at pleasure, raised up or turned to either side, coiled round on itself or twined around any object. With this instrument the elephant collects the herbage on which he feeds and puts it into his mouth; with this he strips the trees of their branches, or grasps his enemy and dashes him to the ground. But this admirable organ is not only adapted for seizing or holding substances of magnitude; it is also capable of plucking a single leaf, or of picking up a straw from the floor. The orifices of the canals of the extremity are encircled by a projecting margin, produced anteriorly into a finger-like process endowed with a high degree of sensibility and exceedingly flexible. It is at once a finger for grasping and a feeler: the division between the two nasal orifices or their elevated sides serves as a point against which to press; and thus it can pick up or hold a small coin, a bit of biscuit, or any trifling thing with the greatest ease.--_Knight's Animal Kingdom._

1079. _Why have bats hooked claws in their wings?_

Because bats are almost destitute of legs and feet; at least those organs are included in their wings. If they alight upon the ground, they have great difficulty in again taking to the wing, as they cannot run or spring to bring their wings in action upon the air. At the angle of each wing there is placed, therefore, a bony hook, by which the bat attaches itself to the sides of rocks, caves, and buildings, laying hold of crevices, joinings, chinks, &c.; and when it takes its flight, _it unhooks itself, and its wings are at once free to strike the air_.

1080. _Why does the bat fly by night?_

Because it lives chiefly upon moths, which are _night-flying insects_.

[Verse: "So are the paths of all that forget God; and the hypocrite's hope shall perish: Whose hope shall be cut off, and whose trust shall be a spider's web."--JOB VIII.]

1081. _Why does the bat sleep during the winter?_

Because, as the winter approaches, the moths and flying insects upon which it feeds, disappear. _If, therefore, it did not sleep through the winter it must have starved._

[Illustration: Fig. 71.--BAT WITH HOOKED WINGS.]

1082. _Why has the spider the power of spinning a web?_

Because, as it lives upon flies, but is _deficient of the power of flying in pursuit of them_, it has been endowed with an instinct _to spread a snare to entrap them_, and with the most wonderful machinery to give that instinct effect.

There are few things better suited to remove the disgust into which young people are betrayed on the view of some natural objects, than this of the spider. They will find that the most despised creature may become a subject of admiration, and be selected by the naturalist to exhibit the marvellous works of the creation. The terms given to these insects, lead us to expect interesting particulars concerning them, since they have been divided into vagrants, hunters, swimmers, and water spiders, sedentary, and mason-spiders; thus evincing a variety in their condition, activity, and mode of life; and we cannot be surprised to find them varying in the performance of their vital functions (as, for example, in their mode of breathing), as well as in their extremities and instruments. Of these instruments the most striking is the apparatus for spinning and weaving, by which they not only fabricate webs to entangle their prey, but form cells for their residence and concealment; sometimes living in the ground, sometimes under water, yet breathing the atmosphere. Corresponding with their very singular organisation are their instincts. We are familiar with the watchfulness and voracity of some spiders, when their prey is indicated by the vibration of the cords of their net-work. Others have the eye and disposition of the lynx or tiger, and after couching in concealment, leap upon their victims. Some conceal themselves under a silken hood or tube, six eyes only projecting. Some bore a hole in the earth, and line it as finely as if it were done with the trowel and mortar, and then hang it with delicate curtains. A very extraordinary degree of contrivance is exhibited in the trap-door spider. This door, from which it derives its name, has a frame and hinge on the mouth of the cell, and is so provided that the claw of the spider can lay hold of it, and whether she enters or goes out, says Mr. Kirby, the door shuts of itself. But the water-spider has a domicile more curious still: it is under water, with an opening at the lower part for her exit and entrance; and although this cell be under water, it contains air like a diving-bell, so that the spider breathes the atmosphere. The air is renewed in the cell in a manner not easily explained. The spider comes to the surface; a bubble of air is attracted to its body; with this air she descends, and gets under her cell, when the air is disengaged and rises into the cell; and thus, though under water, she lives in the air. There must be some peculiar property of the surface of this creature by which she can move in the water surrounded with an atmosphere, and live under the water breathing the air.

[Verse: "The spider taketh hold with her hands, and is in king's palaces."--PROVERBS XXX.]

[Illustration: Fig. 72.--WEB OF THE GEOMETRICAL SPIDER.]

The chief instrument by which the spider performs these wonders is the spinning apparatus. The matter from which the threads are spun is the liquid contained in cells; the ducts from these cells open upon little projecting teats, and the atmosphere has so immediate an effect upon this liquid, that upon exposure to it the secretion becomes a tough and strong thread. Twenty-four of these fine strands form together a thread of the thickness of that of the silk-worm. We are assured that there are three different sorts of material thus produced, which are indeed required for the various purposes to which they are applied--as, for example, to mix up with the earth to form the cells; to line these cells as with fine cotton; to make light and floating threads by which they may be conveyed through the air, as well as those meshes which are so geometrically and correctly formed to entrap their prey.--_Note by Lord Brougham to Paley's Natural Theology._

[Verse: "For every beast of the forest is mine, and the cattle upon a thousand hills."--PSALM L.]

1083. _Why have many insects a great number of eyes?_

Because the orb of the eye is fixed; there is therefore placed over the eye a multiple-lens, which conducts light to the eye from every direction; so that _the insect can see with a fixed eye as readily as it could have done with a movable one_. As many as fourteen hundred eyes, or inlets of light, have been counted in the head of a drone-bee. The spider has _eight eyes_, mounted upon different parts of the head; two in front, two in the top of the head, and two on each side.

1084. _Why have birds of prey no gizzards?_

Because their food _does not require to be ground_ prior to digestion, as does the food of grain-eating birds.

1085. _Why have earth worms no feet?_

Because the undulatory motion of their muscles serves them for fill the purposes of progression _needed by their mode of life_.

1086. _Why have mussels strong tendinous threads proceeding from their shells?_

Because as they live in places that are beaten by the surf of the sea, they _moor their shells_ by those threads to rocks and timbers.

1087. _Why have cockles stiff muscular tongues?_

Because, having no threads to moor themselves, as the mussels have, they _dig out with their tongues a shelter for themselves in the sand_.

1088. _Why do oxen, sheep, deer, &c., ruminate?_

Because they have no front teeth in the upper jaw, the place of which is occupied by a hardened gum. The first process, therefore, consists simply of _cropping_ their food, which is passed into the paunch, to _be brought up again and ground by the back teeth_ when the cropping process is over.

Because, in a wild state, they are constantly exposed to the attacks of carnivorous beasts, and as the mastication of the large amount of vegetable food required for their sustenance would take a considerable time, they are provided with stomachs, by which they are enabled to fill their paunches quickly, and then, retiring to a place of safety, they bring their food up again, and chew it at leisure.

[Verse: "A righteous man regardeth the life of his beast: but the tender mercies of the wicked are cruel."--PROVERBS XII.]

1089. _Why can ruminating animals recover the food from their paunches?_

Because they have a _voluntary power_ over the muscles of the throat, by which they can bring up the food at will.

1090. _Why can they keep the unchewed food in the paunch, from the "cud" they have chewed for nourishment?_

Because their stomachs are divided into three chambers: 1, the _paunch,_ where the unchewed food is stored; 2, the _reticulum_, where portions of the food are received from the paunch, and moistened and rolled into a "cud," to be sent up and chewed; and 3, the _psalterium_, which receives the masticated food, and continues the process of digestion.

In quadrupeds the deficiency of teeth is usually _compensated_ by the faculty of rumination. The sheep, deer, and ox tribe, are without fore-teeth in the upper jaw. These ruminate. The horse and ass are furnished with teeth in the upper jaw, and do not ruminate. In the former class, the grass and hay descend into the stomachs nearly in the state in which they are cropped from the pasture, or gathered from the bundle. In the stomach, they are softened by the gastric juice, which in these animals is unusually copious. Thus softened and rendered tender, they are returned a second time to the action of the mouth, where the grinding teeth complete at their leisure the trituration which is necessary; but which was before left imperfect. I say, the trituration which is necessary; for it appears from experiments, that the gastric fluid of sheep, for example, has no effect in digesting plants, unless they have been previously masticated; that it only produces a slight maceration, nearly as common water would do in a like degree of heat; but that when once vegetables are reduced to pieces by mastication, the fluid then exerts upon them its specific operation. Its first effect is to soften them, and to destroy their natural consistency; it then goes on to dissolve them, not sparing even the toughest parts, such as the nerves of the leaves. I think it very probable, that the gratification also of the animal is renewed and prolonged by this faculty. Sheep, deer, and oxen, appear to be in a state of enjoyment whilst they are chewing the cud. It is then, perhaps, that they best relish their food.--_Paley._

[Verse: "I am like a pelican of the wilderness: I am like an owl of the desert. I watch, and am as a sparrow alone upon the house top."--PSALM CII.]

CHAPTER LV.

1091. _Why do quadrupeds that are vegetable eaters feed so continually?_

Because their food contains but a _small proportion of nutrition_, so that it is necessary to digest a _large quantity_ to obtain sufficient nourishment.

1092. _Why do flesh eating animals satisfy themselves with a rapid meal?_

Because the food which they eat is _rich in nutritious matter_, and more readily digestible than vegetable food; it does not therefore, require the same amount of _grinding with the teeth_.

[Illustration: Fig. 73.--PELICAN WITH DILATED POUCH.]

1093. _Why has the pelican a large pouch under its bill?_

Because it subsists upon fish, generally of the smaller kind, and uses its pouch _as a net_ for catching them; the pouch also serves as a _paunch,_ in which the fish are stored, until the bird ceases from the exertion of fishing, and takes its meal at leisure.

[Verse: "And God created great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and God saw that it was good."--GENESIS I.]

In their wild state they hover and wheel over the surface of the water, watching the shoals of fish beneath, and suddenly sweeping down, bury themselves in the foaming waves; rising immediately from the water by their own buoyancy, up they soar, the pouch laden with the fish scooped up during their momentary submersion. The number of fish the pouch of this species will contain may be easily imagined when we state that it is so dilatable as to be capable of containing two gallons of water; yet the bird has the power of contracting this membranous expansion, by wrinkling it up under the lower mandible, until it is scarcely to be seen. In shallow inlets, which the pelicans often frequent, it nets its prey with great adroitness.

The pelican chooses remote and solitary islands, isolated rocks in the sea, the borders of lakes and rivers, as its breeding place. The nest, placed on the ground, is made of coarse grasses, and the eggs, which are white, are two or three in number. While the female is incubating, the male brings fish to her in his pouch, and the young, when hatched, are assiduously attended by the parents, who feed them by pressing the pouch against the breast, so as to transfer the fish from the former into the throats of the young. This action has doubtless given origin to the old fable of the pelican feeding its young with blood drawn from its own breast.--_Knight's Animal Kingdom._

1094. _Why do the smaller animals breed more abundantly than the larger ones?_

Because the smaller ones are designed to be the food of the larger ones, and are therefore _created in numbers adapted to that end_. An elephant produces but one calf; the whale but one young one; a butterfly lays six hundred eggs; silk-worms lay from 1,000 to 2,000 eggs; the wasp, 5,000; the ant, 4,000 to 5,000; the queen bee, 5,000 to 6,000, or 40,000 to 50,000 in a season; and a species of white ant (_termes fatalis_) produces 86,400 eggs in a day. Birds of prey seldom produce more than two eggs; the sparrow and duck tribe frequently sit upon a dozen; in rivers there prevail a thousand minnows for one pike; and in the sea, a million of herrings for a single shark; while of the animalcules upon which the whale subsists, there must exist hundreds of millions for one whale.

1095. _Why has the whale feathery-like laminæ of whale-bone extending from its jaws?_

Because these feathery bones, lying side by side, form a _sieve, or strainer_, for the large volumes of water which the whale receives into its mouth, drawing off therefrom millions of small animals, which form a jelly-like mass upon which the whale feeds. A whale has been known to weigh as much as 249 tons, and its blubber yielded 4,000 gallons of oil. How many millions of living creatures must have gone to make up that enormous mass of animal matter!

[Verse: "Hast thou given the horse strength? hast thou clothed his neck with thunder? * * He paveth the valley, and rejoiceth in his strength: he goeth on to meet the armed men."--JOB XXXIX.]

1096. _Why have cats, and various other animals, whiskers?_

The whiskers of cats, and of the cat tribe, are exceedingly sensitive, enabling them, when seizing their prey in the dark, to _feel its position most acutely_. These hairs are supplied, through their roots, with branches of the same nerves that give sensibility to the lips, and that in insects _supply their "feelers."_

1097. _Why has the horse a smaller stomach proportionately than other animals?_

Because the horse was created for speed. Had he the ruminating stomach of the ox, he would be quite unfitted for the labour which he now so admirably performs.

1098. _Why has the horse no gall-bladder?_

Because the rapid digestion of the horse, by which its fitness for speed is greatly increased, _does not require the storing up of the bile_ as in other animals in which the digestive process is a slower operation.

1099. _Why do certain butterflies lay their eggs upon cabbage leaves?_

Because the cabbage leaves are _the food of the young caterpillars_; and although the butterfly does not subsist herself upon the leaf, she knows by instinct that the leaf will afford food to her future young; she therefore lays her eggs where her young ones will find food.

This explanation applies to many insects that lay their eggs upon other plants.

1100. _Why have insects long projections from their heads, like horns or feathers?_

Because those organs (the _antennæ_), are those through which come insects _hear_ and others _feel_; and the projecting of these _antennæ_ from their bodies probably enables them to hear or feel more acutely while their wings are in motion, _without the interference of the vibrations of their wings_.

[Verse: "My son, eat thou honey, because it is good; and the honey-comb, which is sweet to thy taste."--PROVERBS XXIV.]

1101. _Why have bees stings?_

Because they gather and store up honey which would constantly attract other insects, and the bees would be robbed of their food but for the sting, _which is given to them for protection_.

1102. _Why have flies fine hairs growing at the extremities of their legs?_

Because they require to cleanse their bodies and wings, and to free them from particles of dust. And as they cannot turn their heads for this purpose, they have hairy feet, which serve as brushes, by which any part of their bodies can be reached and cleaned.

CHAPTER LVI.

1103. _Why when the perfume of flowers is unusually perceptible may wet weather be anticipated?_

Because when the air is damp it _conveys the odours of flowers_ more effectively than it does when dry.

1104. _Why when swallows fly low may wet weather be expected?_

Because the insects which the swallows pursue in their flight are flying low, _to escape the moisture of the upper regions of the atmosphere_.

1105. _Why do ducks and geese go to the water, and dash it over their backs on the approach of rain?_

Because by wetting the outer coat of their feathers before the rain falls, by sudden dashes of water over the surface, they _prevent the drops of rain from penetrating to their bodies through the open and dry feathers_.

1106. _Why do horses and cattle stretch out their necks and snuff the air on the approach of rain?_

Because they smell the _fragrant perfume_ which is diffused in the air by its increasing moistness.

[Verse: "I will remember the works of the Lord: Surely I will remember thy wonders of old."--PSALM LXXVII.]

1107. _Why may change of weather be anticipated when domestic animals are restless?_

Because their skins are exceedingly sensitive to atmospheric influences, and they are oppressed and irritated by _the changing condition of the atmosphere_.

1108. _Why may fine weather be expected when spiders are seen busily constructing their webs?_

Because those insects are highly sensitive to the state of the atmosphere, and when it is setting fine they build their webs, because they know instinctively _that flies will be abroad_.

1109. _Why is wet weather to be expected when spiders hide?_

Because it shows that they are aware that the state of the atmosphere does not _favour the flight of insects_.

1110. _Why if gnats fly in large numbers may fine weather be expected?_

Because it shows that they feel the state of the atmosphere to be favourable, which induces them all to _leave their places of shelter_.

1111. _Why if owls scream during foul weather, will it change to fine?_

Because the birds are pleasurably excited by a favourable _change in the atmosphere_.

1112. _Why is it said that the moping of the owl foretells death?_

Because owls scream when the weather is on the change; and when a patient is lingering on a death bed, the alteration in the state of the atmosphere frequently induces death, because the faint and expiring flame of life has not strength enough _to adapt itself to the change_.

1113. _Why may wet weather be expected when spiders break off their webs, and remove them?_

Because the insects, anticipating the approach of rain, remove their webs for preservation.

[Verse: "There shall the great owl make her nest, and lay, and hatch, and gather under her shadow: there shall the vultures also be gathered, every one with her mate."--ISAIAH XXXIV.]

1114. _Why may we expect a continuance of fine weather when bees wander far from their hives?_

Because the bees feel instinctively that from the state of the atmosphere they may wander far in search of honey, without the danger of being overtaken by rain.

1115. _Why if people feel their corns ache, and their bones rheumatic, may rain be expected?_

Because the dampness of the atmosphere affects its pressure upon the body, and causes a temporary disturbance of the system. All general disturbances of the body, _manifest themselves in those parts which are in a morbid state_--as in a corn, a rheumatic bone, or a decayed tooth.

1116. _Why if various flowers close may rain be expected?_

Because plants are highly sensitive to atmospheric changes, and _close their petals to protect their stamens_.

1117. _Why when moles throw up their hills may rain be expected?_

Because the moles know instinctively, that on the approach of wet, worms move in the ground; the moles therefore become active, _and form their hills_.

1118. _Why is a magpie, when seen alone, said to foretell bad weather?_

Because magpies generally fly in company; but on the approach of wet or cold, one _remains in the nest to take care of the young_, while the other one wanders alone in search of food.

1119. _Why do sea-gulls appear numerous in fine weather_?

Because the fishes swim near to the surface of the sea, and the birds _assemble over the sea to catch the fish, instead of sitting on rocks_, or wading on the shore.

1120. _Why do sea-gulls fly over the land, on the approach of stormy weather?_

Because in stormy weather they cannot catch fish; and the _earth-worms come up on the land_ when the rain falls.

[Verse: "And I said, Oh, that I had wings like a dove! for then would I fly away, and be at rest."--PSALM LV.]

1121. _Why if birds cease to sing, may wet, and probably thunder, be expected._?

Because birds are depressed by an unfavourable change in the atmosphere, and _lose those joyful spirits which give rise to their songs_.

1122. _Why if cattle run around in meadows, may thunder be expected?_

Because the electrical state of the atmosphere has the effect of making them feel uneasy and irritable, and _they chase each other about to get rid of the irritability_.

1123. _Why if birds of passage arrive early, may severe weather be expected?_

Because it shows that the indications of unfavourable weather have set in, in the latitudes from which the birds come, and that they have _taken an early flight to escape it_.

1124. _Why if the webs of the gossamer spider fly about in the autumn, may east winds be anticipated?_

Because an east wind is a dry and dense wind, and suitable to the flight of the gossamer spider; the spider feeling instinctively the dryness of the air, throws out its web, and finds it _more than usually buoyant upon the dense air_.

The observation of the changing phenomena which attend the various states of the weather is a very interesting study, though no general rules can be laid down that can be relied upon, because there are modifying circumstances which influence the weather in various localities and climates. To observe weather indications accurately, no phenomenon should be taken alone, but several should be regarded together. The character and the duration of the weather of the preceding days, the direction of the wind, the forms of the clouds, the indications of the barometer, the rise or fall of the thermometer, and the instinctive forewarnings of birds, beasts, insects, and flowers, should all be taken into account. Although no direct material advantages attend such a study, it induces a habit of observation, and develops the inductive faculty of the mind, which, when applied to more significant things, may trace important effects to their greater causes.

[Verse: "Go to the ant, thou sluggard; consider her ways, and be wise."--PROV. VI.]

CHAPTER LVII.

1125. _Why can gossamer spiders float through the air?_

Because, having no wings, and being deficient in the active muscular powers of other spiders, they have been endowed with the power of spinning a web which is so light that it floats in the air, and bears the body of the gossamer spider from place to place. Each web acts as a balloon, and the spider attached thereto is a little _aeronaut_.

1126. _Why do crickets make a peculiar chirping sound?_

Because they have hard wing cases, by the friction of the edges of which they cause their peculiar noise, _to make known to each other where they are_, in the dark crevices in which they hide.

[Illustration: Fig. 74.--GLOW-WORM USING HIS BRUSH.]

1127. _Why has the glow-worm a brush attached to its tail?_

Because it is necessary to keep its back very clean, that _the light which its body emits may not be dimmed_.

1128. _Why does the glow-worm emit a light?_

Because the female glow-worm is without wings, but the male is a winged insect. The female, therefore, is endowed with the power of displaying a phosphorescent light. The light is only visible by night, but it is, nevertheless, beautifully adapted for the purpose stated, because the _male is a night-flying insect_, and never ventures abroad by day.

[Verse: "They that go down to the sea in great ships, that do business in great waters these see the works of the Lord, and his wonders in the deep."--PSALM CVII.]

There exists some difference of opinion between naturalists upon the uses of the light of a glow-worm; there are some who doubt that it is exhibited to attract the flying insect. The objectors, however, offer no explanation of the luminous properties of the worm. Sir Charles Bell says the preponderance of the argument is decidedly in favour of the explanation we have given.

1129. _Why does not the iris of the fish's eye contract_?

Because the diminished light in water is _never too strong for the retina_.

1130. _Why is the eye of the eel covered with a transparent horny covering?_

Because, as the eel lives in holes, and pushes its head into mud, and under stones, &c., it needed such a covering to _defend the eye_.

1131. _Why is the whale provided with an eye, having remarkably thick and strong coats?_

Because, when he is attacked by the sword-fish and the shark, he is almost helpless against his enemies, as they fix themselves upon his huge carcase. He therefore dives with them down to a depth where the pressure of the water is so great that they cannot bear it. The eye of the whale is expressly organised _to bear the immense pressure of extreme ocean depths_, without impairing the sight.

1132. _Why have fishes no eyelids?_

Because the water in which they swim keeps their eyes moist. Eyelids would therefore be _useless to them_.

1133. _Why have fishes the power of giving their eye-balls very sudden motion?_

Because, having no eyelids (such organs being unnecessary to keep their eyes moist), they still need the power of freeing their eyes from the contact of foreign matters; and this is secured to them by the power they have of giving the eyeball a very rapid motion, which causes reaction in the fluid surrounding it, and _sweeps the surface_.

This motion may frequently be seen in the eyes of fishes, in glass globes.

[Verse: "And God made the beast of the earth after his kind, and cattle after their kind, and everything that creepeth upon the earth after his kind: and God saw that it was good."--GENESIS I.]

1134. _Why is the lachrymal secretion of the horse's eye thick and glutinous?_

Because, as his eye is large, and constantly exposed to dust on journeys, it is provided with a _viscid secretion_, which cleanses the eye, and more instantly and securely removes the dust, than a _watery_ secretion would.

1135. _Why does the lower bill of the sea-crow project beyond the upper one?_

Because the bird obtains his food by _skimming along the water_, into which he dips his bill, and lifts his food out.

1136. _Why do the mandibles of the cross-bill overlap each other?_

Because the bird requires a peculiar bill, to enable it to _split seeds into halves_, and to tear the open cones of the fir-tree.

1137. _Why are the tails of fishes so much larger than their fins?_

Because their tails are their _chief instruments of motion_, while their fins are employed simply to direct their progress, and steady their movements.

1138. _Why have oxen, and other quadrupeds a tough ligament called the "pax-wax," running from their backs to their heads?_

Because their heads are of considerable weight; and having frequent occasion to lift them, they are provided with an elastic ligament, which is fastened at the middle of their backs, while its other extremity is attached to the head. This enables them to raise their heads easily; otherwise the effort to do so would be a work of great labour. To the horse, the pax-wax acts as a natural bearing-rein, assisting it to hold its head in that position which adds to the grace and beauty of the animal.

In carving beef, this ligament may be seen passing along the vertebræ of the neck, the chuck, and the fore ribs.

[Verse: "He shall feed his flock like a shepherd; he shall gather the lambs with his arm, and carry them in his bosom, and shall gently lead those that are with young."--ISAIAH XL.]

1139. _Why have the females of the kangaroo and opossum tribes pouches, or pockets, formed in the skin of their breasts for the reception of their young?_

Because their young ones are remarkably _small and helpless_; in fact, more so than those of any other animal of equal proportions. Besides which, the full grown animals have very long hind-legs, and they progress by a series of extraordinary leaps. It would consequently be impossible for their helpless young ones to follow them: God has therefore given to female kangaroos and opossums curious pockets, _formed out of their own skin_, in which they place their little young ones, and bear them through their surprising leaps with the greatest ease and safety.

CHAPTER LVIII.

1140. _What is the difference between an animal, a plant, and a mineral?_

The great naturalist, Linnæus, used to say that animals _grow_, _live_, and _feel_; plants _grow_ and _live_; and minerals _grow_.

Animals are here defined to enjoy _three_ conditions of existence; plants _two_ conditions; and minerals _one_ condition.

This definition has, in latter days, been held to be unsatisfactory, since there _are a few plants_ that are _supposed to feel_, and _a few animals_ that are supposed to have even _less feeling_ than the _sensitive plants_ alluded to.

The concise definition by Linnæus, nevertheless, is true, as far as regards a _vast majority_ of the bodies constituting the three great kingdoms of nature. And it may be sufficient to say that

_Animals_--grow, live, feel, and move.

_Plants_--grow and live.

_Minerals_--grow, by the addition of particles of inorganic matter.

If we now state the few exceptions that are admitted to this definition, we shall bring the explanation as near to the truth, as the present state of knowledge will permit.

[Verse: "And God said, Behold, I have given you every herb bearing seed, which is upon the face of all the earth, and every tree, in the which is the fruit of a tree, yielding seed; to you it shall be for meat."--GENESIS I.]

1141. _Why is it understood that some plants feel?_

Because the _sensitive plant_ closes its leaves on being touched; the _Venus's fly trap_ closes its leaves upon flies that alight upon them; others _close_ upon the approach of rain, and at sunset, and _open_ at sunrise, and turn towards the sun during its daily transit.

1142. _Why is it understood that some plants move?_

Because certain _sea-weeds_ throw off undeveloped young plants, which move through the water by the aid of fine _cilia_, or muscular hairs, until they find a suitable place upon which to attach themselves.

The roots of plants will penetrate through the ground in the direction of water, and of favourable soil.

1143. _Of what elementary substances are plants composed?_

Of carbon, oxygen, hydrogen, and nitrogen.

1144. _Whence do plants derive those substances?_

From the air, the earth, and water.

1145. _How do plants obtain carbon?_

They obtain it chiefly from the air, in the form of _carbonic acid gas_. The carbon, of the carbonic acid gas, which is thrown out by the breath of animals, and by other processes in nature, is _absorbed by the leaves of plants_, and the _oxygen_ which had united with the carbon to form the _carbonic acid gas_, is again set free for the use of animals.

1146. _How do plants obtain oxygen?_

They obtain it from the _atmospheric air_. But as they do not require a large amount of oxygen for their own use, _they throw off the amount which is in excess_, after having separated it from the other elements with which it was combined when taken up by them. From the humble blade of grass, to the stately tree of the forest, plants operate to purify the air, and to correct and counteract the corruption of the air, by the myriads of animals inhabiting the earth.

It has been generally stated that plants in rooms purify the air by absorbing carbonic acid _by day_, and releasing a part of the oxygen; but that, as the presence of light is necessary to produce this action, they do not restore oxygen to the air, by night, but, on the contrary, give off carbonic acid gas. Therefore it has been stated that plants in rooms by night are unhealthy. Mr. Robert Hunt, one of the ablest chemists of the present time, makes the following remarks upon this subject in his "Poetry of Science:"--

[Verse: "The heavens declare the glory of God: and the firmament showeth his handy work. Day unto day uttereth speech, and night unto night showeth knowledge."--PSALM XIX.]

"The power of decomposing carbonic acid is a vital function which belongs to the leaves and bark. It has been stated, on the authority of Leibig, that during the night the plant acts only as a mere bundle of fibres--that it allows of the circulation of carbonic acid and its evaporation, unchanged. In his eagerness to support his chemical hypothesis of respiration, the able chemist neglected to enquire if this was absolutely correct. The healthy plant never ceases to decompose carbonic acid during one moment of its existence; but during the night, when the excitement of light is removed, and the plant reposes, its vital powers are at their minimum of action, and a much less quantity is decomposed than when a stimulating sun, by the action of its rays, is compelling the exertion of every vital function."

In hot, swampy countries, where vegetation is very rapid, and the soil loaded with decomposing carbonic matter, the plants absorb more carbonic acid than they require, and they _then_ evolve carbonic acid gas from their leaves. Hence such climates as the West Indies are injurious to _life_, though favourable to _vegetation_.

1147. _How do plants obtain hydrogen?_

They obtain _hydrogen_ in combination with _oxygen_ in water, and with _nitrogen_, in the form of _ammonia_, as which it exists in animal manures.

1148. _How do plants obtain nitrogen?_

From the _atmospheric air_, and from the _soil_, in which it is combined with other elements.

1149. _How do plants apply these elements to the formation of their own structures?_

When those substances which form the food of plants are absorbed, either by their leaves or their roots, they are converted, with the aid of water, into a _nutritive sap_, which answers the same purposes in _plants_ as _blood_ does in _animals_.

1150. _How is the nutritive sap applied to the growth and enlargement of the plant?_

Every seed contains a small amount of nutrition, sufficient for the sustentation of the _germ of the plant_, until those vessels are formed, by which the nutritive elements can be absorbed and used for the further development of the living structure.

The earth, penetrated by the sun's rays, warms the sleeping germ, and quickens it into life. For a short time the germ lives upon the seed, which, moistened and warmed by the soil, yields a kind of glutinous sap, out of which the first members of the plant are formed. And then the tender leaf, looking up to the sky, and the slender rootlet penetrating the soil, begin to draw their sustenance from the vast stores of nature.

[Verse: "He causeth the grass to grow for the cattle, and herb for the service of man: that he may bring forth food out of the earth."--PSALM CIV.]

1151. _Of what do vegetable structures consist?_

Of _membranes_, or thin tissues, which, being variously arranged, form cells, tubes, air passages, &c. Of _fibres_, which form a stronger kind of membrane, and which is variously applied to the production of the organs of the plants. And of _organs_, formed by those elementary substances, by which the plants absorb, secrete, and grow, and fulfil the conditions of their existence.

1152. _Why are seeds generally enveloped in hard cases?_

Because the covering of the seed, like the shell of an egg, is designed _to preserve the germ_ within from the influence of external agencies, until the time for development has arrived, and the conditions of germination are fulfilled.

1153. _Why does a seed throw out a root, before it forms a leaf?_

Because moisture, which the root absorbs from the earth, is necessary to enable the germ _to use the nutrition which the seed itself contains_, and out of which the leaf must be eliminated. Moisture forms a kind of gluten, in which the starch of the seed is dissolved, and converted into sugar, the sugar into carbonaceous sap, and the sap into cellular tissue and woody fibre, as the leaves present themselves to the influence of the air and light.

1154. _Why does a plant grow?_

Because, as soon as membranes and vessels are organised in the young germ, the nutritive fluid, formed by its first organs, _begins to move through the fine structures_, and from that time the plant commences to incorporate with its own substance the elements with which it is surrounded, that are suitable to its development.

[Verse: "Can the rush grow up without mire? can the flag grow without water? Whilst it is yet in his greenness, and not cut down, it withereth before any other herb."--JOB VIII.]

CHAPTER LIX.

1155. _Why, if we break the stem of a hyacinth, do we see a glutinous fluid exude?_

Because, by breaking the stem, we rupture the vessels of the plant, and cause the nutritive fluid to escape. The sap of the plant is _analogous to the blood of man_, and the vessels, to the arteries and veins of the animal body.

1156. _Why, if we split the petal of a tulip, do we see cells containing matter of various colours?_

Because, by splitting the petal of the flower, we disclose the anatomy of its structure, and bring to view those cells, or organs, of the vegetable body, by which _the different colouring matters are secreted_.

1157. _Why, if we break a pea-shell across, do we discover a transparent membrane which may be removed from the green cells underneath?_

Because we separate from the cellular, or fleshy part of the shell, the membrane, _which forms the epidermis_, and answers to the skin of the animal body.

1158. _Why, if we cut through a cabbage stump, do we find an outer coat of woody fibre, and an inner substance of cellular matter?_

Because the woody fibre _forms a kind of skeleton_, which supports the internal stricture of the plant, and gives form and character to its organisation. The woody fibre of plants is analogous to the bony structure of animal bodies.

1159. _Why, if we cut across the stem of a plant do we see numerous tubes arranged in parallel lines?_

Because we thereby bring to view _the vessels formed by the membranes and fibres_ of the vegetable body, for the transmission of the fluids, by which the structure is sustained.

[Verse: "It was planted in a good soil by great waters, that it might bring forth branches, and that it might bear fruit, that it might be a goodly vine."--EZEKIEL XVII.]

Skeleton leaves, and seed vessels of plants, form exceedingly interesting objects, and serve to illustrate the wonderful structure of plants. With patience and care, they may be produced by any person, and will afford an interesting occupation. The leaves should be gathered when they are in perfection--that is, when some of the earliest leaves begin to fall from the trees. Select perfect leaves, taking care that they are not broken, or injured by insects. Lay them in pans of _rain water_, and expose them to the air to undergo decomposition. Renew the water from time to time, taking care not to damage the leaves. They need not be examined more than once a week, and then only to see that the water is sufficient to cover them. Give them sufficient time for their soft parts to become decomposed, then take them out, and laying them on a white plate with a little water, wash away carefully, with a camel-hair pencil, the green matter that clings to the fibres. The chief requirement is _patience_ on the part of the operator, to allow the leaves and seed vessels sufficient time to decompose. Some leaves will take a few weeks, and others a few months, but a large panful may be put to decompose at the same time, and there will always be some ready for the process of cleansing. When they are thoroughly cleaned, they should be bleached, by steeping for a short time in a weak solution of chloride of lime. They should then be dried, and either pressed flat, or arranged in bouquets for preservation under glass shades. The result will amply reward the perseverance of the operator.

1160. _Why are clayey soils unfavourable to vegetation?_

Because the soil is _too close and adhesive_ to allow of the free passage of air or water to the roots of the plants; it also obstructs the expansion of the fibres of the roots.

1161. _Why are sandy soils unfavourable to vegetation?_

Because they consist of particles that have _too little adhesion to each other_; they do not retain sufficient moisture for the nourishment of the plants; and they allow too much solar heat to pass to the roots.

1162. _Why are chalk soils unfavourable to vegetation?_

Because they do not absorb the solar rays, _and are therefore cold to the roots of plants_.

1163. _Why are mixed soils favourable to vegetation?_

Because they contain the _elements of nutrition_ essential to the development of the vegetables, and the plants absorb from them those constituents which are necessary to their growth.

1164. _Why do farmers sow different crops in rotation?_

Because every plant takes something from the soil, and gives something back; but all kinds of plants do not absorb nor restore the elements in the same proportions. Therefore a succession of crops of one kind would soon impoverish the soil; but a succession of crops of different kinds will compensate the soil, in some degree, for the nourishment withdrawn.

[Verse: "He watereth the hills from his chambers; the earth is satisfied with the fruit of thy works."--PSALM CIV.]

1165. _Why do farmers manure their lands?_

Because, as soils vary, and crops impoverish the soils, the farmer employs manure _to restore fertility_, and to _adapt the soils to the wants of the plants_ he desires to cultivate.

It is remarkable that Nature herself points out to man the necessity for changing the succession of vegetable growths.

When plants have exhausted the soil upon which they grow, they will push their roots far in search of sustenance, and in time migrate to a new soil, while other plants will spring up and thrive upon the area vacated. When a forest in North America is destroyed by fire, the trees that grow afterwards are unlike those that the fire consumed, and evidently arise from seeds that have long lain buried in the earth, waiting the time when the ascendancy of the reigning order of plants should cease.

1166. _Why are grasses so widely diffused throughout nature?_

Because they form the _food_ of a very large portion of the animal kingdom. They have therefore been abundantly provided. No spot of earth is allowed to remain idle long. When the foot of man ceases to tread down the path, grass immediately begins to appear; and by its universality and the hardihood of its nature, it clothes the earth as with a carpet.

Many grasses, whose leaves are so dry and withered that the plants appear dead, revive and renew their existence in the spring by pushing forth new leaves from the bosom of the former ones.--_Withering's Botany._

Grasses are Nature's care. With these she clothes the earth; with these she sustains its inhabitants. Cattle feed upon their leaves; birds upon their smaller seeds; men upon the larger; for, few readers need be told that the plants which produce our bread-corn, belong to this class. In those tribes which are more generally considered as grasses, their extraordinary means and powers of preservation and increase, their hardiness, their almost unconquerable disposition to spread, their faculties of reviviscence, coincide with the intention of nature concerning them. They thrive under a treatment by which other plants are destroyed. The more their leaves are consumed, the more their roots increase. The more they are trampled upon, the thicker they grow. Many of the seemingly dry and dead leaves of grasses revive, and renew their verdure in the spring. In lofty mountains, where the summer heats are not sufficient to ripen the seeds, grasses abound which are viviparous, and consequently able to propagate themselves without seed. It is an observation, likewise, which has often been made, that herbivorous animals attach themselves to the leaves of grasses; and, if at liberty in their pastures to range and choose, leave untouched the straws which support the flowers.--_Paley._

[Verse: "For the earth bringeth forth fruit of herself; first the blade, then the ear, after that the full ear in the corn."--MARK V.]

CHAPTER LX.

1167. _Why do some plants droop, and turn to the earth after sunset?_

Because, when the warmth of the son's rays is withdrawn, they turn downwards, and _receive the warmth of the earth by radiation_.

1167. _Why does the young ear of corn first appear enfolded in two green leaves?_

Because the light and air would _act too powerfully for the young ear_; two leaves therefore join, and embrace the ear, and protect it until it has acquired strength, when they divide, and leave the ear to swell and ripen.

1168. _Why are the seeds of plants usually formed within the corollas of flowers?_

Because the petals of the flowers, surrounding the seeds, _afford them protection until they are ripened_, when the flower dies, and the petals fall to the ground.

1169. _Why does the flower of the poppy turn down during the early formation of seed?_

Because the heat would probably be too great for the seed in its early stage. The plant is therefore provided with a curious _curve in its stalk_, which turns the flower downward. But when the seeds are prepared for ripening, _the stalk erects itself_, and the seeds are then presented to the ripening influences of the sun.

1170. _Why have plants of the pea tribe, a folding blossom called the "boat," or "keel?"_

Because, within that blossom the pea is formed, and the shape of the blossom is exactly suited to that of the pea which is formed therein. The blossom is itself protected by external petals; and when the wind blows, and threatens to destroy the parts upon which the seeds depend, the plants _turn their backs to the wind_, and shelter the seed.

[Verse: "The fruit of the righteous is a tree of life; and he that winneth souls is wise."--PROVERBS XI.]

1171. _Why are the leaf buds enclosed in scales which fall off as the leaf opens?_

Because the scales _serve as a shelter_ to the tender structure of the young leaf. The scales are rudimentary leaves, formed at the end of the previous season, and which, being undeveloped then, serve to guard the young leaves of the future year.

In trees, especially those which are natives of colder climates, this point is taken up earlier. Many of these trees (observe in particular the _ash_ and the _horse-chestnut_) produce the embryos of the leaves and flowers in one year, and bring them to perfection the following. There is a winter therefore to be gotten over. Now what we are to remark is, how nature has prepared for the trials and severities of that season. These tender embryos are, in the first place wrapped up with a compactness, which no art can imitate; in which state they compose what we call the bud. This is not all. The bud itself is enclosed in scales; which scales are formed from the remains of past leaves, and the rudiments of future ones. Neither is this the whole. In the coldest climates, a third preservative is added, by the bud having a _coat_ of gum or resin, which, being congealed, resists the strongest frosts. On the approach of warm weather this gum is softened, and ceases to be an hindrance to the expansion of the leaves and flowers. All this care is part of that system of provisions which has for its object and consummation, the production and perfecting of the seeds.--_Paley._

1172. _Why are the seeds of many plants enclosed in a rich juice, or pulp?_

Because the matter by which the seed is surrounded, as well as being intended for the _nourishment and care of the seed_, is designed for the use of man and of animals, by whom the seed is set free to take its place in the earth.

By virtue of this process, so necessary, but so diversified, we have the seed, at length, in stone-fruits and nuts, incased in a strong shell, the shell itself enclosed in a pulp or husk, by which the seed within is, or hath been, fed; or, more generally (as in grapes, oranges, and the numerous kinds of berries), plunged overhead in a glutinous syrup, contained within a skin or bladder; at other times (as in apples and pears) embedded in the heart of a firm fleshy substance; or (as in strawberries) pricked into the surface of a soft pulp.

These and many more varieties exist in what we call _fruits_. In pulse, and grain, and grasses; seeds (as in the pea tribe) regularly disposed in parchment pods, which, though soft and membranous, completely exclude the wet even in the heaviest rains; the pod also, not seldom, (as in the bean), lined with a fine down; at other times (as in the senna) distended like a blown bladder; or we have the seed enveloped in wool (as in the cotton-plant), lodged (as in pines) between the hard and compact scales of a cone, or barricadoed (as in the artichoke and thistle) with spikes and prickles; in mushrooms, placed under a pent-house; in ferns, within slits in the back part of the leaf; or (which is the most general organisation of all) we find them covered by strong, close tunicles, and attached to the stem according to an order appropriated to each plant, as is seen in the several kinds of grains and of grasses.

[Verse: "And I will send grass in thy fields for thy cattle, that thou mayest eat, and be full."--DEUTERONOMY XI.]

In which enumeration, what we have first to notice is, unity of purpose under variety of expedients. Nothing can be more _single_ than the design; more _diversified_ than the means. Pellicles, shells, pulps, pods, husks, skin, scales armed with thorns, are all employed in prosecuting the same intention. Secondly; we may observe, that in all these cases, the purpose is fulfilled within a just and _limited_ degree. We can perceive, that if the seeds of plants were more strongly guarded than they are, their greater security would interfere with other uses. Many species of animals would suffer, and many perish, if they could not obtain access to them. The plant would overrun the soil; or the seed be wasted for want of room to sow itself. It is, sometimes, as necessary to destroy particular species of plants, as it is, at other times, to encourage their growth. Here, as in many cases, a balance is to be maintained between opposite uses. The provisions for the presentation of seeds appear to be directed, chiefly against the inconstancy of the elements, or the sweeping destruction of inclement seasons. The depredation of animals, and the injuries of accidental violence, are allowed for in the abundance of the increase. The result is, that out of the many thousand different plants which cover the earth, not a single species, perhaps, has been lost since the creation.

When nature has perfected her seeds, her next care is to disperse them. The seed cannot answer its purpose, while it remains confined in the capsule. After the seeds therefore are ripened, the pericarpium opens to let them out, and the opening is not like an accidental bursting, but for the most part, is according to a certain rule in each plant. What I have always thought very extraordinary; nuts and shells, which we can hardly crack with our teeth, divide and make way for the little tender sprout which proceeds from the kernel. Handling the nut, I could hardly conceive how the plantule was ever to get out of it. There are cases, it is said, in which the seed-vessel, by an elastic jerk, at the moment of its explosion, casts the seeds to a distance. We all, however, know, that many seeds (those of most composite flowers, as of the thistle, dandelion, &c.) are endowed with what are not improperly called _wings_; that is, downy appendages, by which they are enabled to float in the air, and are carried oftentimes by the wind to great distances from the plant which produces them. It is the swelling also of this downy tuft within the seed-vessel that seems to overcome the resistance of its coats, and to open a passage for the seed to escape.

But the _constitution_ of seeds is still more admirable than either their preservation or their dispersion. In the body of the seed of every species of plant, or nearly of every one, provision is made for two grand purposes: first, for the safety of the _germ_; secondly, for the temporary support of the future plant. The sprout, as folded up in the seed, is delicate and brittle beyond any other substance. It cannot be touched without being broken.

Yet in beans, peas, grass-seeds, grain, fruits, it is so fenced on all sides, so shut up and protected, that whilst the seed itself is rudely handled, tossed into sacks, shovelled into heaps, the sacred particle, the miniature plant remains unhurt. It is wonderful, also, how long many kinds of seeds, by the help of their integuments, and perhaps of their oils, stand out against decay. A grain of mustard-seed has been known to lie in the earth for a hundred years; and as soon as it had acquired a favourable situation, to shoot as vigorously as if just gathered from the plant. Then, as to the second point, the temporary support of the future plant, the matter stands thus. In grain, and pulse, and kernels, and pipins, the germ composes a very small part of the seed. The rest consists of a nutritious substance, from which the sprout draws its aliment for some considerable time after it is put forth; viz., until the fibres, shot out from the other end of the seed, are able to imbibe juices from the earth, in a sufficient quantity for its demand. It is owing to this constitution that we see seeds sprout, and the sprouts make a considerable progress, without any earth at all.

[Verse: "Say not ye, There are four months, and then cometh harvest? behold, I say unto you, Lift up your eyes, and look on the fields; for they are white already to harvest."--JOHN IV.]

From the conformation of fruits alone, one might be led, even without experience, to suppose, that part of this provision was destined for the utilities of animals. As limited to the plant, the provision itself seems to go beyond its object. The flesh of an apple, the pulp of an orange, the meat of a plum, the fatness of the olive, appear to be _more_ than sufficient for the nourishing of the seed or kernel. The event shows, that this redundancy, if it be one, ministers to the support and gratification of animal natures; and when we observe a provision to be more than sufficient for one purpose, yet wanted for another purpose, it is not unfair to conclude that both purposes were contemplated together.--_Paley._

1173. _Why have climbing plants tough curly tendrils?_

Because, _having no woody stalks of their own_ to support them, they require to take hold of surrounding objects, and raise themselves from the ground by climbing. Their spiral tendrils are, therefore, so many hands, assisting them to rise from the earth.

1174. _Why does the pea put forth tendrils, and the bean not?_

Because the bean has in its stalk _sufficient woody fibre to support itself_, but the pea has not. We do not know a single tree or shrub having a firm strong stem sufficient for its support which is _also_ supplied with tendrils.

1175. _Why do the ears of wheat stand up by day, and turn down by night?_

Because, when the ear is becoming ripe, the cold dew falling into the ear, might _induce blight_; the ears therefore turn down to the earth, and _receive warmth by radiation_.

1176. _Why have grasses, corn, canes, &c., joints, or knots in their stalks?_

Because a long hollow stem would be liable to bend and break. But the joints are so many points where the fibres are bound together, and the structure _greatly strengthened_.

[Verse: "Then shall the earth yield her increase; and God, even our own God, shall bless us."--PSALM XLVII.]

1177. _Why have the berries of the mistletoe a thick viscid juice?_

Because the mistletoe is a _parasitical_ plant, growing upon the bark of other trees. It will not grow in the ground; its seeds are therefore filled with an exceedingly sticky substance, which serves to attach them to the bark of trees, to which the berries attach themselves at once, by throwing out tough fibres; and the next year the plant grows.

[Illustration: Fig. 75.--THE MISTLETOE.]

1178. _How are the seeds of the mistletoe transferred from its own stem to the bark of trees?_

Various birds, and particularly the _missel thrush_, feed upon the berries. As the bird moves in pursuit of its food, the viscid berries attach themselves to its feathers, and in this way the thrush is the instrument which conveys the seed to the spot to which it adheres, and from which the tree ultimately grows.

1179. _What is the circulation of the sap in plants?_

The circulation of the sap is the movement of the nutritive juices by which the plant is sustained. There is a slow uninterrupted movement of the sap from the root through the stems to the leaves, and downwards from the leaves through the bark to the root.

[Verse: "For the sun is no sooner arisen with a burning heat, but it withereth the grass, and the flower thereof falleth, and the grace of the fashion of it perisheth: so also shall the rich man fade away in his ways."--JAMES I.]

1180. _Why does the sap of plants thus ascend and descend?_

Because it _conveys upward_ from the ground some of the matter by which the plant is to be nourished, and which must undergo digestion in the leaves; and it _brings downward_ from the leaves the matters absorbed, for the nourishment of the plant, and discharges through the root the substances which the plant cannot use.

The movement of the sap is most active in the spring; but in the depths of the winter it almost ceases.

There are other motions of the sap in plants, which are called _special_, in distinction from the ascending and descending of the sap, which is called _general_, or common to all plants. The special movements of the sap are peculiar to certain plants, in some of which a fluid, full of little green cells, is found to have a rotatory motion; in other plants, a milky fluid is found to move through particular tissues of the vegetable structure.

1181. _Why are the leaves of plants green?_

Because they secrete a carbonaceous matter, named _chlorophyll_, from which they derive their green colour.

1182. _Why are the hearts of cabbages, lettuces, &c., of a pale yellow colour?_

Because the action of _light_ is necessary to the formation of _chlorophyll_; and as the leaves are folded upon each other, they exclude the light, and the green matter is not formed.

1183. _Why do leaves turn brown in the autumn?_

Because, when their power of decomposing the air declines, the _oxygen_ absorbed in the carbonic acid gas, _lodges in the leaf_, imparting to it a red or brown colour.

1184. _Why do succulent fruits, such as gooseberries, plums, &c., taste acid?_

Because, in the formation of juices, a considerable amount of _oxygen_ is absorbed, and the oxygen imparts acidity to the taste.

[Verse: "The earth is full of the goodness of the Lord."--PSALM XXXIII.]

1185. _Why do ripe fruits taste sweet, and unripe fruits taste sour?_

Because the juices of the ripe fruit contain a large proportion of _sugar_, which in the unripe fruit has not been formed.

1186. _Why do some leaves turn yellow?_

Because they retain an excess of _nitrogen_. Leaves undergoing decay turn either yellow, red, crimson, or violet. Yellow is due to the excess of _nitrogen_; red and crimson to various proportions of _oxygen_; violet to a mixture of _carbon_; and green to _chlorophyll_.

1187. _Why do leaves fall off in the autumn?_

Because they have supplied for a season the natural wants of the tree. Every part has received nutrition through the spring and summer months; and the wants of the tree being supplied, the chief use of the leaf ceases, and it falls to the ground to decay, and enrich the soil.

1188. _Why do plants suffer from the smoke of cities?_

Because the smoke _injures the porous structure of the leaves_, and interferes with their free respiration.

CHAPTER LXI.

1189. _Why are vegetable productions so widely diffused?_

Because they everywhere form the _food of the animal creation_. Without them, neither man nor beast could exist. Even the flesh-eating animals are sustained by them, since they live by preying upon the bodies of vegetable-eaters.

They also enrich and beautify the earth. They present the most charming diversities of proportions and features. From the cowslip, the primrose, and the blue-bell of our childish days, to the broad oak under which we recline, while children gambol round us, they are all beautiful or sublime, and eminently useful in countless ways to man.

They spread a carpet over the surface of the earth; they cling to old ruins, and cover hard rocks, as though they would hide decay, and give warmth to the coldness of stone. In tropical climates they supply rich fruits full of cool and refreshing juices, and they spread out upon the crests of tall trees those broad leaves which shelter the native from the scorching heat of the sun.

They supply our dwellings with furniture of every kind, from the plain deal table, to the handsome cabinet of satin or rose-wood; they afford rich perfumes to the toilette, and luscious fruits and wines to the desert; they charm the eye of the child in the daisied field; they adorn the brow of the bride; they are laid in the coffin with the dead; and, as the cypress or the willow bend over our graves, they become the emblems of our grief.

[Verse: "The glory of the Lord shall endure for ever: the Lord shall rejoice in his works."--PSALM CIV.]

1190. _What is mahogany?_

Mahogany is the wood of trees brought chiefly from South America and Spain. The finest kind is imported from St. Domingo, and an inferior kind from Honduras.

We all know the beauty of mahogany wood. But we do not all know that mahogany was first employed in the repair of some of Sir Walter Raleigh's ships at Trinidad in 1597. The discovery of the beauty of its grain for furniture and cabinet work was accidental. Dr. Gibbons, a physician of eminence, was building a house in King-street, Covent-garden; his brother, captain of a West Indiaman, had brought over some planks of mahogany as ballast, and he thought that the wood might be used up in his brother's building, but the carpenters found the wood too hard for their tools, and objected to use it. Mrs. Gibbons shortly afterwards wanted a small box made, so the doctor called upon his cabinet-maker, and ordered him to make a box out of some wood that lay in his garden. The cabinet-maker also complained that the wood was too hard. But the doctor insisted upon its being used, as he wished to preserve it as a memento of his brother. When the box was completed, its fine colour and polish attracted much attention; and he, therefore, ordered a bureau to be made of it. This was done, and it presented so fine an appearance that the cabinet-maker invited numerous persons to see it, before it was sent home. Among the visitors was her Grace the Duchess of Buckingham, who immediately begged some of the wood from Mr. Gibbons, and employed the cabinet-maker to make her a bureau also. Mahogany from this time became a fashionable wood, and the cabinet-maker, who at first objected to use it, made a great success by its introduction.

1191. _What is rose-wood?_

Rosewood is the wood of a tree which grows in Brazil. It is, generally speaking, too dark for large articles of furniture, but is admirably adapted for smaller ones. It is expensive, and the hardness of the wood renders the cost of making articles of it very high.

[Verse: "I am come up to the height of the mountains, to the sides of Lebanon, and will cut down the tall cedars thereof, and the choice fir trees thereof."--II. KINGS XXIII.]

Respecting the other woods used in the manufacture of furniture, we have nothing special to say, except of the oak--the emblem of our native land. This tree yields a most useful and durable wood, and as it not only defends our country by supplying our "wooden walls," but gives to us the floors of our houses, furnishes our good substantial tables, and comfortable arm-chairs, it will be well for us to know a few facts about this celebrated tree. It is said that there are no less than one hundred and fifty species of the oak. The importance of the growth of oaks may be gathered from the fact, that the building of a 70-gun ship would take forty acres of timber. The building of a 70-gun ship is estimated to cost about £70,000. Oak trees attain to the age of 1,000 years. The oak enlarges its circumference from 10-1/2 inches to 12 inches in a year. The interior of a great oak at Allonville, in Normandy, has been converted into a place of worship. An oak at Kiddington, served as a village prison. A large oak at Salcey, was used as a cattle fold; and others have served as tanks, tombs, prisons, and dwelling-houses.

The _Mammoth tree_, which is exhibiting at the Crystal Palace, is one of the great wonders of the vegetable creation. It is the grand monarch of the Californian forest, inhabiting a solitary district on the elevated slopes of the Sierra Nevada, at 5,000 feet above the sea-level. From 80 to 90 trees exist, all within the circuit of a mile, and these varying from 250 to 320 feet in height, and from 10 to 20 feet in diameter. The bark is from 12 to 15 inches in thickness; the branchlets are somewhat pendent, and resemble those of cypress or juniper, and it has the cones of a pine. Of a tree felled in 1853, 21 feet of the bark from the lower part of the trunk were put in the natural form as a room, which would contain a piano, with seats for forty persons; and on one occasion 150 children were admitted. The tree is reputed to have been above 3,000 years old; that is to say, it must have been a little plant when Samson was slaying the Philistines. The portion of the tree exhibiting at the palace is 103 feet in height, and 32 feet in diameter at the base.

1192. _What is tea?_

Tea is the leaf of a shrub (_Thea Chinensis_). The plant usually grows to the height of from three to six feet, and resembles in appearance the well-known myrtle. It bears a blossom not unlike that of the common dog-rose. The climate most congenial to it is that between the 25th and 33rd degrees of latitude. The growth of good tea prevails chiefly in China, and is confined to a few provinces. The _green_ and _black_ teas are mere varieties, depending upon the culture, time of gathering, mode of drying, &c. _Coffee was used in this country before tea._ In 1664, it is recorded, the East India Company bought 2lb. 2oz. of coffee as a present for the king. In the year 1832, there were 101,687 licensed tea dealers in the United Kingdom. Green tea was first used in 1715. A dispute with America about the duty upon tea led to the American war, out of which arose American independence. The consumption of tea throughout the whole world is estimated at above 52,000,000 lbs., of which the consumption of Great Britain alone amounts to 30,000,000. (_See_ 1225).

[Verse: "Every man should eat and drink, and enjoy the good of all his labour, it is the gift of God."--ECCLESIASTES III.]

1193. _What is coffee?_

Coffee is the berry of the coffee plant, which was a native of that part of Arabia called Yemen, but it is now extensively cultivated in India, Java, the West Indies, Brazil, &c. (_See_ 1224).

The first coffee-house in London was opened in 1652, under the following circumstances. A Turkey merchant named Edwards, having brought along with him from the Levant, some bags of coffee, and a Greek servant who was skilful in making it, his house was thronged with visitors to see and taste this new beverage. Being desirous to gratify his friends without putting himself to inconvenience, he allowed his servant to open a coffee-house, and to sell coffee publicly.

Here we have another illustration of the great results springing from trifling causes. Coffee soon became so extensively used that taxes were imposed upon it. In 1660 a duty of 4d. a gallon was imposed upon all coffee made and sold. Before 1732 the duty upon coffee was 2s. a pound; it was afterwards reduced to 1s. 6d., at which it yielded to the revenue, for many years, £10,000 per annum. The duty has been gradually reduced, and the consumption has gone on increasing, until at last above 25,000,000 of pounds are consumed annually! Fancy this great result springing from a "friendly coffee party" that assembled in the year 1652.

1194. _What is chocolate?_

It is a cake prepared from the cocoa-nut. The nut is first roasted like coffee, then it is reduced to powder and mixed with water, the paste is then put into moulds and hardened. The properties are very healthful, but its consumption is very insignificant, as compared with tea or coffee. The cocoa tree grows chiefly in the West Indies and South America.

1195. _What is cocoa?_

Cocoa is also a preparation from the seeds or beans of the cocoa tree. But the best form of cocoa for family use is to obtain the beans pure, as they are now commonly sold ready for use, and to break them and then grind them in a large coffee mill.

1196. _What is chicory?_

Chicory is the root of the common endive, dried and roasted as coffee, for which it is used as a substitute. Some persons prefer the flavour of chicory admixed with coffee. But very opposite opinions prevail respecting the qualities of chicory. We believe it to be perfectly healthful, and attribute the prejudice that prevails against it, to its having been used, from its cheapness, to adulterate coffee.

[Verse: "He that tilleth the land shall have plenty of bread: but he that followeth after vain persons shall have poverty enough."--PROVERBS XXVIII.]

1197. _What is sugar?_

Sugar is a sweet granulated substance, which may be derived from many vegetable substances, but the chief source of which is the sugar cane. The other chief sources that supply it are the maple, beet-root, birch, parsnip, &c. It is extensively used all over the world. Sugar is supposed to have been known to the ancient Jews. It was found in the East Indies by Newcheus, Admiral of Alexander, 325 B.C. It was brought into Europe from Asia.

The art of sugar refining was first practised in England, in 1659, and sugar was first taxed by name by James II., 1685. Sugar is derived from the West Indies, Brazil, Surinam, Java, Mauritius, Bengal, Siam, the Isle de Bourbon, &c. &c. Before the introduction of sugar to this country, honey was the chief substance employed in making sweet dishes; and long after the introduction of sugar it was used only in the houses of the rich. The consumption in England in 1700 reached only 10,000 tons; in 1834 it had reached 180,000 tons. The English took possession of the West Indies in 1672, and in 1646 began to export sugar. In 1676 it is recorded that 400 vessels, averaging 150 tons, were employed in the sugar trade of Barbadoes. Jamaica was discovered by Columbus, and was occupied by the Spaniards, from whom it was taken by Cromwell, in 1656, and has since continued in our own possession. When it was conquered there were only three sugar plantations upon it. But they rapidly increased. Until the abolition of slavery in the West Indies, the production of sugar was almost exclusively limited to slave labour. (_See_ 1226).

1198. _What is wheat?_

Wheat, rye, barley, oats, millet, and maize, all belong to the natural order of grain-bearing plants. They all grow in a similar manner, and all yield starch, gluten, and a certain amount of phosphates. They are commonly spoken of as _farinaceous foods_.

[Verse: "I clothed thee also with broidered work, and shod thee with badgers' skin, and I girded thee about with fine linen, and I covered thee with silk."--EZEKIEL XVI.]

From the Sacred writings we learn that unleavened bread was common in the days of Abraham. In the earlier periods of our own history, people had no other method of making bread than by roasting corn, and beating it in mortars, then wetting it into a kind of coarse cake. In 1596, rye bread and oatmeal formed a considerable part of the diet of servants, even in great families. In the time of Charles the First, barley bread was the chief food of the people. In many parts of England it was more the custom to make bread at home than at present. In 1804, there was not a single public baker in Manchester. In France, when the use of yeast was first introduced, it was deemed by the faculty of medicine to be so injurious to health that its use was prohibited under the severest penalties. Herault says that, during the siege of Paris by Henry the Fourth, a famine raged, and bread sold at a crown a pound. When this was consumed, the dried bones from the charnel house of the Holy Innocents were exhumed, and a kind of bread made therefrom. Bread-street, in London, was once a bread market. From the year 1266, it had been customary to regulate by law the price of bread in proportion to the price of wheat or flour at the time. This was called the assize of bread; but, in 1815, it was abolished. In the year 272 there was a famine in Britain so severe that people ate the bark of trees; forty thousand persons perished by famine in England in 310! In the year 450 there was a famine in Italy so dreadful that people ate their own children. A famine, commencing in England, Wales, and Scotland, in 954, lasted four years. A famine in England and France, in 1193, led to a pestilential fever, which lasted until 1195. In 1315 there was again a dreadful famine in England, during which people devoured the flesh of horses, dogs, cats, and vermin! In the year 1775, 16,000 people died of famine in the Cape de Verds. These are only a few of the remarkable famines that have occurred in the course of history. Let us thank God that we live in times of abundance, when improved cultivation, the pursuit of industry, and the settlement of the laws, render such a calamity as a famine almost an impossibility.

1199. _What is cotton?_

Cotton is a species of vegetable wool, produced by the cotton shrub, called, botanically, _Gossypium herbaceum_, of which there are numerous varieties. It grows naturally in Asia, Africa, and America, and is cultivated largely for purposes of commerce.

The precise time when the cotton manufacture was introduced into England is unknown; but probably it was not before the 17th century. Since then, what wonderful advances have been made! The cotton trade and manufacture have become a vast source of British industry, and of commerce between nations. It was some years ago calculated that the cotton manufacture yielded to Great Britain one thousand millions sterling. The names of Hargreaves, Arkwright, Crompton, Cartwright, and others, have become immortalised by their inventions for the improvement of the manufacture of cotton fabrics. Little more than half a century has passed since the British cotton manufactory was in its infancy--now it engages many millions of capital--keeps millions of work people employed; freights thousands of ships that are ever crossing and re-crossing the seas; and binds nations together in ties of mutual interest. The present yearly value of cotton manufactures in Great Britain is estimated at £34,000,000. About £6,044,000 of the above sum is distributed yearly among working people as wages.

1200. _What is silk?_

Silk, though not directly a vegetable product, is, nevertheless, indirectly derived from the vegetable creation, since it is a thread spun by the silk-worm from matter which the worm derives from the _mulberry leaf_.

[Verse: "And there was a man in Maon, whose possessions were in Carmel; and the man was very great, and he had three thousand sheep, and a thousand goats: and he was shearing his sheep in Carmel."--I SAMUEL XXV.]

Silk is supplied by various parts of the world, including China, the East Indies, Turkey, &c., where the silk-worm has been found to thrive. The attempts that have been hitherto made to cultivate it in this country have proved unsuccessful. At Rome, in the time of Tiberius, a law passed the senate which, as well as prohibiting the wearing of massive gold jewels, also forbade the men to debase themselves by wearing silk. There was a time when silk was of the same value as gold--weight for weight--and it was thought to grow upon trees. It is recorded that silk mantles were worn by some noble ladies at a ball at Kenilworth Castle, 1286. It was first manufactured in England in 1604. In the reign of Elizabeth, the manufacture of silk in England made rapid strides. In 1666, there were 40,000 persons engaged in the silk trade. The silk throwsters of the metropolis were enrolled in a fellowship in 1562, and were incorporated in 1629. In 1685, a considerable impetus was given to the English silk manufactures. Louis the Fourteenth of France revoked the edict of Nantes. The edict of Nantes was promulgated by Henry the Fourth of France in 1598. It gave to the Protestants of France the free exercise of their religion. Louis the Fourteenth revoked this edict in 1685, and thereby drove the Protestants as refugees to England, Holland, and parts of Germany, where they established various manufactures. Many of these French refugees settled in Spitalfields, and there founded extensive manufactories, which soon rivalled those of their own country; and thus the intolerance of the king was justly punished. What important facts we see connected with the simple thread of the silk-worm!

1201. _What is wool?_

Wool is a kind of soft hair or coarse down, produced by various animals, but chiefly by sheep.

This is another of the useful productions of nature, for which we are indirectly indebted to the vegetable kingdom; for were it not for the rich pastures forming the green carpet of the earth, it would be impossible for man to keep large flocks of sheep for the production of wool. Wool, like the hair of most animals, completes its growth in a year, and then exhibits a tendency to fall off. For the production of wool in England and Wales it has been estimated that there are no less than 27,000,000 sheep and lambs; and, in Great Britain and Ireland, the total number is estimated at 82,000,000. Wool was not manufactured in any quantity in England until 1331, when the weaving of it was introduced by John Kempe and other artizans from Flanders. The exportation or non-exportation of wool has from time to time formed a vexed subject for legislators. Woollen clothes were made an article of commerce in the reign of Julius Cæsar. They were made in England prior to 1200. Blankets were first made in England in 1340. The art of dyeing wools was first introduced into England in 1608. The annual value of the raw material in wool is set down at £6,000,000; the wages of workmen engaged in the wool trade, £9,600,000. The number of people employed is said to be 500,000.

1202. _What is starch?_

Starch is one of the most useful products of the vegetable kingdom. As a rule, _a vegetable, if nutritious at all, is so according to the amount of starch which it contains_. It is most abundantly found in the seeds of plants, and especially in the _wheat_ tribe.

It is also met with in the cellular tissues of plants, and especially in such underground stems as the _potatoe_, _carrot_, _turnip_, _&c._, and the stems of the _sago-palm_ fig, &c. It is also found in the _bark_ of some trees.

[Verse: "Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning."--JAMES I.]

1203. _Why is the horse chestnut, though containing a great quantity of starch, unfit for food?_

Because (like many other vegetable productions) it contains with the starch an _acrid juice_, which renders it unhealthy; and although the juice can be separated from the starch, the process is too expensive to be made generally available.

The starch which is used for domestic purposes is an artificial preparation, and does not properly represent the starch of nutrition. A better idea of it is afforded by _the meal of a flowery potatoe_. The starch used by laundresses is frequently prepared from diseased potatoes. This does not impair the quality of the starch, for the purposes of the laundress, and the reason why potatoes that are diseased are thus applied is, that it is one method of saving some part of their value. The finest kinds of starch are prepared from rice. It is prepared by breaking the pulp, and disengaging the starch from the cells; and it is then put through other processes to remove the fragments of the broken cells. But in the flowery meal of the potatoe, the starch cell may be seen entire.

CHAPTER LXII.

1204. _What are vegetable oils and fats?_

Vegetable oils and fats constitute, next to starch and sugar, the most important secretion of the vegetable creation. There are very few plants from which some amount of oil cannot be obtained; and those which are famed for yielding it owe their celebrity rather to the abundance that they yield, and the peculiar qualities of their oil, than to the secretion of oil being rare--for probably there is no plant without it.

Oil is most commonly found in seeds, as _rape-seed_, _linseed_, &c., but it is found also in leaves, as in the rose, sweet-briar, peppermint, &c., where its presence may be recognised by the distinguishing perfume; and it is also found in the wood of a few trees, such as the sassafras and the sandal-wood; the bark frequently yields an oily secretion.

[Verse: "Ointment and perfume rejoice the heart; so doth the sweetness of a man's friend by hearty counsel."--PROVERBS XXVII.]

The London and North Western Railway Company alone use about 50,000 gallons of oil yearly.

1205. _Why are fat and oil found most abundantly in the bodies of animals in cold climates_?

Because they contribute to keep the _bodies of animals warm_, not only by their non-conducting property _keeping in_ the heat of the animals, but by supplying _carbon_ abundantly to combine with _oxygen_ during respiration, and thereby developing _animal heat_.

1206. _Why are oil and fat-forming trees found most abundantly in hot climates?_

Because, in hot countries, the formation of large quantities of fat in animal bodies would oppress living creatures with heat; fats and oils are, therefore, produced in those countries chiefly by vegetables, and are used externally by the Asiatics and Africans as an _external_ unction for _cooling the skin_, and as _perfumes_ which give inspiriting properties to the air, rendered oppressive by excess of heat.

1207. _Why are succulent fruits most abundant in tropical climates?_

Because they are rendered necessary in those climates by the _excessive heat_, and are found to have a most beneficial effect in cooling, purifying the blood of the inhabitants of tropical countries; while the grandeur of their foliage, and the richness of their flowers, are in perfect keeping with the intensity of light and heat, and serve, by throwing dense shades over the earth, to cool its surface, and to offer to living creatures a pleasant retreat from the rays of the burning sun.

The following sketch of _Botanical Geography_ should be read attentively after the reader has gone through the whole of the Chapters of "Reasons." The technical terms employed in the course of the article are nearly all explained at 1212, and should be committed to memory at the commencement of the perusal. _Mimosa_ means a sensitive plant; _concentric zones_, circular lines spreading from a centre; _arborescent_, resembling trees; _Gramineæ_, grass-like. The botanical names represent individual plants.

[Verse: "Blessed is the man that walketh not in the counsel of the ungodly, nor standeth in the way of sinners, nor sitteth in the seat of the scornful:"]

1208. When treating of the geographical distribution of vegetables, we have to mark the general arrangements indicated, and the agencies that have evidently operated in promoting the diffusion of floral tribes. Vegetation occurs over the whole globe, therefore, under the most opposite conditions. Plants flourish in the bosom of the ocean as well as on land, under the extremes of cold and heat in polar and equatorial regions, on the hardest rocks and the soft alluvium of the plains, amidst the perpetual snow of lofty mountains, and in springs at the temperature of boiling water, in situations never penetrated by the solar rays, as the dark vaults of caverns, and the walls of mines, as well as freely exposed to the influences of light and air. But these diverse circumstances have different species and genera. There is only one state which seems fatal to the existence of vegetable life--the entire absence of humidity.

1209. By species we understand so many individuals as intimately resemble each other in appearance and properties, and agree in all their permanent characters, which are founded in the immutable laws of creation. An established species may frequently exhibit new varieties, depending upon local and accidental causes, but these are imperfectly, or for a limited time, if at all, perpetuated.

1210. A genus comprises one or more species similar to each other, but essentially differing in formation, nature, and in many adventitious qualities from other plants. A tribe, family, group, or order, comprises several genera.

1211. The known number of species in the vegetable kingdom has been gradually enlarged by the progress of maritime and inland discovery; but owing to great districts of the globe not having yet been explored by the botanist, the interior of Africa, and Australia, with sections of America, Asia, and Oceanica, it is impossible to state the exact amount. The successive augmentation of the catalogue appears from the numbers below:

Species. Theophrastus 500 Pliny 1,000 Greek, Roman, and Arabian botanists 1,400 Bauhin 6,000 Linnæus 8,800 Persoon 27,000 Humboldt and Brown 38,000 De Candolle 56,000 Lindley 86,000 Hinds 89,000

1212. Vegetable forms are divided into three great classes which differ materially in their structure:--1. Cryptogamous plants--those which have no flowers, properly so called, mosses, lichens, fungi, and ferns: as distinguished from those which are phænogamous, or flower-bearing, to which the two following classes belong. 2. Endogenous plants, which have stems increasing from within, also called Monocotyledons, from having only one seed-lobe, as the numerous grasses, lilies, and the palm family. 3. Exogenous plants, which have stems growing by additions from without, also called Dicoteledons, from the seed consisting of two lobes, the most perfect, beautiful, and numerous class, embracing the forest trees, and most flowering shrubs and herbs.

1213. The exogens furnish examples of gigantic size, and great longevity. In South America on the banks of the Atabapo, Humboldt measured a _Bombax caiba_ more than 120 feet high, and 15 in diameter; and near Cumana, he found the _Zamang del Guayra_, a species of mimosa, the pendant branches of the hemispherical head having a circumference of upwards of 600 feet. The _Adansonia_, or baobab of Senegal, though attaining no great height, rarely more than fifty feet, has a trunk with a diameter sometimes amounting to 34 feet; while the _Pinus Lambertiana_, growing singly on the plains west of the Rocky Mountains, has been found 250 feet high, 60 feet in circumference at the base, 4-1/2 feet in girth at the height of 190 feet, yielding cones 11 inches round, and 16 long. The _Ficus Indicus_, or banian tree, sending out shoots from its horizontal branches, which reaching the ground take root, and form new stems till a single tree multiplies almost to a forest, has been observed covering an area of 1700 square yards.

[Verse: "He shall be like a tree planted by the rivers of water, that bringeth forth his fruit in season: his leaf also shall not wither; and whatsoever he doeth shall prosper."--PSALM I.]

1214. From the number of concentric zones observed in a transverse section of the stems De Caudolle advances proof of the following ages:

Elm 335 years. Cypress about 350 " Cheirostemon " 400 " Ivy 450 " Larch 576 " Orange 630 " Olive 700 " Oriental Plane " 720 " and upwards. Cedar of Lebanon " 800 " Oak 810, 1080, 1500 " Lime 1076, 1147 " Yew 1214, 1458, 2588, 2880 " Taxodium 4000 to 6000 " Baobab 5150 "

1215. Admitting, with Professor Henslow, that De Candolle overrated the ages of these trees one-third, they are examples of extraordinary longevity. Yew trees upwards of 700 years old remain at Fountains Abbey, Yorkshire, as there is historic evidence of their existence in the year 1133. But a yew in the churchyard of Darley-in-the-Dale, Derbyshire, is considered by Mr. Bowman as 2000 years old.

1216. The cryptogamous plants afford the most numerous examples of wide diffusion. A lichen indigenous in Cornwall, _sticta aurata_, is also a native of the West India Islands, Brazil, St Helena, and the Cape of Good Hope; while 38 lichens and 28 mosses are common to Great Britain and Australia, though the general vegetation of the two districts is remarkably discordant. Some species of endogenous plants are also widely distributed, the _Phleum alpinum_ of Switzerland occurring without the slightest difference at the Strait of Magellan, and the quaking grasses of Europe in the interior of Southern Africa. But only in very few instances are the same species of exogenous plants met with in regions far apart from each other; and generally speaking, in passing from one country to another, we encounter a new flora; for if the same genera occur, the species are not identical, while in districts widely separated the genera are different.

1217. The cryptogamic plants, mosses, lichens, ferns, and fungi, are to the whole mass of phænogamic vegetation in the following proportions in different districts: Equatorial latitudes, 0 deg. to 10 deg.; on the plains, 1-25th, on the mountains, 1-5th; mean latitudes, 45 deg. to 52 deg. 1/2; high latitudes, 67 deg. 70 deg., proportion about equal. Thus the proportion of the flowerless vegetation to the flowering increases from the equator to the poles. But the family of ferns, _filices_, viewed singly, forms an exception to this law, decreasing as we depart from equinoctial countries, being 1-20th in equatorial and 1-70th in mean latitudes, and not found at all in the high latitudes of the new world.

[Verse: "To give unto them beauty for ashes, the oil of joy for mourning, the garment of praise for the spirit of heaviness; that they might be called Trees of righteousness, The planting of the Lord, that he might be glorified."--ISAIAH LXI.]

1218. In equinoctial and tropical countries, where a sufficient supply of moisture combines with the influence of light and heat, vegetation appears in all its magnitude and glory. Its lower orders, mosses, fungi, and confervæ, are very rare. The ferns are aborescent. Reeds ascend to the height of a hundred feet, and rigid grasses rise to forty. The forests are composed of majestic leafy evergreen trees bearing brilliant blossoms, their colours finely contrasting, scarcely any two standing together being of the same species. Enormous creepers climb their trunks; parasitical orchidæ hang in festoons from branch to branch, and augment the floral decoration with scarlet, purple, blue, rose, and golden dyes. Of plants used by man for food, or as luxuries, or for medicinal purposes, occurring in this region, rice, bananas, dates, cocoa, cacao, bread-fruit, coffee, tea, sugar, vanilla, Peruvian bark, pepper, cinnamon, cloves, and nutmegs, are either characteristic of it as principally cultivated within its limits, or entirely confined to them.

1219. Rice (_Oryza-sativa_), the chief food of, perhaps, a third of the human race, is cultivated beyond the tropics, but principally within them, only where there is a plentiful supply of water. It has never been found wild; its native country is unknown; but probably southern Asia.

1220. Bananas, or plantains (_Musa sapientum et paradisiaca_), are cultivated in intertropical Asia, Africa, and America. The latter species occur in Syria. The banana is not known in an uncultivated state. Its produce is enormous, estimated to be on the same space of ground to that of wheat, as 133 to 1, and to that of potatoes as 44 to 1.

1221. Dates (_Phoenix dactylifera_), and cocoa (_Cocos nucifera_), belonging to the family _Palmæ_. The palms, remarkable for their elegant forms and importance to man, contribute more than any other trees to impress upon the vegetation of tropical and equinoctial countries its peculiar physiognomy. The date palm is a native of northern Africa, and is so abundant between the Barbary states and the Sahara, that the district has been named Biledul erid, the land of dates. As the desert is approached, the only objects that break the monotony of the landscape are the date palm, and the tent of the Arab. It accompanies the margin of the mighty desert in all its sinuosities from the shores of the Atlantic to the confines of Persia, and is the only vegetable affording subsistence to man that can grow in such an arid situation. The annual produce of an individual is from 150 to 260lbs. weight of fruit. The cocoa palm furnishes annually about a hundred cocoa-nuts. It is spread throughout the torrid zone; but occurs most abundantly in the islands of the Indian archipelago. The family of palms is supposed to contain a thousand species, some of large size, forming extensive forests.

1222. Cacao (_Theobrama cacao_), from the seeds of which chocolate is prepared, grows wild in central America, and is also extensively cultivated in Mexico, Guatemala, and on the coast of Cumana.

1223. Bread-fruit tree (_Artocarpus incisa_), a native of the South Sea Islands, and Indian archipelago, grows also in Southern Asia, and has been introduced into the tropical parts of America; but the fruit is not equal to the banana as an article of human food.

[Verse: "And they returned and prepared spices and ointments; and rested the Sabbath-day, according to the commandment."--LUKE XXIV.]

1224. Coffee (_Coffea Arabica_). The bush has probably for its native region the Ethiopian Highlands, from whence it was taken in the fifteenth century to the Highlands of Yemen, the southern part of the Arabian peninsula. It has been introduced, and is now extensively cultivated in British India, Java, Ceylon, the Mauritius, Brazil, and the West Indies, but the quality is inferior, which makes the climate of the Mocha coffee district of importance, as peculiarly favourable to the plant. It grows there on hills described by Niebuhr as being soaked with rain every day from the beginning of June to the end of September, which is carefully collected for the purpose of irrigation during the dry season. Forskhal gives the following temperatures in the district:

Boit el Fakih March 16, 7 A.M. 76 deg. 1 P.M. 95 deg. " " 18, " 77 " 95 Hodeida " 18, " 72 " 92-3/4 Bulgosa, a village in the hills " 20, " 69-1/2 " 85-1/2

Boit el Fakih 10 P.M. 81 deg. " " 81 Hodeida " 78 Bulgosa, a village in the hills " 73

1225. Tea (_Thea Chinensis_). The plant is indigenous in China, Japan, and Upper Assam. In the latter country, it has recently been found in a wild state, and is in process there of extensive cultivation. As the plant is hardy, its culture has very lately been attempted in the South of France, and apparently with complete success. A similar experiment on the burning plains of Algeria completely failed, all the plants being killed by the heat, notwithstanding every precaution. Tea was first introduced into Europe by the Dutch in 1666. The leaves of the coffee-plant have long been used as a substitute for tea, by the lower classes in Java and Sumatra; and recently, Professor Blume, of Leyden, exhibited samples of tea prepared from coffee-leaves, agreeing entirely in appearance, odour, and taste, with the genuine Chinese production.

1226. Sugar-cane (_Saccharum officinaram_), a species of _Gramineæ_, occurs to some extent without the tropics, having been cultivated centuries ago in Europe, as at present scantily in the South of Spain. But it properly belongs to the torrid zone, and has for its principal districts, the Southern United States, the West Indies, Venezuela, Brazil, the Mauritius, British India, China, the Sunda and Philippine Islands. The plant was found wild in several parts of America on the discovery of that continent, and occurs in a wild state on many of the islands of the Pacific.

1227. Vanilla (_Vanilla aromatica_), the fruit of which forms the well-known aromatic, grows wild principally in Mexico.

1228. Peruvian bark (_Cinchona officinalis_), a forest tree, of which there are several species, furnishing the valuable medicine so called. It is exclusively confined to South America, and grows chiefly on the Andes of Loxa and Venezuela.

1229. Pepper (_Piper nigrum_) belongs exclusively to the Malabar coast, where it has been found wild, Sumatra, which produces the greatest quantity, Borneo, the Malay peninsula, and Siam. Other species of _Piperaceoe_ occur in tropical America.

1230. Cinnamon (_Laurus Cinnamomum_), a small tree yielding the aromatic bark, is found native only in the island of Ceylon; but another species occurs in Cochin China.

[Verse: "I am the true vine, and my Father is the husbandman."--JOHN XV.]

1231. Clove (_Myrtus caryophyllus_), an evergreen small tree, the dried flower-buds of which form the celebrated aromatic, grows naturally in the Moluccas, whence it has been conveyed to other tropical districts. The island of Amboyna, one of that group, is the principal seat of its cultivation. The lowest temperature there is 72 degs.; the mean temperature of the year 82 degs.

1232. Nutmeg (_Myrstica moschata_) grows naturally in several islands of the eastern archipelago, but is principally cultivated in the Banda Isles.

Tropical families and forms successively vanish with an increase of distance from the equator, and new phases of vegetation mark the transition from hot to temperate climates. Vividly green meadows, abounding with tender herbs, replace the tall rigid grasses which form the impenetrable jungle; and instead of forests composed of towering evergreen trees, woods of the deciduous class appear, which cast their leaves in winter, and hybernate in the colder season, the oak, ash, elm, maple, beech, lime, alder, birch, and sycamore. The cultivation of the vine becomes characteristic, with the perfection of the cereal grasses, and a larger proportion of herbaceous annuals and cryptogamic plants.

1233. The vine (_Vitis vinifera_) is less impatient of a cold winter than a cool summer. Hence its northern limit, which coincides with lat. 47 deg. 30 min. on the west coast of France, rises in the interior, where, though the winters are colder, the summers are warmer, to lat. 49 degs., cuts the Rhine at Coblentz in lat. 50 deg. 20 min., and ascends to 52 deg. 31 min. in Germany.

1234. Receding further from the equator, magnificent forests of the fir and pine tribe prevail, as in the central parts of Russia, on the southern shores of the Baltic, in Scandinavia, and North America. But some of the cereals are no longer cultivatable, and several timber-trees common to the temperate zone do not reach its northern limits. Gradually all ligneous vegetation disappears entirely as higher latitudes are approached, the woods having first dwindled to mere dwarfs in struggling with the elements, hostile to that state which nature destined them to assume. The limit of the forests is a sinuous line running along the extreme north of the old world; and extending from Hudson's Bay, lat. 60 deg., to the Mackenzie River, lat. 68 deg., and thence to Behring's Strait. The dwarf birch (_Betula nana_), a mere bush, is the last tree found on drawing near the eternal snow of the pole. At the island of Hammerfest, lat. 70 deg. 40 min., near the North Cape, it rises to about the height of a man, in sheltered hollows between the mountains, its lower branches trailing on the ground, affording a shelter to the ptarmigan. In the polar zone, some low flowering annuals, saxifrages, ranunculi, gentians, chickweeds, and others, flourish during the brief ardent summer; a few perennials also accommodate themselves to the rigorous climate by spreading laterally, never rising higher than four or five inches from the ground; till finally no development of vegetable life is met with, but lichens, and the microscopic forms that colour the snow.

1235. In Europe, wheat ceases with a line connecting Inverness in Scotland, lat. 58 deg., Drontheim in Norway, lat. 64 deg., and Petersburgh in Russia lat. 60 deg. 15 min. Oats reach a somewhat higher latitude. Barley and rye ascend to lat. 70 deg., but require a favourable aspect and season to produce a crop.

1236. The northern limit of the growth of oak, lat. 61 deg., falls short of that of wheat. The oak makes a singular leap at the confines of Europe and Asia, disappearing towards the Ural mountains. This is the case also with the wild-nut and apple. The oak and the wild-nut, however, re-appear suddenly in Eastern Asia, on the banks of the Argoun and the Amour; and the apple occurs again in the Aleutian Isles.

[Verse: "He hath made the earth by his power, he hath established the world by his wisdom, and hath stretched out the heavens by his discretion."--JEREMIAH X.]

1237. The following are the northern limits of several trees in Scandinavia:

Lat. Beech, _Fagus silvatica_ 60 deg. 0 min. Hard Oak, _Quercus robur_ 61 " 0 " Common Elm, _Ulmus campestris_ 61 " 0 " Common Lime, _Tilia communis_ 61 " 0 " Common Ash, _Fraxinus excelsior_ 62 " 0 " Fruit trees 63 " 0 " Hazel, Corylus, _avellana_ 64 " 0 " Spruce Fir, _Abies excelsa_ 67 " 40 " Service Tree, _Sorbus aucuparia_ 70 " 0 " Scotch Fir, _Pinus silvestris_ 70 " 0 " White Birch, _Betula alba_ 70 " 40 " Dwarf Birch, _Betula nana_ 71 " 0 "

1238. Thus distinct vegetable regions are observed on passing from south to north through different climatic zones, defined as to their limits by the isothermal curves, and not by the parallels of latitude. Similar changes of vegetation mark a perpendicular transit through varying climates. A succession of plants appear on the tropical mountains which rise above the snow line, corresponding to those which are encountered in mean and high latitudes. The higher we ascend, the more does the number of the phænogamic class diminish in proportion to the cryptogamic, till only members of the latter family are found, whose further progress upward is arrested by the everlasting snow. The last lichen met with by Saussure on Mont Blanc, _Silene acaulis_, was also observed by M. Brevais in the neighbourhood of Bosekop, lat. 69 deg. 58 min. where it was vegetating on the seashore, shaded by the last pines of Europe.

1239. Isolated mountains display to the best advantage the effort of climatic change of vegetation.

1240. Etna is divided into three great regions: _La Regione Culta_, or fertile region; _La Regione Sylvosa_, or woody region; _La Regione Deserta_, the bare or desert region. But each of these is susceptible of sub-divisions, defined by the presence of certain families of plants, forming seven botanical zones.

1. The sub-tropical zone, which does not rise more than 100 feet above the level of the sea, is characterised by the palm, banana, Indian fig, sugar-cane, varieties of mimosa and acacia, which with us are only found in conservatories.

2. The hilly zone, rises about 2,000 feet, characterised by the orange, lemon, shaddock, maize, cotton, and grape plants.

3. The woody zone lies between the height of 2,000 and 4,000 feet, where the cork-tree flourishes, several kinds of oak, the maple, and enormous chestnuts.

4. The zone between the height of 4,000 and 6,000 feet is distinguished by the beech, Scotch fir, birch, and, among small plants, by clover, sandwort, chickweed, dock, and plantain.

5. The sub-alpine zone, between the elevation of 6,000 and 7,500 feet, produces the barberry, soap-wort, toad-flax, and juniper.

6. The zone between 7,500 and 9,000 feet, has almost all the plants of the preceding, with the fleshy and jagged groundsel.

[Verse: "In the mountain of the height of Israel will I plant it; and it shall bring forth boughs, and bear fruit, and be a goodly cedar: and under it shall dwell all fowl of every wing; in the shadow of the branches thereof shall they dwell."--EZEKIEL XVII.]

7. The narrow zone between 9,000 and 9,200 feet, only produces a few lichens, beyond which, there is complete sterility.

1241. The Peak of Teneriffe exhibits five botanical districts, thus distinguished by Von Buch:

1. The region of Africa forms, 0--1,248 feet, comprising palms, bananas, the sugar-cane, various species of arborescent _Euphorbiæ_, _Mesembryanthema_, the _Dracæna_, and other plants, whose naked and tortuous trunks, succulent leaves, and bluish-green tints, are distinctive of the vegetation of Africa.

2. Region of Vines and Cereals, 1,248--2,748 feet, comprising also the olive, and the fruit-trees of Europe.

3. Region of Laurels, 2,748--4,350 feet, including lauri of four species, the wild olive, an oak, the iron-tree, the arbutus, and other evergreens. The ivy of the Canaries and various twining shrubs cover the trunks of the trees, and numerous species of fern occur, with beautiful flowering plants.

4. Region of the Pines, 4,350--6,270, characterised by a vast forest of trees resembling the Scotch fir, intermixed with juniper.

5. Region of the Retama, 6,270--11,061 feet, a species of broom, which forms oases in the midst of a desert of ashes, ornamented with fragrant flowers, and furnishing food to the goats, which run wild on the Peak. A few gramineous and cryptogamic plants are observed higher, but the summit is entirely destitute of vegetation.

1242. There are many plants which can accommodate themselves to the most diverse climates and localities; and therefore ascend from the plains close to the boundary of vegetable life on the highest mountains. But it is the general law in these cases for such plants to be singularly modified in appearance and anatomical structure as they ascend. The spring gentian, _Gentiana verna_, is one of the exceptions, which Raymond found unaltered at all heights in the Pyrenees.

1243. Trees, plants, and bushes, of humbler growth, which occur on the plains and at great heights, are usually much smaller in the latter situation. The leaves, and everything green about them, dwindle with the increased elevation; and the pure, well defined green is exchanged for an ill-defined light yellow. Singular enough, those parts which seem most capable of resisting cold, as the leaves and stalks, are uniformly subjected to a diminution of their vital functions; while the flowers remain of the same size, are never deformed, and become more dense and richer in their colours. While the _Myosotis silvestris_ becomes stunted, its flowers assume an intense blue--the admiration of the traveller. The flowers of the pale primrose have a much deeper colour on the top of the Faulhorn, while the plant itself is much smaller than its congener on the Swiss plains. The observations of M. Parrot, among others, are to this effect on the flora of the Caucasus, of Ararat, the Swiss and Italian Alps, and the Pyrenees. The arctic flora is similarly distinguished.

1244. The preceding references to different climatic states are, however, perfectly inadequate to explain the phenomena of vegetable distribution. While an analogy is often observable between the plants of different regions under corresponding circumstances of latitude, elevation, and soil, the species are generally found to be different; and usually the botanical character of countries not widely apart from each other, is totally different, though under the same parallels.

[Verse: "From the rising of the sun, unto the going down of the same, the Lord's name is to be praised."--PSALM CXIII.]

1245. Some plants are entirely confined to one side of our planet. The beautiful genus _Erica_, or heath, of which there are upwards of 300 species, occurs with breaks over a narrow surface, extending from a high northern latitude to the Cape of Good Hope. But the whole continent of America does not contain a single native specimen; nor has a _Poenia_ been found in it, except a solitary one to the west of the Rocky Mountains. On the other hand, the New World contains many families, as the _Cacti_, which are not found naturally in the Old.

1246. Some plants occur in a single specific locality, frequently a contracted area, and nowhere else. The beautiful _Disa grandiflora_ is limited to a spot on the top of the Table Mountain at the Cape; and the celebrated cedar of Lebanon appears to be restricted in its spontaneous growth to the Syrian mountains. The small island of St. Helena has an indigenous flora, with a few exceptions different from that of the rest of the globe.

1247. Mountain chains of no great width very commonly divide a totally distinct botany. There is a marked difference in the vegetation of the Chilian and opposite side of the Andes, though the climate as well as the soil is nearly the same, and the difference of longitude very trifling. In North America, two completely different classes of vegetation appear on the two sides of the Rocky Mountains. A variety of oaks, palms, magnolias, azaleas, and magnificent rhododendrons occur on the eastern side, all of which are unknown on the western, the region of the giant pine.

1248. The distinct vegetation possessed by various parts of the globe, has led to its division into botanical kingdoms or phyto-geographical regions, named in general after the genera that are either peculiar to them, or predominant in them. The arrangement of M. Schouw, which is usually adopted, discriminates twenty-five great provinces of characteristic vegetation upon the surface of the earth.

In constituting any portion of the globe into a phyto-geographical region, M. Schouw has proceeded upon the following principles:--1. That at least one-half of the species should be indigenous in it. 2. That a-quarter of the genera should also be peculiar to it, or at least should have a decided maximum. 3. That individual families of plants should either be exclusively confined to the region, or have their maxima there.

1249. The phenomena of botanical geography, and the facts of geology, are mutually illustrative. The existing dry land having been upheaved above the waters at different epochs, it may be reasonably inferred that each portion on its emergence received a vegetable creation in harmony with its position. The ultimate constitution of the general surface into different botanical kingdoms would hence follow, each of which has preserved its primitive features, while adjoining, and even far distant foci, have to some extent intermingled their respective products, under control of the natural agencies of diffusion.

1250. The agents that involuntarily officiate in the diffusion of vegetable products are the atmosphere, the waters, and many animals.

1. The impulsion of the atmosphere in its calmest state, is quite sufficient to transport to considerable distances seeds furnished with downy appendages or winglets, as is the case with many plants, with the minute sporules of cryptogamia, which are light as the finest powder. When ordinary breezes convey the sand-dust of the Sahara a thousand miles or more from the desert, it may be conceived that seeds, which are comparatively heavy, are borne far from home by the hurricane. Two Jamaica lichens, which had never been seen in France before, were found by De Candolle growing on the coast of Brittany, the offspring of sporules which had been swept over the Atlantic.

[Verse: "He shall come down like rain upon the mown grass, as showers that water the earth."--PSALM LXXII.]

2. The mountain torrent washes down into the valley the seeds that have accidentally fallen into it, or have been swept away by its overflows; and hence the plants of the High Alps occur on the plains of Switzerland, which are entirely wanting in France and Germany. Rivers answer the same purpose more extensively, and also the oceanic currents. The nicker-tree, one of the leguminous tribe, has been raised from seed borne across the Atlantic by the Gulf stream.

3. Animals of the sheep and goat kinds, with the horse, deer, buffalo, and others, widely disperse several species of plants, the seeds of which, furnished with an apparatus of barbs and hooks, adhere to their coating. Seeds also of various kinds pass through the digestive organs of birds, uninjured as to their vitality. The little squirrel buries the acorn in the ground for winter provender, and sows an oak, if prevented from returning to the spot.

1251. Plants capable of extended naturalisation, and serviceable as articles of food or luxury, have been widely disseminated by the human race in their migrations. The cerealia afford a striking example. These important grasses known to the ancients, wheat, barley, oats, and rye, were the gifts of the Old World to the New. They are also importations into Europe; but the loose reports of the ancients, and the diligent researches of the moderns, alike leave us in ignorance of their native seat. Probability points to the conclusion that they have spread from the neighbourhood of the great rivers of Western Asia, the primitive location of the human family; and it is not impossible that in that imperfectly explored district, or further east on the Tartarian table-land, some of the cereals may yet be found growing spontaneously. The first wheat sown in North America, consisted of a few grains accidentally found by a negro slave of Cortes, among the rice taken for the support of his army. In South America the first wheat was brought to Lima by one of the early colonists, a Spanish lady, Maria d'Escobar. An ecclesiastic, Jose Rixi, was the first to sow it in the neighbourhood of Quito.

1252. Maize, or Indian corn (_Zea mays_), has been dispersed in the Old World from the New; and also a more important product, the potato (_Solanum tuberosum_), the use of which now extends from the extremity of Africa to Lapland. In Chili, the native country of the plant, it occurs at present in a wild state. The Spaniards imported it into Spain, and from thence it was communicated to Italy. It was first made known in England at a subsequent period from Virginia, having been received there from the Spanish colonists in South America, as it is not a native of intervening Mexico.

1253. The grape-vine, so extensively spread over Europe, is probably not indigenous in any part of it. It chiefly owes its diffusion there to the Romans, who received it from the Greeks, to whom it most likely immediately came from the country between the Black and Caspian Seas. The Romans introduced most of the finer European fruit-trees, some from Africa, as the pomegranate, but the great majority from Western Asia, as the orange, fig, cherry, peach, apricot, apple, and pear. A variety of the plum, the damson, or damascene, came from the neighbourhood of Damascus during the Crusades. The name of the damask-rose points to the importation of the plant from the same quarter into Europe.

[Verse: "To every thing there is a season, and a time to every purpose under heaven."--ECCLESIASTES III.]

The ocean as well as the land has different botanical regions; and changes of the vegetation are observed with the depth analogous to the variations of terrestrial plants with the height. Marine vegetation seems to have its vertical extent determined by the range of light in water, which varies with the power of the sun and the transparency of the water.

CHAPTER LXIII.

1254. _What are vegetable gums?_

Vegetable gums are secretions of plants which are generally _soluble in water_, and which subserve various useful purposes. _Gum Arabic_ is one of the most important of this class of vegetable productions.

_Gutta-percha_ is an invaluable substance lately added to the list of known vegetable productions. It is obtained by cutting the bark of trees of the class called _Sapotacea_. Its proper name is gutta Pulo Percha, gutta meaning gum, and Pulo Percha is the island whence it is obtained. But gutta-percha is not, strictly speaking, a gum.

_India-rubber_ is also a vegetable secretion, improperly called elastic gum. It is obtained from the milky juice of various trees and plants, especially from the syringe tree, of Cayenne.

1255. _What are vegetable resins?_

_Vegetable_ resins are derived from the secretions of plants, and are generally distinguished from gums by being _insoluble in water_, but being soluble in spirits.

When one of these substances is soluble in either water or spirits it is called a _gum-resin_.

1256. _What are vegetable acids?_

Vegetable acids are chiefly obtained from _fruit_; but also abundantly from _wood_, by distillation.

[Verse: "Thou art the God that doest wonders."--PSALM LXXVII.]

1257. _What is tannin?_

Tannin is a vegetable production, obtained chiefly from the oak-bark, and from a variety of other vegetable sources. It possesses the peculiar chemical property which renders it valuable in tanning leather.

1258. _What is opium?_

Opium is the produce of the _poppy_, and is obtained from the seed.

1259. _What are vegetable dyes?_

Vegetable dyes are the various colours derived from the secretions of plants, such as _indigo_, _madder_, _logwood_, _alkanet-root_, _&c._

1260. _What is silica?_

Silica is a mineral substance, commonly known as _flint_; and it is one of the wonders of the vegetable tribes, that, although flint is so indestructible that the strongest chemical aid is required for its solution, plants possess the power of _dissolving and secreting_ it. Even so delicate a structure as the wheat straw dissolves silica, and every stalk of wheat is covered with a perfect, but inconceivably thin coating of this substance.

Amid all the wonders of nature which we have had occasion to explain, there is none more startling than that which reveals to our knowledge the fact that a flint stone consists of the mineralised bodies of animals, just as coal consists of masses of mineralised vegetable matter. The animals are believed to have been infusorial animalculæ, coated with silicous shells, as the wheat straw of to-day is clothed with a glassy covering of silica. The skeletons of animalculæ which compose flint may be brought under microscopic examination. Geologists have some difficulty in determining their opinions respecting the relation which these animalculæ bear to the flint stones in which they are found. Whether the animalculæ, in dense masses, form the flint; or whether the flint merely supplies a sepulchre to the countless millions of creatures that, ages ago, enjoyed each a separate and conscious existence, is a problem that may never be solved. And what a problem! The buried plant being disentombed, after having lain for ages in the bowels of the earth, gives us light and warmth; and the animalcule, after a sleep of ages, dissolves into the sap of a plant, and wraps the coat it wore, probably "in the beginning, when God created the heavens and the earth, and when the earth first brought forth living creatures," around the slender stalk of waving corn!

1261. _Why is silica diffused over the stems of wheat, grasses, canes, &c.?_

Because it affords strength, density, and durability, to structures that are very light, and which, but for this beautiful provision, would be exceedingly perishable.

[Verse: "For in this mountain shall the hand of the Lord rest, and Moab shall be trodden down under him, even as straw is trodden down for the dunghill."--ISAIAH XXV.]

1262. _Why is guano a productive manure?_

Because it contains, with other suitable elements, an abundance of the _silicous skeletons of animalculæ_.

1263. _Why does a wheat-crop greatly exhaust the soil?_

Because, as well as the _carbon, and the salts_, which form the straw and the grain, it draws off from the soil a great amount of _silica_.

1264. _Why is straw frequently used as a manure?_

Because it gives back, with other substances, a _considerable proportion of silica_, in that form which adapts it to the use of the succeeding crop.

1265. _Why is the structure of herbaceous plants less consolidated than that of woody plants?_

Because, for the most part, herbaceous plants last only _a single year_; they, therefore, do not require the enduring qualities of plants that have to sustain the influences of the elements for a succession of seasons.

1266. _Why are the stalks of plants of light structure generally cylindrical?_

Because the cylindrical form is stronger than any other; _a hollow cylinder_, with moderately thick walls, _is stronger than a solid rod_, containing the same amount of material.

1267. _Why do the stalks of plants become hollow?_

Because the parallel and perpendicular fibres of the stalk are developed _more rapidly than the horizontal_. The growth of the plant, therefore, consists of a kind of _divergence from the centre_.

1268. _Why are the stomata, or pores of leaves, generally placed on their under surface?_

Because, being placed on the under surface, they are _shaded_ from the action of the _sun's rays_, and so carry on the function of respiration more actively than if subjected to direct heat; they are also protected from the injurious _effects of dust_; and are moistened by _evaporation from the earth's surface_.

[Verse: "The trees of the Lord are full of sap: and the cedars of Lebanon which he hath planted."--PSALM CIV.]

1269. _Why have plants a formation of pith in their centre?_

The pith is the chief organ of nutriment, especially in the young plant. It is the structure which first conveys fluids to, and receives them from, the newly-formed leaf. It communicates with every branch, leaf, bud, and flower; and also with the bark, through the _medullary rays_, which radiate horizontally from the centre of the plant. It is the centre of the movements of the sap which occur in the horizontal vessels; and it holds an important influence over the life of the plant.

1270. _Why are trees covered with bark?_

Because the bark serves to protect the woody structure, and also to give a passage to the descending sap which flows abundantly in the spring, and out of which the woody fibre is formed. It is also, from its peculiar nature, well fitted to endure the changes of the seasons for many years; and from its non-conducting properties it serves to maintain the equal temperature of the vital parts of the tree.

1271. _What is cork?_

Cork is the bark of a description of _oak-tree_, which grows in great abundance in Spain, Italy, and France.

1272. _Why does the cork-tree release its own bark?_

Because it possesses a bark which is exceedingly _useful to man_; and it seems, therefore, to have been the design of providence that the tree should cast it off, to be applied to the wants of the human family; for the cork-tree does not discharge its bark by the mere cracking, or exfoliation, of its substance; the tree retains the bark for a number of years, until it has attained that consistency and thickness which renders it useful, and then the tree forms within the bark a series of tabular cells, which _cut off the connection of the bark with the internal structure_, after which it peels off in large sheets.

[Verse: "And all the trees of the field shall know that I the Lord have brought down the high tree, have exalted the low tree, have dried up the green tree, and have made the dry tree to flourish: I the Lord have spoken, and have done it."--EZEK. XVII.]

Man assists this evident intention of nature, by slitting the bark from the top of the tree to its base; but even were this not done, the bark would be cast off by the tree itself.

Another proof of design in this useful adaptation of the cork-tree is to be found in the fact, that it thrives under treatment that would destroy other trees. The cork-tree will endure the barking process for _seven or eight successive years_.

CHAPTER LXIV.

1273. _Why are there curious markings in walnut, mahogany, rose-wood, satin-wood, &c.?_

Because those markings are produced by the various _structure of the vessels_ by which the wood is formed; and by successive zones of wood, which indicate the periods of growth.

The inclosure of zone within zone is owing to the mode in which the wood is produced, and the position in which it is deposited. Wood is formed by the leaves during the growing season, and passes down towards the root between the bark and the wood of the previous year (if any), or in the position in which cambium is effused; and, as the leaves more or less surround the whole stem, the new layer at length completes a zone, and perfectly encloses the wood of all former years. This is the explanation of the term _exogenous_, which is derived from two words signifying to grow outwardly, for the stem increases in thickness by successive layers on the outer side of the previously-formed wood. That this is the mode of growth has been abundantly proved by experiment, and demonstrated by accidental discoveries. Thus, if a plate of metal be inserted between the bark and wood, it will, in progress of time, become inclosed by the new wood which has overlaid them. So in like manner if letters be cut deeply through the bark and into the wood, the spaces will not be filled up from the bottom, but may be seen in subsequent years overlaid by new wood. A statement appeared in a daily paper, during the past year, to the effect that in cutting down a tree a cat had been discovered inclosed in the wood of the trunk. These facts prove that the wood is applied from without. Again, if a branch be stripped of its leaves down to a certain point, it will not grow above that point; and so, in like manner, if branches be stripped from one side of a tree, the tree will not grow on that side. If a circle of bark be removed from a branch above and also below a leaf, it will be found that increase of size will occur below, but not above that bud; and so, likewise, whenever a ring of bark is removed from a tree, the new woody fibre will not proceed from the lower but from the upper edge.--_Orr's Circle of the Sciences._

[Verse: "And when he saw a fig tree in the way, he came to it, and found nothing thereon, but leaves only, and said unto it, Let no fruit grow on thee henceforward for ever. And presently the tree withered away."--MATTHEW XXI.]

1274. _Why have trees with large trunks a great number of leafy branches?_

Because it is _by the leaves_ that the secretion is formed which supplies the _woody fibre_. The number of leaves on a tree, therefore, generally bears a relation to the size of its trunk, and the number of its branches.

1275. _Why have poplar-trees comparatively few branches and leaves?_

Because their trunks are comparatively _small_, although they grow to a great height.

1276. _Why had the mammoth-tree comparatively few leaves in relation to the immense size of its bark?_

Because the woody texture of this tree (_Wellingtonea gigantea_) is _exceedingly light and porous_. It is, in fact, lighter than cork, and, therefore, requires less leaf-produce in its formation.

1277. _Why have oak-trees an abundance of leaves?_

Because their wood is _so dense_ that they require a larger amount of the wood-forming secretion which is supplied by the leaves.

1278. _Why are the trunks of trees round?_

Because, generally speaking, the leaves are distributed upon branches around the trees in every direction. They consequently send down the wood-forming principle on all sides. When a trunk is unduly developed on one side, it may generally be traced to the unequal distribution of the branches.

1279. _What are exogenous stems?_

Exogenous stems are those that grow by the addition of wood _on their outer surface_, underneath the bark.

1280. _What are endogenous stems?_

Endogenous stems are those that _grow inwardly_, from the centre. Trees of this class, of which palms are the best example, are almost peculiar to tropical climates.

1281. _Why do endogenous stems chiefly abound in tropical climates?_

Because, probably, the excessive heat of those climates would interfere with the _formation of wood from the sap_ upon the outer surface.

The vascular structure of endogenous stems lying more abundantly towards their centre, tends to _conserve the juices_ which in hot climates are so highly valued. Palm-wine is a delicious and cooling beverage, and is procured from various kinds of palms, but especially from the cocoa-nut palm. Even the fresh sap is very refreshing. The juice is procured by cutting the tree in the upper part, and attaching a vessel to the opening, to receive the sap. Its flow is increased by cutting off a slice of the wood daily.

[Verse: "I have caused thee to multiply as the bud of the field, and thou hast increased and waxen great, and thou art come to excellent ornaments."--EZEKIEL XVI.]

1282. _Why have endogenous stems no bark?_

Because, one of the chief functions of the bark in exogenous trees, is to _protect the sap_ from which the wood is formed on the outward surface; and as there is no such external flow of sap in endogenous trees, the bark is _unnecessary to them_, and is therefore withheld. They are furnished instead with a thin cuticle.

1283. _Why do endogenous stems grow to a great height?_

Because, as the stem grows from the centre, it soon reaches that limit of diameter _which its vascular structure is calculated to support_; and, therefore, the wood-forming sap is deposited chiefly at the top of the stem, causing it to grow to a considerable height.

1284. _Why do the various vegetable fruits ripen in succession?_

Because the Author of Nature has thus arranged its economy, _in order that the wants of living creatures may be adequately provided for_. Some vegetable productions arrive at their perfection in the spring; others in summer; and others in autumn. Among the latter are many that require to come slowly to maturity after they are gathered; by these the winter season is provided for, and a surplus of the winter stock goes to supply the natural deficiency of spring.

[Verse: "O sing unto the Lord a new made song; for he hath done marvellous things."--PSALM XCVIII.]

1285. _Why, when seeds are sown, and germination begins, does the leaf-germ seek the light, and the root-germ grow down into the earth?_

Because the Creator has endowed every single seed with a _vital instinct which governs its development_. The rootlet could more easily grow upward than downward, because of the looser earth, and of the exciting influences of light and moisture. Yet it takes the contrary course, leaving the leaf-germ to come up to meet the sun-light, and to send down to the stem and roots, the matter needed for their growth.

Frequently, indeed, when seeds are thrown into the earth, their natural position is reversed, and when the germs first start from the seed, the _root-germ_ is directed _upward_ and the _leaf-germ downward_. What then occurs? They each turn, and, in doing so, frequently cross each other. Each goes to its particular duty--the duty that God appointed.

CHAPTER LXV.

1286. _Why are the seeds of plants indigestible?_

Because they are encased in a hard covering upon which the gastric juice of animals takes no effect. This provision has been made by the Creator, _for the preservation of seeds_, the productions of which are so essential to animal life.

The gastric juice can dissolve any other part of the plant, even the woody fibre, and yet upon the _seed_ it takes no effect. When, however, the seed is _crushed_, and, thereby, the vital principle destroyed, so that no plant can spring from it, the gastric juice acts upon it, and it is soon dissolved.

Hence graminivorous birds are provided with gizzards _to break the protecting coats of the grain_; and animals that feed on seeds and nuts _strip them of their shells and husks_.

It is remarkable that in the _succulent fruits_, such as the strawberry, the raspberry, currant, apple, orange, melon, &c., and which, from their very nature, are likely to attract animals to use them, and in eating which _the seeds are likely to be swallowed_, they are fortified by a doubly-protective coating; the pips of the apple, orange, &c., and the seeds of the strawberry and raspberry, pass through the digestive organs, not only unharmed, but their germinating powers are even improved by the warmth and trituration of the stomach. Indeed, the stomachs of quadrupeds and birds have been made the vehicles of propagating plants, and distributing them to the widest geographical latitudes. It is even said of some seeds that they will not germinate until they have passed through the digestive organs of an animal.

[Verse: "And it was commanded them that they should not hurt the grass of the earth, neither any green thing, neither any tree."--REVELATION IX.]

1287. _Why do animals that graze, crop the tender blades of grass, but avoid the tall stems?_

Because they are tempted by the greater sweetness and tenderness of the young blades; and in this temptation a very important end is served; for, by avoiding the stems that have grown up, _the animals spare the matured plant by which seeds are borne_, and by which the supply of food is to be continued.

1288. _Why do the eggs of butterflies lie dormant during the winter?_

Because the _coldness of the winter_ would be fatal to the life of the young insects; and the absence of vegetation would leave the caterpillars to _perish of starvation_, if they were developed during the winter months.

[Illustration: Fig. 76.--CATERPILLAR FEEDING.]

1289. _Why do caterpillars appear in the spring?_

Because the increasing warmth of the sun developes the living embryo, _at the same time that it developes the vegetable germ_. The warmth, therefore, that calls the caterpillar from its embryo sleep, also kindles the germinating power of the vegetable upon which it is destined to feed. The worm awakes and finds the bountiful table of nature spread for it.

[Verse: "Thou shalt plant vineyards, and dress them, but shalt neither drink of the wine, nor gather the grapes: for the worms shall eat them."--DEUTERONOMY XXVIII.]

1290. _Why does the caterpillar eat voraciously?_

Because it _grows rapidly_, and a large amount of vegetable matter is necessary to supply the rapid growth of its animal substance. Caterpillars in the course of a month devour 60,000 times their own weight of aliment.

[Illustration: Fig. 77.--THE UNDER SIDE OF THE CHRYSALIS OF THE PEACOCK BUTTERFLY.]

[Illustration: Fig. 78.--THE SAME CHRYSALIS, WITH PART OF ITS SHEATH RAISED TO SHOW THE PARTIALLY-FORMED WINGS, &c.]

1291. _Why do caterpillars pass into the state of the chrysalis?_

Because they are thereby prepared for the new existence which they are about to enjoy; _new organs must be perfected in them_ to adapt them to the altered conditions of their lives.

Because, also, in the transformation of their bodies, differing materially from the laws of existence that pertain to other creatures, the Creator affords another illustration of his Omnipotence.

Because, also, during the stage that the insect sleeps in the chrysalis, the flowers and their sweet juices, upon, which the fly is to feed, are being prepared for it, just as, when it was sleeping in the egg, the green food was being prepared for the caterpillar. When, therefore, the beautiful fly spreads its silken wings, it finds a _second time_ that, while it has slept, its meal has been prepared, and it now flies away joyously to feed upon the milk and honey of beautiful flowers which, at the time it passed into the chrysalis, had not yet unfolded their petals.

[Verse: "For the moth shall eat them up like a garment, and the worm shall eat them like wool: but my righteousness shall be for ever, and my salvation from generation to generation."--ISAIAH LI.]

[Illustration: Fig. 79.--THE PEACOCK BUTTERFLY.]

Paley observes, that "the _metamorphosis_ of insects from grubs into moths and flies, is an astonishing process. A hairy caterpillar is transformed into a butterfly. Observe the change. We have four beautiful wings where there were none before; a tubular proboscis, in the place of a mouth with jaws and teeth; six long legs, instead of fourteen feet. In another case, we see a white, smooth, soft worm, turned into a black, hard, crustaceous beetle, with gauze wings. These, as I said, are astonishing processes, and must require, as it should seem, a proportionably artificial apparatus. The hypothesis which appears to me most probable, is that, in the grub, there exists at the same time three animals, one within another, all nourished by the same digestion, and by a communicating circulation; but in different stages of maturity. The latest discoveries made by naturalists, seem to favour this supposition. The insect, already equipped with wings, is descried under the membranes both of the worm and nymph. In some species, the proboscis, the antennæ, the limbs, and wings of the fly, have been observed to be folded up within the body of the caterpillar; and with such nicety as to occupy a small space only under the two first wings. This being so, the outermost animal, which, besides its own proper character, serves as an integument to the other two, being the farthest advanced, dies, as we suppose, and drops off first. The second, the pupa or chrysalis, then offers itself to observation. This also, in its turn, dies; its dead and brittle husk falls to pieces, and makes way for the appearance of the fly or moth. Now, if this be the case, or indeed whatever explication be adopted, we have a prospective contrivance of the most curious kind; we have organisations _three deep_; yet a vascular system, which supplies nutrition, growth, and life, to all of them together."

[Verse: "That which the palmer-worm hath left hath the locust eaten; and that which the locust hath left hath the canker-worm eaten; and that which the canker-worm hath left hath the caterpillar eaten."--JOEL I.]

Lord Brougham, in a note upon the above, does not support Paley's view. He says "It is more than probable that the parts which are to appear in the perfect insect do _not_ exist in the larvæ, where there is not much difference between the larva and pupa, excepting at the time just previous to its becoming a pupa, at which time the larva is motionless and torpid. The caterpillar of a moth, when about to turn into a pupa, provides for the protection of the latter state, either by surrounding itself with a web, or by some other means. Soon after this is accomplished, the caterpillar becomes motionless, or nearly so; it can neither eat nor crawl. At this time, and _not before_, the parts of the pupa are forming within the skin of the caterpillar, which may be easily seen by dissection."

It appears to the author, however, that Paley is partially right, and Lord Brougham totally wrong, in these remarks. When Lord Brougham asserts that the parts of the pupa are forming within the skin of the caterpillar at that time when the transformation begins, "and not before, which may be easily seen by dissection," he forgets, that although in some instances it is the first moment when, to the human eye, the organs of the new creature _become perceptible_, that the "_three deep_" nature which Paley attributes to the _grub_, must really have existed _in the egg_--that the _butterfly_ originated _in the egg_, as certainly as did the _caterpillar_, or the _chrysalis_, and that unless that egg had possessed its three mysterious embryos, it would have been impossible for the grub to have progressed to the stages of transformation. No one has ever known the embryo of a bird's egg to pass through three distinct and dissimilar states of existence; nor has any one ever known the embryo of the butterfly's egg to stop short at either of the stages, if the proper conditions of its existence and development were supplied to it. _Why?_ Because the embryo of the insect has a _threefold_ nature, while that of the bird is _single_.

[Verse: "They shall cut down her forest, saith the Lord, though it cannot be searched; because they are more than the grasshoppers, and are innumerable."--JEREMIAH XLVI.]

CHAPTER LXVI.

1292. _Why does the caterpillar become torpid when passing into the state of the chrysalis?_

Because in all probability, where the difference between the first and the ultimate form is considerable, the organs of the insect having to undergo great changes, it would suffer considerable pain. Torpor comes upon the insect, it is thrown into a state similar to that of a person who has inhaled chloroform; and after what has, in all probability, proved a pleasant dream, the insect awakes to find itself changed and beautified.

1293. _Why are the pupæ of grasshoppers and other insects, when about to undergo transformation, still active and sensitive?_

Because, as there is but a _slight difference_ between the form which they have in the pupa state, and that which they ultimately assume, they do not require the state of torpidity to save them from pain, nor to arrest their movements while their organs are being changed. With them _the outer skin is thrown off_, and they are then perfect insects.

1294. _Why do caterpillars, when about to pass through the chrysalis state, attach themselves to the leaves of plants, &c.?_

Because they know instinctively that for a time they will be _unable to controul their own movements, and to avoid danger_. They therefore choose secure and dry places, underneath leaves, or in the crevices of old and dry walls, and there they firmly attach themselves, to await the time of their liberation.

1295. _Why do insects attach their eggs, to leaves &c.?_

Because, as the eggs have to be preserved during the winter, the insect attaches them to some surface which will be a _protection to them_. Generally speaking, the eggs are attached to the permanent stems of plants, and not to those leafy portions which are liable to fall and decay. The spider _weaves a silken bag_ in which it deposits its eggs, and then it hangs the bag in a sheltered situation. Nature keeps her butterflies, moths, and caterpillars, locked up during the winter, in their egg-state; and we have to admire the various devices to which, if we may so speak, the same nature has resorted for the _security_ of the egg. Many insects enclose their eggs in a silken web; others cover them with a coat of hair, torn from their own bodies; some glue them together; and others, like the moth of the silk-worm, glue them to the leaves upon which they are deposited, that they may not be shaken off by the wind, or washed away by rain; some again make incisions into leaves, and hide an egg in each incision; whilst some envelope their eggs with a soft substance, which forms the first aliment of the young animal; and some again make a hole in the earth, and, having stored it with a quantity of proper food, deposit their eggs in it.

[Verse: "Lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through and steal."--MATT. VI.]

1296. _Why do butterflies fly by day?_

Because they are _organised to enjoy light and warmth_, and they live upon the sweets of flowers which by day are most accessible.

1297. _Why do moths fly by night?_

Because they are _organised to enjoy subdued light_ and cool air; and as they take very little food during the short life they have in the winged state, they find sufficient by night. Some of the moths, like that of the silk-worm, take no food from the time they escape from the chrysalis until they die.

Because, also, they form the food of bats, owls, and other of the night-flying tribes.

1298. _Why are the bodies of moths generally covered with a very thick down?_

Because, as they fly by night, they are liable to the effects of cold and damp. The moths, therefore, are nearly all of them covered with a very thick down, quite distinguishable from the lighter down of butterflies.

1299. _Why do moths fly against the candle flame?_

Because their eyes are organised _to bear only a small amount of light_. When, therefore, they come within the light of a candle, their sight is overpowered and their vision confused; and as they cannot distinguish objects, they pursue the light itself, and fly against the flame.

[Verse: "Let him that glorieth glory in this that he understandeth and knoweth me, that I am the Lord which exercise loving-kindness, judgment, and righteousness in the earth: for in these things I delight, saith the Lord."--JER. IX.]

1300. _Why do insects multiply so numerously?_

Because they form the food of larger animals, and especially of birds. A single pair of sparrows and a nest of young ones have been estimated to consume upwards of _three thousand_ insects in a week.

1301. _Why does the "death-watch" make a ticking noise?_

Because the insect is one of the beetle tribe, having a horny case upon its head, _with which it taps upon any hard substance_, the ticking is the call of the insect to its species, just as the noise made by the cricket is a note of communication with other crickets.

There is a superstition connected with the death-watch, which, like most superstitions, is based upon the theory of _probabilities_. The death-watch is usually heard in the spring of the year, and a superstition runs to the effect that some one in the house will die before the year has ended. Persons who are superstitious are never very strict in the interpretation of their predictions; and therefore, whether a person dies in the house or out of it, in the same room where the death-watch was heard, or across the wide Atlantic, so that there be some kind of relationship, or even acquaintance, between the person who hears the omen, and the person dying, the event is sure to be connected with the prophetic sounds of the death-watch. Little weens the small timber-boring beetle, when he is tapping gently to call his mate, and perhaps peeping into every corner and crevice to find her, that he is sending dismay into the heart of some superstitious listener, who, in ignorance of a simple fact, overwhelms herself with an imaginary grief.

1302. _Why are insects in the first stage, after leaving the egg, said to be in the "larva" state?_

Because the term larva is derived from the Latin _larvated_, meaning masked, clothed as with a mask; the term is meant to express that the future insect is disguised in its first form.

1303. _Why are insects in the second state said to be in the "pupa" state?_

Because the term is derived from the Latin _pupa_, from a slight resemblance in the manner in which the insects are enclosed, to that in which it was the fashion of the ancients to _bandage their infants_.

1304. _Why are insects in the "pupa" stage also called "chrysalides?"_

Because, as the Latin term implies, it is adorned with gems. Many chrysalides are _studded with golden and pearl-like spots_.

[Verse: "Thou hast set all the borders of the earth: thou hast made summer and winter."--PSALM LXXIV.]

1305. _Why are the perfect insects said to be in the "nymph" state?_

Because their joyful existence, and their beautiful forms, give them a fancied resemblance to the _nymphs of the heathen mythology_. The nymphs were supposed goddesses of the mountains, forests, meadows, and waters.

This term has generally, but very improperly, been also applied to the pupa state, so that _pupa_, _chrysalis_, and _nymph_ have all been employed to represent one state. This is obviously an error, as there is nothing in the condition of the _pupa_ or _chrysalis_ that can at all accord with the mythological idea of a _nymph_, and which, in reference to the beautiful and joyous fly, finds a much truer application.

CHAPTER LXVII.

1306. _Whence does the snail obtain its shell?_

Young snails come from the egg _with a shell upon their backs_.

1307. _How does the shell grow with the increase of size of the animal?_

The soft slime which is yielded by the body of the animal, _hardens upon the orifice of the shell_, and thus increases its size.

[Illustration: Fig. 80.--COMMON GARDEN SNAIL.]

1308. _Why is the shell spiral?_

Partly because of its original formation; but also because, _as the shell grows_, the opening is elongated; and thrown up, causing the spiral body of the shell to turn, and so to wind its growth around the centre.

[Verse: "Notwithstanding they hearkened not unto Moses; but some of them left it until the morning, and it bred worms, and stank: and Moses was wrath with them."--EXODUS XVI.]

1309. _Why has the snail four tentacula attached to its head?_

Because the insect, having no other limbs, is provided with those projecting members, the lower two serving as _feelers_ and the upper two also as _feelers_ and _eyes_. These, projecting in the front of the animal, impart to it a consciousness of surrounding objects, and especially of those which lie in its path.

1310. _Why is the snail able to move, without feet?_

Because it has attached to its body a fringe of muscular skin, which is capable of considerable contraction and expansion, and by alternately stretching and shortening this, the snail is able to draw himself along.

1311. _Why do we see no snails in the winter time?_

Because they bury themselves in the ground, or in holes, where they remain _in a torpid state_ for several months. Before they enter into the torpid state, they form with their slimy secretion, and with some earthy matters which they collect, a strong cement with which they seal up the opening to their shells.

1312. _Why can snails live in shells thus sealed?_

Because they leave, in the thin wall by which they close themselves in, _a small hole_, too small to admit water, but large enough to let in sufficient air to carry on their feeble respiration during their winter sleep.

1313. _Why do insects abound in putrid waters, and in decaying substances?_

Because they have been endowed with appetites and with constitutions that enable them to live upon and to enjoy corrupt matter. In this point of view the maggots of flies are exceedingly useful; a dead carcass is speedily threaded by them in every direction; thus that corrupt matter which, in a large mass, would poison the air, is taken up in small portions by millions of living bodies, and by them _dispersed_, and becomes innoxious.

[Verse: "For he maketh small the drops of water: they pour down rain according to the vapour thereof."--JOB XXXV.]

1314. _Why do we see, in tanks of rain water, insects rising to the surface?_

Because numerous insects pass through their first stages of existence in _water_, and among them the common gnat. The gnats of the previous season having deposited their eggs on the sides of the water-butt, the warm water developes them, and the larvæ of the gnats appear (Fig. 81; _c_ natural size of larva; _b_ larva magnified).

[Illustration: Fig. 81.--LARVA AND PUPA OF GNAT.

(_Greatly magnified._)]

1315. _Why do they continually rise to the surface of the water?_

Because they require to breathe air, and therefore they come up to the surface, where, elevating the tube (_b_) above the surface of the water, they are enabled to breathe.

1316. _Why do some appear to have larger heads than others?_

Those that have apparently larger heads, and that breathe through tubes attached to their heads (_d_) are in the _pupa_, or second stage of development, and underneath the large shield by which their heads are marked, their wings, feet, &c., are being formed.

[Verse: "Because thy loving kindness is better than life, my lips shall praise thee."--PSALM LXIII.]

1317. _Why, when the water is disturbed, do the larvæ descend more rapidly than the pupæ?_

Because the pupæ are in a torpid condition, awaiting the formation of their perfect organs.

1318. _Why are the flies able to escape from the water?_

Because, as their formation becomes perfected, and the fluids of the body of the pupa become absorbed in the production of the light texture of the wings, &c., _the body and its case become lighter than the water_, and rise and float upon the surface. The pupa-case then forms a natural boat, from which the fly emerges, and spreading its wings, enters upon the final state of its existence.

[Illustration: Fig. 82.--THE PERFECT GNAT. ESCAPING FROM THE PUPA-CASE.

_(Greatly magnified.)_]

This interesting metamorphosis may be seen going on in the summer time, in every pond, brook, and reservoir. A fine sunny morning calls up millions of these little boats from beneath the surface, and the diver within that wonderful little bell breaks its sealed doors, and flies away to enjoy the bright sunshine.

1319. _Why are beetles denominated "coleoptera?"_

Because they have wings protected by horny sheaths; the term _coleoptera_ signifies _wings in a sheath_.

[Verse: "They shall lie down in the dust; and the worms shall cover them."--JOB XXI.]

1320. _Why have beetles hard horny wing-cases?_

Because they live underground, or in holes excavated in wood, &c. If, therefore, their wings were not protected by a hard and firm covering, they would be constantly _liable to destruction_ from the movement of the insect within hard and rough bodies.

[Illustration: Fig. 83.--STAG-BEETLE, SHOWING ITS WINGS UNFOLDED, AND THE WING-CASES OPEN.]

The _elytra_, or scaly wings of the genus of scarabæus, or beetle, furnish an example of this kind. The true wing of the animal is a light, transparent membrane, finer than the finest gauze, and not unlike it. It is also, when expanded, in proportion to the size of the animal, very large. In order to protect this delicate structure, and, perhaps, also to preserve it in a due state of suppleness and humidity, a strong, hard case is given to it, in the shape of the horny wing which we call the elytron. When the animal is at rest, the gauze wings lie folded up under this impenetrable shield. When the beetle prepares for flying, he raises the integument, and spreads out his thin membrane to the air. And it cannot be observed without admiration, what a tissue of cordage, _i. e._ of muscular tendons, must run in various and complicated, but determinate directions, along this fine surface, in order to enable the animal, either to gather it up into a certain precise form, whenever it desires to place its wings under the shelter which nature hath given to them, or to expand again their folds when wanted for action.

[Verse: "The Lord is good; his mercy is everlasting; and his truth endureth to all generations."--PSALM C.]

In some insects, the elytra cover the whole body; in others, half; in others only a small part of it; but in all, they completely hide and cover the true wings. Also,

Many, or most of the beetle species lodge in holes in the earth, environed by hard, rough substances, and have frequently to squeeze their way through narrow passages; in which situation, wings so tender, and so large, could scarcely have escaped injury, without both a firm covering to defend them, and the capacity of folding themselves up under its protection.

1321. _Why have many of the beetle tribe large strong horns?_

Because, as they live in holes in the earth, or in excavations in wood, they use their horns to _dig out their places of retreat_.

1322. _Why has the giraffe a small head?_

Because, being set upon the end of a very long neck, the animal would be _unable to raise it_ if it were heavy.

1323. _Why has the giraffe a long neck?_

Because it _feeds upon the branches of tall trees_.

1324. _Why has the giraffe a long and flexible tongue?_

Because it is thereby enabled to lay hold of the tender twigs and branches, _and draw them into its mouth_, avoiding the coarser parts of the branches.

1325. _Why are the nostrils of the giraffe small and narrow, and studded with hairs?_

Because the hairs and the peculiar shape of the nasal passages are designed as a protection against the insects which inhabit the boughs of the trees upon which the giraffe feeds; and also against the sands of the desert, which storms raise into almost suffocating clouds.

* * * * *

[Illustration: Fig. 84.--GIRAFFE FEEDING.]

[Verse: "Bless the Lord, all his works, in all places of his dominion: bless the Lord, O my soul."--PSALM CIII.]

1326. The distribution of animals, or _Zoological Geography_, is of great interest, and should be carefully studied in connection with _Botanical Geography_ (_see_ 1208). The highest department of the animal kingdom (writes the Rev. W. Milner) commences with the class of _Birds_, which may be naturally divided into the three great orders of ærial, terrestrial, and aquatic. Aggregation into immense flocks is a distinguishing feature of several species, especially of the aquatic order, which form separate colonies, building their nests in the same state, though other spots equally adapted are at no great distance. Hence the Vogel-bergs, or bird rocks of the northern seas, one of which at Westmannsharn in the Faroe group of islands, seldom intruded upon by man, presents a most extraordinary spectacle to the visitor. The Vogel-berg lies in a frightful chasm in the precipitous shores of the island, which rise to the height of a thousand feet, only accessible from the sea by a narrow passage. Here congregate a host of birds. Thousands of guillemots and auks swim in groups around the boat which conveys man to their domain, look curiously at him, and vanish beneath the water to rise in his immediate neighbourhood. The black guillemot comes close to the very oars. The seal stretches his head above the waves, not comprehending what has disturbed the repose of his asylum, while the rapacious skua pursues the puffin and gull. High in the air the birds seem like bees clustering about the rocks, whilst lower they fly past so close that they might be knocked down with a stick. But not less strange is the domicile of this colony. On some low rocks scarcely projecting above the water sit the glossy cormorants, turning their long necks on every side. Next are the skua gulls, regarded with an anxious eye by the kittiwakes above. Nest follows nest in crowded rows along the whole breadth of the rock, and nothing is visible but the heads of the mothers and the white rocks between. A little higher on the narrow shelves sit the guillemots and auks, arranged as on parade, with their white breasts to the sea, and so close that a hailstone could not pass between them. The puffins take the highest station, and, though scarcely visible, betray themselves by their flying backwards and forwards. The noise of such a multitude of birds is confounding, and in vain a person asks a question of his nearest neighbour. The harsh tones of the kittiwakes are heard above the whole, the intervals being filled with the monotonous note of the auk, and the softer voice of the guillemot. When Graba, from whose travels this description is principally drawn, visited the Vogel-berg, he was tempted by the sight of a crested cormorant to fire a gun, but what became of it, he remarks, it was impossible to ascertain. The air was darkened by the birds roused from their repose. Thousands hastened out of the chasm with a frightful noise, and spread themselves over the ocean. The puffins came wandering from their holes, and regarded the universal confusion with comic gestures. The kittiwakes remained composedly in their nests, whilst the cormorants tumbled headlong into the sea. Similar great congregations of the feathered race appear where the shores are rocky high, and precipitous, but this is strikingly the case, where

----"The northern ocean, in vast whirls, Boils round the naked melancholy isles Of farthest Thule; and the Atlantic surge Pours in among the stormy Hebrides. Who can recount what transmigrations there Are annual made? what nations come and go? And how the living clouds on clouds arise? Infinite wings! till all the plume-dark air And rude resounding shore are one wild cry."

[Verse: "He rained flesh upon them as dust, and feathered fowls like as the sand of the sea."--PSALM LXXVIII.]

1327. Most terrestrial birds, unacquainted with man, exhibit a remarkable tameness, and are slow in acquiring a dread of him, even after repeated lessons that danger is to be apprehended from his neighbourhood. Mr. Darwin speaks of a gun as almost superfluous in the unfrequented districts of South America, for with its muzzle he pushed a hawk off the branch of a tree. Once, while lying down, a mocking thrush alighted on the edge of a pitcher, made of the shell of a tortoise, which he was holding in his hand, and began very leisurely to sip the water, even allowing him to handle it while seated on the vessel. In Charles Island, which had been colonised about six years, he saw a boy sitting by a well with a switch in his hand, with which he killed the doves and finches as they came to drink; and for some time had been constantly in the habit of waiting by the well for the same purpose, to provide himself with his dinners. In the Falkland Islands, at Bourbon, and at Tristan d'Acunha, the same tameness was noticed by the early visitors. On the other hand, the small birds in the arctic regions of America, which have never been persecuted, exhibit the anomalous fact of great wildness. From a review of various facts, Mr. Darwin concludes, "first, that the wildness of birds with regard to man is a particular instinct directed against him, and not dependent on any general degree of caution arising from other sources of danger; secondly, that it is not acquired by individual birds in a short time, even when much persecuted; but that in the course of successive generations it becomes hereditary. Comparatively few young birds in any one year have been injured by man in England, yet almost all, even nestlings, are afraid of him; many individuals, however, both at the Galapagos and at the Falklands, have been pursued and injured by man, but yet have not learned a salutary dread of him."

[Verse: "As a bird that wandereth from her nest; so is a man that wandereth from his place."--PSALM XXVII.]

1328. Numerous species of birds may be regarded as the favourites of nature on account of the gracefulness given to their shape, and the richly-coloured plumage with which they are adorned, as evidenced in the gaudy liveries of many of the parrot tribe, and the forms and hues of the birds of paradise. But they are especially interesting to man for the faculty of song with which they are endowed; in some, "most musical, most melancholy," in others, sprightly and animating, inspiriting the sons of toil under the burdens peculiar to their station. It deserves to be remarked, as an instance of compensation and adjustment, that whilst the birds of the temperate zone are far inferior to those of tropical climes in point of beauty, they have far more melodious notes in connection with their less attractive appearance.

1329. From the powerful means of locomotion possessed by several of the bird tribe, and their great specific levity, air being admitted to the whole organisation as water to a sponge, it might be inferred, that the entire atmosphere was intended to be their domain, so that no species would be limited to a particular region. The common crow flies at the rate of twenty-five miles an hour; the rapidity of the eider-duck, _Anas mollissima_, is equal to ninety miles an hour; while the swifts and hawks travel at the astonishing speed of a hundred and fifty miles in the same time. It is true that some species have a very extensive range, as the nightingale, the common wild goose, and several of the vulture tribe. The same kind of osprey or fishing-eagle that wanders along the Scottish shores appears upon those of the south of Europe, and of New Holland. The lammergeyer haunts the heights of the Pyrenees, the mountains of Abyssinia, and the Mongolian steppes; and the penguin falcon occurs in Greenland, Europe, America, and Australia. In general, however, like plants and terrestrial quadrupeds, the birds are subject to geographical laws, definite limits circumscribing particular groups. The common grouse of our own country affords a striking exemplification of this arrangement, as it is nowhere met with out of Great Britain; and other examples occur of a very scanty area containing a species not to be found in any other region. The celebrated birds of paradise we exclusively confined to a small part of the torrid zone, embracing New Guinea and the contiguous islands; and the beautiful Lories are inhabitants of the same districts, being quite unknown to the New World. Parroquets are chiefly occupants of a zone extending a few degrees beyond each tropic, but the American group is quite distinct from the African, and neither of these have one in common with the parrots of India. The great eagle is limited to the highest summits of the Alps; and the condor, which soars above the peak of the loftiest of the Andes, never quits that chain. Humming-birds are entirely limited to the western hemisphere, where a particular species is sometimes bounded by the range of an island, while others are more extensively spread, the _Trochilus flammifrons_, common to Lima, being observed by Captain King upon the coast of the Straits of Magellan, in the depth of winter, sucking the flowers of a large fuchsia, then in bloom in the midst of a shower of snow. Among the birds incapable of flight, which rival the quadrupeds in their size, the intertropical countries of the globe have their distinct species, presenting similar general features of organisation, as the ostrich of Africa and Arabia, the cassowary of Java and Australia, and the touyou of Brazil. In the arctic regions, we meet with species peculiar to them, the _Strix laeponicus_ or Lapland owl, and the eider-duck, an inhabitant of the shores, from whose nests the eider-down is obtained. Several families of maritime birds are likewise limited to particular oceanic localities. Approaching the fortieth parallel of latitude, the albatross is seen flitting along the surface of the waves, and soon afterwards the frigate and other tropical birds appear, which never wander far beyond the torrid zone. It thus appears, that, notwithstanding the great locomotive powers of birds, particular groups have had certain regions assigned to them as their sphere of existence, which they are adapted to occupy, and to which they adhere in the main, though it is easy to conceive of natural causes occasionally constraining to a migration into new and even distant territories. Captain Smyth informed Mr. Lyell, that when engaged in his survey of the Mediterranean, he encountered a gale in the Gulf of Lyons, at the distance of between twenty and thirty leagues from the coast of France, which bore along many land-birds of various species, some of which alighted on the ship, while others were thrown with violence against the sails. In this manner, many an islet in the deep, after ages of solitude and silence, uninterrupted except by the wave's wild dash, and the wind's fierce howl, may have received the song of birds, forced by the tempest from their home, and compelled to seek a new one under its direction.

[Verse: "There is a path which no fowl knoweth, and which the vulture's eye hath not seen."--JOB XVIII.]

1330. There is no feature more remarkable in the economy of birds than the periodical migrations, so systematically conducted, in which five-sixths of the whole feathered population engage. In the case of North America, according to an estimate by Dr. Richardson, the passenger-pigeons form themselves into vast flocks for the journey, one of which has been calculated to include 2,230,000,000 individuals. We are familiar with the cuckoo as our visitor in spring, and with the house-swallow as our guest through the summer, the latter usually departing in October to the warmer regions of the south, wintering in Africa, returning again when a more genial season revives its insect food. By cutting off two claws from the feet of a certain number of swallows, Dr. Jenner ascertained the fact of the same individuals re-appearing in their old haunts in the following year, and one was met with even after the lapse of seven years. The arctic birds migrate farther south, when the seas, lakes, and rivers become covered with unbroken sheets of ice; the swans, geese, ducks, divers, and coots flying off in regular phalanxes to regions where a less rigorous winter allows of access to the means of life. Hence, soon after, we lose the swallows, we gain the snipes and other waders, which have fled from the hard frozen north to our partially frozen morasses, where their ordinary nutriment may still be obtained. The equinoctial zone, where the seasonal change is that of humidity and drought furnishes an example of the same phenomenon. As soon as the Orinoco is swollen by the rains, overflows its banks, and inundates the country on either side, an innumerable quantity of aquatics leave its course for the West India islands on the north, and the valley of the Amazon on the south, the increased depth of the river, and the flooded state of the shores, depriving them of the usual supply of fish and insects. Upon the stream decreasing, and retiring within its bed, the birds return.

[Verse: "The Lord is my light and my salvation; whom shall I fear? the Lord is the strength of my life; of whom shall I be afraid?"--PSALM XXVII.]

1331. A comparison between the quadrupeds of the Old and New Worlds is in every point strikingly in favour of the former. Not only has the western continent no animals of such giant bulk as those of the eastern, but no examples of such high organisation, such power and courage, as the African lion and the Asiatic tiger display. Buffon's remark must indeed be considerably modified, respecting the cowardice of the American feline race; for the jaguar of the woods about the Amazon, when attacked by man, will not hesitate to accept his challenge, will even become the assailant, nor shrink from an encounter against the greatest odds. The following passages from the writings of Humboldt show that this transatlantic animal is not to be despised:--

"The night was gloomy; the Devil's Wall and its denticulated rocks appeared from time to time at a distance, illuminated by the burning of the savannahs, or wrapped in ruddy smoke. At the spot where the bushes were the thickest, our horses were frightened by the yell of an animal that seemed to follow us closely. It was a large jaguar, that had roamed for three years among these mountains. He had constantly escaped the pursuit of the boldest hunters, and had carried off horses and mules from the midst of enclosures; but, having no want of food, had _not yet_ attacked men. The negro who conducted us uttered wild cries. He thought he should frighten the jaguar; but these means were of course without effect. The jaguar, like the wolf of Europe, follows travellers even when he will not attack them; the wolf in the open fields and in unsheltered places, the jaguar skirting the road, and appearing only at intervals between the bushes."

The same illustrious observer also remarks,--

"Near the Joval, nature assumes an awful and savage aspect. We there saw the largest jaguar we had ever met with. The natives themselves were astonished at its prodigious length, which surpassed that of all the tigers of India I had seen in the collections of Europe."

Still these were extraordinary specimens of the race, and leave the fact undoubted, that the most formidable of the western _Feræ_ has no pretensions to an equality with his congener, the tyrant of the jungles of Bengal.

1332. In vain also we look among the tribes of America for a rival in outward appearance to the giraffe, so remarkable for its height, its swan-like neck, gentle habits, and soft expressive eye; while of the animals most serviceable to mankind--the horse, the ox, the ass, the goat, and the hog--not a living example of either was known there before its occupancy by the Europeans. But, however inferior the animal race of the New may be as compared to those of the Old world, the balance between the two appears to have been pretty equal in remote ages; geological discovery has disproved the assertion of Buffon, that the creative force in America in relation to quadrupeds never possessed great vigour, and has established the fact, that it is only the more recent specimens of its energy that are upon an inferior scale. The relics of the unwieldly magatherium, of the gigantic sloth, and armadillo-like animals, discovered in great abundance imbedded in its soil, prove that at a former period it swarmed with monsters of equal bulk with those that now roam in the midst of Africa and Asia. The estuary deposit that forms the plains westward of Buenos Ayres, and covers the gigantic rocks of the Bando Oriental, appears to be the grave of extinct gigantic quadrupeds.

[Verse: "But wild beasts of the desert shall lie there; and their houses shall be full of doleful creatures; and owls shall dwell there, and satyrs shall dance there."--ISAIAH XIV.]

1333. There are various animals which are very widely dispersed, enduring the extremes of tropical heat and of polar cold, which are either in a wild condition or in a state of domestication. Wild races, considered to be varieties of the domestic dog, occur in India, Sumatra, Australia, Beloochistan, Natolia, Nubia, various parts of Africa, and both the Americas; while in subjection to man, the dog is his faithful companion, and has followed his steps into every diversity of climate and of situation to which he has wandered. The north temperate zone of the Old Continent appears to be the native region of the ox, which passes in Lapland within the arctic circle, and has been spread over South America since its first introduction by the Spaniards. The horse, originally an inhabitant of the temperate parts of the Old World, has shared in a similar dispersion, and now exists in the high latitude of Iceland, in the desolate regions of Patagonia, and roams wild in immense herds over the Llanos of the Orinoco, leading a painful and restless life in the burning climate of the tropics. Humboldt draws a striking picture of the sufferings of these gifts of the Old World to the New, returned to a savage state in their western location.

"In the rainy season, the horses that wander in the savannah, and have not time to reach the rising grounds of the Llanos, perish by hundreds amidst the overflowings of the rivers. The mares are seen, followed by their colts, swimming, during a part of the day, to feed upon grass, the tops of which alone wave above the waters. In this state they are pursued by the crocodiles; and it is by no means uncommon to find the prints of the teeth of these carnivorous reptiles on their thighs. Pressed alternately by excess of drought and of humidity, they sometimes seek a pool, in the midst of a bare and dusty soil, to quench their thirst; and at other times flee from water and the overflowing rivers, as menaced by an enemy that encounters them in every direction. Harassed during the day by gad-flies and mosquitoes, the horses, mules, and cows find themselves attacked at night by enormous bats, that fasten on their backs, and cause wounds which become dangerous, because they are filled with acaridæ and other hurtful insects. In the time of great drought, the mules gnaw even the thorny melocactus (melon-thistle), in order to drink its cooling juice, and draw it forth as from a vegetable fountain. During the great inundations, these same animals lead an amphibious life, surrounded by crocodiles water-serpents, and manatees. Yet, such are the immutable laws of nature, their races are preserved in the struggle with the elements, and amid so many sufferings and dangers. When the waters retire, and the rivers return into their beds, the Savannah is spread over with a fine odoriferous grass; and the animals of old Europe and Upper Asia seem to enjoy, as in their native climates the renewed vegetation of spring."

1334. The first colonists of La Plata landed with seventy-two horses, in the year 1535, when, owing to a temporary desertion of the colony, the animals ran wild; and in 1580, only forty-five years afterwards, it had reached the Straits of Magellan. The ass has a more restricted range than the horse, not being capable of enduring so great a degree of cold, though usually far from being considered a delicate animal. To the warmer parts of the temperate zone, between the 20th and the 40th parallels of latitude, the ass seems best adapted, not propagating much beyond the 60th, and only occurring in a state of degeneration beyond the 52nd. The sheep and goat tribe are widely spread, equally supporting the extremes of temperature. According to Zimmerman, the _Argali_ or _Mouflon_, the original race of sheep, still exists on all the great mountains of the two continents; and the _Capricorn_ and _Ibex_, the ancestors of the common goat inhabit the high European elevations. From the 64th degree of north latitude the hog is met with all over the old continent, and also in the islands of the Indian Ocean, peopled by the Malay race; and since its introduction into the New World, it has diffused itself over it, from the 50th parallel north as far as Patagonia. Originally the cat was not known in America, nor in any part of Oceanica; but it has now spread into almost every country of the globe. Among animals entirely wild, the most extensively diffused, are the fox, hare, squirrel, and ermine; but the species are different in every region of the world; nor is there perhaps one example to be found of a species perfectly identical naturally existing in distant localities of the earth.

[Verse: "His going forth is from the end of the heaven, and his circuit unto the ends of it; and there is nothing hid from the heat thereof."--PSALM XIX.]

Respecting the _internal constitution and heat of the earth_, differences of opinion, and some very wild speculation have existed. We find in Humboldt's "Cosmos" the following remarks:--

1335. "It has been computed at what depths liquid and even gaseous substances, from the pressure of their own superimposed strata, would attain a density exceeding that of platinum, or of iridium; and in order to bring the actual degree of ellipticity, which was known within very narrow limits, into harmony with the hypothesis of the infinite compressibility of matter, Leslie conceived the interior of the Earth to be a hollow sphere, filled with "an imponderable fluid of enormous expansive force." Such rash and arbitrary conjectures have given rise, in wholly unscientific circles, to still more fantastic notions. The hollow sphere has been peopled with plants and animals, on which two small subterranean revolving planets, Pluto and Proserpine, were supposed to shed a mild light. A constantly uniform temperature is supposed to prevail in these inner regions, and the air being rendered self-luminous by compression, might well render the planets of this lower world unnecessary. Near the north pole, in 82 deg. of latitude, an enormous opening is imagined, from which the polar light visible in Aurora streams forth, and by which a descent into the hollow sphere may be made. Sir Humphry Davy and myself were repeatedly and publicly invited by Captain Symmes to undertake this subterranean expedition; so powerful is the morbid inclination of men to fill unseen spaces with shapes of wonder, regardless of the counter-evidence of well-established facts, or universally recognised natural laws. Even the celebrated Halley, at the end of the 17th century, hollowed out the earth in his magnetic speculations; a freely rotating subterranean nucleus was supposed to occasion, by its varying positions, the diurnal and annual changes of the magnetic declination. It has been attempted in our own day, in tedious earnest, to invest with a scientific garb that which, in the pages of the ingenious Holberg, was an amusing fiction."

The following are among the speculations which Humboldt thus severely but justly condemns:--

"The increase of temperature observed is about 1 deg. Fahr. for every fifteen yards of descent. In all probability, however, the increase will be found to be in a geometrical progression as investigation is extended; in which case the present crust will be found to be much thinner than we have calculated it to be. And should this be found to be correct, the igneous theory will become a subject of much more importance, in a geological point of view, than we are at present disposed to consider it. Taking, then, as correct, the present observed rate of increase, the temperature would be as follows:

Water will boil at the depth of 2,430 yards. Lead melts at the depth of 8,400 yards. There is red heat at the depth of 7 miles. Gold melts at 21 miles. Cast iron at 74 miles. Soft iron at 97 miles.

And at the depth of 100 miles there is a temperature equal to the greatest artificial heat yet observed; a temperature capable of fusing platina, porcelain, and indeed every refractory substance we are acquainted with. These temperatures are calculated from Guyton Morveau's corrected scale of Wedgwood's pyrometer; and if we adopt them, we find that the earth is fluid at the depth of 100 miles from the surface, and that even in its present state very little more than the soil on which we tread is fit for the habitation of organised beings."

The above is to be found in Mr. Timbs's "Things not Generally Known," a little book which professes to set people right upon points on which they are in error!

[Verse: "He hath filled the hungry with good things; and the rich he hath sent empty away."--LUKE I.]

Upon this subject Mr. Hunt, in his "Poetry of Science," says:--

1336. "A question of great interest, in a scientific point of view, is the temperature of the centre of the earth. We are, of course, without the means of solving this problem; but we advance a little way onwards in the inquiry by a careful examination of subterranean temperature at such depths as the enterprise of man enables us to reach. These researches show us, that where the mean temperature of the climate is 50 deg., the temperature of the rock at 59 fathoms from the surface is 60 deg.; at 132 fathoms it is 70 deg; at 239 fathoms it is 80 deg.; being an increase of 10 deg. at 59 fathoms deep, or 1 deg. in 35.4 feet; of 10 deg. more at 73 fathoms deeper, or 1 deg. in 43.8 feet; and of 10 deg. more at 114 fathoms still deeper, or 1 deg. in 64.2 feet.

Although this would indicate an increase to a certain depth of about one degree in every fifty feet, yet it would appear that the rate of increase diminishes with the depth. It appears therefore probable, that the heat of the earth, so far as man can examine it, is due to the absorption of the solar rays by the surface. The evidences of intense igneous action at a great depth cannot be denied, but the doctrine of a cooling mass, and of the existence of an incandescent mass, at the earth's centre, remains but one of those guesses which active minds delight in."

Upon the subject of _hunger_ and _thirst_, by which living creatures are prompted to feast upon the bounties of nature, Sir Charles Bell says, in "Appendix to Paley's Natural Theology:"--

1337. "Hunger is defined to be a peculiar sensation experienced in the stomach from a deficiency of food. Such a definition does not greatly differ from the notions of those who referred the sense of hunger to the mechanical action of the surfaces of the stomach upon each other, or to a threatening of chemical action of the gastric juice on the stomach itself. But an empty stomach does not cause hunger. On the contrary, the time when the meal has passed the stomach is the best suited for exercise, and when there is the greatest alacrity of spirits. The beast of prey feeds at long intervals; the snake and other cold-blooded animals take food after intervals of days or weeks. A horse, on the contrary, is always feeding. His stomach, at most, contains about four gallons, yet throw before him a truss of tares or lucerne, and he will eat continually. The emptying of the stomach cannot, therefore, be the cause of hunger.

"The natural appetite is a sensation related to the general condition of the system, and not simply referable to the state of the stomach; neither to its action, nor its emptiness, nor the acidity of its contents; nor in a starved creature will a full stomach satisfy the desire of food. Under the same impulse which makes us swallow, the ruminating animal draws the morsel from its own stomach.

1338. "Hunger is well illustrated by thirst. Suppose we take the definition of thirst--that it is a sense of dryness and constriction in the back part of the mouth and fauces; the moistening of these parts will not allay thirst after much fatigue or during fever. In making a long speech, if a man's mouth is parched, and the dryness is merely from speaking, it will be relieved by moistening, but if it comes from the feverish anxiety and excitement attending a public exhibition, his thirst will not be so removed. The question, as it regards thirst, was brought to a demonstration by the following circumstance. A man having a wound low down in his throat, was tortured with thirst; but no quantity of fluid passing through his mouth and gullet, and escaping by the wound, was found in any degree to quench his thirst.

"Thirst, then, like hunger, has relation to the general condition of the animal system--to the necessity for fluid in the circulation. For this reason, a man dying from loss of blood suffers under intolerable thirst. In both thirst and hunger, the supply is obtained through the gratification of an appetite; and as to these appetites, it will be acknowledged that the pleasures resulting from them far exceed the pains. They gently solicit for the wants of the body; they are the perpetual motive and spring to action."

[Verse: "Let us hear the conclusion of the whole matter; Fear God, and keep his commandments: for this is the whole duty of man."--ECCLESIASTES XII.]

* * * * *

Our task draws near to a conclusion; and we hope that those who have followed our teachings will thirst after further knowledge; that they will henceforward regard the great Book of Nature as the work of an Almighty Hand, and endeavour to find, for everything that Nature does, the _Reason Why_.

A high perception of the wisdom of the Divine Being, must necessarily be the result of an intelligent contemplation of the Divine works. To the ignorant, the name of God is an unmeaning word; it may inspire fear, but it does not develope love. To the dark mind of the untaught man, God is no more than one of those mysterious existences that awe the superstitious, and deter the wicked. There is no grafting of the soul of the man upon the eternal love. But knowledge brings man into communion with that Almighty wisdom which is the fountain of all truth and happiness. To the enlightened man, God is the sun of all goodness, around whom the attributes of Power, Wisdom, and Love, radiate and fill the universe. As man's physical eye cannot withstand the light of the sun, neither can man's spiritual eye see the whole glory of God. But as we can rejoice in the sunshine, and interpret the mission of the sunbeam, so can we find happiness in the Divine presence, and gather wisdom by the contemplation of the Creator's works.

Nature is a great teacher. What a lesson may be gathered from the germination of a seed; how uniformly the germs obey their destiny. However carelessly a seed may be set in the ground, the germ which forms the root, and that which is the architect of the stem, will seek their way--the one to light, the other to darkness--to fulfil their duty. The obstruction of granite rocks, cannot force the rootlet upward, nor drive the leaflet down. They may kill the germs by exhausting their vital powers in an endeavour to find the proper elements; but no obstruction can make a single blade of grass do aught but strive to fulfil the end for which it was created. Would that man were equally true to the purpose of his existence, and suffered neither the rocks of selfishness, nor the false light of temptation, to force or allure him from duty to his God.

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* * * * *

ESTELLE GRANT; OR, THE LOST WIFE.

Large 12mo., cloth. Price $1 00.

This is a book so thoroughly excellent, so exalted in its character, so full of exquisite pictures of society, and manifesting so much genius, skill, and knowledge of human nature, that no one can possibly read it without admitting it to be, in every way, a noble book. The story, too, is one of stirring interest; and it either sweeps you along with its powerful spell, or beguiles you with its tenderness, pathos, and geniality.

* * * * *

THE PILGRIMS OF WALSINGHAM.

A Romance of the Middle Ages, from the accomplished pen of Agnes Strickland.

Large 12mo., pp. 460. Price $1 00.

Truly a charming book! Full of the profoundest interest, yet not one improbable incident--not one prurient idea. You will sooner find spots upon the leaves of the silvery lily than an impure sentence in a book by this author.--_Buffalo Courier._

* * * * *

NA MOTU; OR, REEF ROVINGS IN THE SOUTH SEAS.

A Narrative of Adventures in the Hawaiian, Georgian, and Society Islands, with original illustrations.

BY EDWARD T. PERKINS

12mo. Cloth. $1 00

Na Motu is the quaint title of a handsome volume of voyage and adventure in the South Seas. Mr. Perkins, the author, a schoolmate of Ike Marvel, has spent several years before the mast, and on the salt water in other capacities, and his style is characterized by a straightforward, honest nonchalance and idiomatic flavor, redolent of Old Ocean from stem to stern. His daguerreotype of nautical dialogues is only a little too perfect, occasionally, for good taste; a large portion of his experience being gained on a whaling ship.--_New York Church Jour._

* * * * *

_SAM SLICK'S YANKEE COURTSHIP._

RECENTLY PUBLISHED,

WISE SAWS;

OR,

SAM SLICK IN SEARCH OF A WIFE.

By the Author of "Sam Slick In England," "Nature and Human Nature," "Sam Slick's Sayings and Doings," &c.

In One Elegant Volume, neatly bound in Muslin;

_Price 75 Cts.--in Paper 50 Cts._

Extract from the Preface:

* * * * "Fun has no limits. It is like the human race and face; there is a family likeness among all the species, but they all differ. New combinations produce new varieties. A man who has an eye for fun sees it in everything. * * * There is a work called 'The Horse,' and another 'The Cow,' and 'The Dog,' and so on; why should'nt there be one on 'The Galls?' They are about the most difficult to choose and to manage of any created critter, and yet there aint any dependable directions about pickin' and choosin' of them. Is it any wonder then so many fellows get taken in when they go for to swap hearts with them? Besides; any one can find a gentleman that keeps a livery stable to get him a horse to order, but who can say, 'This is the Gall for your money!'"

CONTENTS.

Introductory Letter, Chat with the President, Stealing a Speech, Everything in General, and Nothing in Particular, The black Hawk: or Life in a Fore-and-After, Old Blowhard, The Widow's Son, The Language of Mackerel, The Best-natured Man in the World, The Bait-Box, The Water-Glass; or a Day-Dream of Life, Old Sarsaparilla Pills, Our Colonies and Sailors, The House that Hope Built, The House without Hope, An Old Friend with a New Face, Chat in a Calm, The Sable Island Ghost, The Witch of Eskisoony, Jericho beyond Jordan, Three Truths for One Lie, Aunt Thankful & her Room, A Single Idea, An Excellent Plan of Reform, Goose Van Dam, A Hot Day, A Pic-Nic at La Haire, A Narrow Escape.

Published by DICK & FITZGERALD, 18 Ann St., N. Y.

_And for sale by all the principal Booksellers._

* * * * *

THE ARTIST'S BRIDE;

OR, THE PAWNBROKER'S HEIR.

A Novel, by EMERSON BENNET.

12mo. Cloth,--420 pages,--Price 1 00.

"We have perused this work with some attention, and do not hesitate to pronounce it one of the very best productions of the talented author. There is not a page that does not glow with thrilling and interesting incident, and will well repay the reader for the time occupied in perusing it. The characters are most admirably drawn, and are perfectly natural throughout. We have derived so much gratification from the perusal of this charming novel, that we are anxious to make our readers share it with us: and, at the same time, to recommend it to be read by all persons who are fond of romantic adventures. Mr. Bennett is a spirited and vigorous writer, and his works deserve to be generally read; not only because they are well written, but that they are, in most part, taken from events connected with the history of our own country, from which much valuable information is derived, and should, therefore, have a double claim upon our preference, over those works where the incidents are gleaned from the romantic legends of old castles and foreign climes."--_Louisville Journal._

* * * * *

DICK TARLETON;

OR,

THE LAST OF HIS RACE.

Containing 112 very large octavo pages. Price 25 cts. and the book sent free of postage. This well written work has been pronounced by good judges to be the best of Mr. Smith's production. This is saying a great deal, considering that gentleman is the author of "Minnie Grey," and "Woman and her Master,"--works which have become famous with novel readers.

* * * * *

CYRILLA;

A ROMANCE.

BY THE AUTHOR OF THE INITIALS.

Large Octavo.--Price 50c.

Every person who has read that charming novel, "The Initials," should purchase a copy of "Cyrilla." It is one of the best novels that has been published in the past ten years. There is, probably no work of fiction now before the public that surpasses it for power, pathos, depth of plot, delineation of character and brilliancy of sentiment. It forcibly shows that "Many who have perished have erred and sinned for woman."

Copies of the above books sent by mail free of postage. Send cash orders to

DICK & FITZGERALD, No. 18 Ann St., New York.

* * * * *

New Works by Miss E. Marryatt.

(DAUGHTER OF CAPTAIN MARRYATT.)

HENRY LYLE; OR LIFE AND EXISTENCE.

12mo. Cloth, Price $1 00.

* * * * *

TEMPER; A TALE.

12mo. Cloth, Price $1 00.

The above novels, by the talented daughter of the late Captain Marryatt, were written in compliance with the wishes of her father, expressed a short time previous to his death; and the fair authoress alludes to this circumstance by way of apology, in the preface to "Temper." We predict for them a wide spread popularity. They are original in style, truly moral and religious in tone, and are calculated to accomplish much good, as the author aims some telling blows at the tendency of the present generation towards _Infidelity_, and other modern evils.

* * * * *

Works by the Author of "Zaidee."

ADAM GRAEME OF MOSSGRAY.

12mo. Cloth. Price $1 00.

The characters are painted in bold relief, and seem to live, move and speak before you. Not one is overdrawn, and yet each comes up to the popular standard, in point of interest, individualization, and spirit. The tale is, indeed, "sad, high and working; full of state and woe;" but it is pleasant enough for all that, and the sober, truthful earnestness with which it is related, will at once communicate itself to the mind of the most fastidious and hypercritical peruser of modern volumes.

* * * * *

MAGDALEN HEPBURN;

A Story of the Scottish Reformation.

12mo. Cloth, Price $1 00.

This charming novel, by the author of "Zaidee," will be welcomed by all who have had the pleasure of reading the former production. The quaint originality, the healthy and cheerful religious tone, and charming simplicity and good sense of this volume, will render it a general and permanent favorite. A work which will be read as long as any volume of our time. We know of no fiction, in fact, that we would sooner recommend; for, while it will fascinate all who merely read for amusement, it will delight as well as improve those who seek for something even in a novel. It is fascinating from beginning to ending, and no reader will lay it down, after perusal, without wishing the author had extended its pages.

* * * * *

A REPLY TO "DRED," AND "UNCLE TOM."

TIT FOR TAT;

A NOVEL,

BY A LADY OF NEW ORLEANS.

12mo. Cloth, Price $1. Sent free of postage.

This the title of a most wonderful book, written by a lady of New Orleans, and issued from the press for the perusal of all persons whose minds have been poisoned by the pernicious exaggerations of American life and Negro Slavery to be found in "Uncle Tom's Cabin" and "Dred." The lady of New Orleans has done her work manfully. The book shows clearly that those who cry out against Negro Slavery, and utter the rankest falsehoods about that institution, are the supporters and proprietors of a system of _white_ slavery more cruel and debasing in its character and operations than the most skilful romancist could imagine. All this is shown in a Tale abounding with spirited and dramatic scenes and incidents. "TIT FOR TAT" embraces forty chapters of astonishing interest. MILLIONS of copies of this work should be circulated.

OPINIONS OF THE PRESS.

"It recounts, in a forcible manner, the evils of the English social system.... We only wish it furnished any sufficient apology for our shortcomings."--_Commercial, Buffalo._

"One of the must powerfully written novels of the day."--_Springfield Republican._

"It is a poem in all its parts; fervid, womanly and eloquent."--_Galveston News._

"She shows clearly that those who cry out against Negro Slavery are the supporters of a system of white Slavery, most cruel and depraved."--_Savannah News._

This is "carrying the war into Africa" with a vengeance. It is more than "a Roland for an Oliver." It is more caustic than even "Change for Dickens' American Notes. By a lady." "Dred, a Tale of the Dismal Swamp," the offspring of foreign influence; British influence; subsidising and Anglicising the Yankee pen of Harriet Beecher Stowe, is answered most effectually in a tale of white slavery, far more dismal than all the caricatures that have ever been painted of Negro servitude in the South. Our bane and antidote are both before us. "Tit for Tat" is confined to England and the English, and is, therefore, a more direct and appropriate reply to the Duchess of Sutherland's minion. The bold, startling pictures are drawn from real life, and their darkest shadows do not exaggerate the depths of degradation and misery into which the fairest specimens of God's handiwork are plunged; white men capable of appreciating misery in its highest forms, and of enjoying all its benefits and refinements. And all the suffering and woe depicted by the author with masculine vigor are the direct results of the cruel oppression of the aristocracy, to whom Mrs. Stowe plays the flunkey, flattering in their vices, the tyrants who wallow in luxury upon the toil and blood of the people.--_New York Citizen._

Copies of the above books sent per mail free of postage. Send cash orders to

DICK & FITZGERALD. No. 18 Ann Street, New York.

+--------------------------------------------------------------------+ | TRANSCRIBER'S NOTE: | | | | Minor typographical errors have been corrected without note. | | | | Punctuation and spelling were made consistent when a predominant | | form was found in this book; otherwise they were not changed. | | | | Ambiguous hyphens at the ends of lines were retained. | | | | Mid-paragraph illustrations have been moved between paragraphs and | | some illustrations have been moved closer to the text that | | references them. The List of Illustrations paginations were | | changed accordingly. | | | | Italicized words are surrounded by underline characters, | | _like this_. | | | | Each page starts with a biblical quotation. These were enclosed in | | square brackets, labeled verses and moved between paragraphs when | | needed. | | | | Other Notes: | | | | p. ix: LESSON XI follows LESSON XII. This was not changed. | | | | p. 151: Two consecutive questions are labeled 666. This was not | | changed. | | | | p. 177: Question 892 was mislabeled. Changed to 802. | | | | p. 232: "F. The muscle situated underneath, ..." This muscle is | | shown above in Fig. 61. | | | | pp. 199 and 201 each have a question 869. This was not changed. | | | | p. 251: "... that required to be removed..." changed to "... that | | are required to be removed"... | | | | p. 253: "... destruction of the optic,..." changed to | | "... destruction of the optic nerve,..." | | | | p. 286: "... that minerals, and animals..." changed to | | "... animals..." | | | | p. 293: Two consecutive questions are labeled 666. This was not | | changed. | +--------------------------------------------------------------------+