A Guide to the Scientific Knowledge of Things Familiar

by Ebenezer Cobham Brewer

PART II.

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AIR.

CHAPTER XVIII.

Q. Of what is atmospheric air composed?

A. Principally of two gases, oxygen and nitrogen; mixed together in the following proportion: viz. 1 part of oxygen, to 4 parts of nitrogen.

Q. What are the uses of the oxygen of the air?

A. It is the oxygen of the air which supports combustion, and sustains life.

Q. What is meant when it is said, that the oxygen of the air “supports combustion?”

A. It means this; that it is the oxygen of the air which makes fuel burn.

Q. How does the oxygen of the air make fuel burn?

A. The fuel being decomposed (by heat) into hydrogen and carbon; the carbon combines with the oxygen of the air, and produces combustion.

Q. What does the combination of carbon and oxygen produce?

A. The carbon of the fuel combining with the oxygen of the air makes carbonic acid gas. (see pp. [36], [37]).

Q. What becomes of the hydrogen of the fuel?

A. Hydrogen (being very inflammable) burns with a blaze, and is the cause of the flame which is produced by combustion. (see p.[34]).

Q. What becomes of the nitrogen of the air, amidst all these changes and combinations?

A. The nitrogen of the air escapes, and is absorbed by the leaves of grass, trees, and various other vegetables.

Q. What is meant when it is said, that oxygen “sustains life”?

A. It means this: if a person could not inhale oxygen, he would die.

Q. What good does this inspiration of oxygen do?

A. 1st—It gives vitality to the blood: and

2ndly—It is the cause of animal heat.

Q. How is food converted into blood?

A. After it is swallowed, it is dissolved in the stomach into a grey pulp; it then passes into the intestines, and is converted by the “bile” into a milky substance (called chyle).

Q. What becomes of the milky substance, called chyle?

A. It is absorbed by the vessels called “lacteals,” and poured into the veins on the left side of the neck.

Q. What becomes of the chyle after it is poured into the veins?

A. It then mingles with the blood, and is itself converted into blood.

Q. How does the oxygen we inhale mingle with the blood?

A. The oxygen of the air mingles with the blood in the lungs, and converts it into a bright red colour.

Q. What colour is the blood before it is oxydized in the lungs?

A. A dark purple. The oxygen turns it to a bright red.

Q. Why are persons so pale who live in close rooms and cities?

A. The blood derives its redness from the oxygen of the air inhaled; but, as the air in close rooms and cities is not fresh, it is deficient in oxygen, and cannot turn the blood to a beautiful bright red.

Q. Why are persons who live in the open air and in the country, of a ruddy complexion?

A. As the blood derives its bright red colour from the oxygen of the air inhaled, therefore, country-people (who inhale fresh air) are more ruddy than citizens.

Q. Why is not the air in cities so fresh as that in the country?

A. Because it is impregnated with the breath of its numerous inhabitants, the odour of its sewers, the smoke of its fires, and many other impurities.

Q. How does the combination of oxygen with the blood produce animal heat?

A. The principal element of the blood is carbon, which (combining with the oxygen of the air inhaled) produces carbonic acid gas, (in the same way as burning fuel.) (see pp.[33],[36]).

Q. What becomes of the nitrogen of the air, after the oxygen enters the blood?

A. The nitrogen is exhaled, and taken up by the leaves of trees and other vegetables. (see p.[35]).

Q. Why does the vitiated air (after the oxygen has been absorbed) come out of the mouth, and not sink into the stomach?

A. The vitiated air (being heated by the heat of the body) ascends naturally, and passes by the heavier fresh air (which we inhale) without obstruction or injury.

Q. If (both in combustion and in respiration) the oxygen of the air is consumed, and the nitrogen rejected—Why are not the proportions of the air destroyed?

A. Because the upper surface of vegetable leaves (during the day) gives out oxygen and absorbs nitrogen, and thus the proper balance is perpetually restored.

Q. Show how God has made animal and vegetable life dependent on each other?

A. Animals require oxygen to keep them alive, and draw it from the air by inspiration; the upper surface of leaves (all day long) gives out oxygen, and thus supplies the air with the very gas required by man and other animals.

Q. Do not animals exhale the very gas needed by vegetables?

A. Yes; animals reject the nitrogen of the air (as not suited to the use of animal life), but vegetables absorb it, as it is the food they live on; and thus the vegetable world restores the equilibrium of the air, disturbed by man and other animals.

Q. Is air a good conductor?

A. No; air is a very bad conductor.

Q. How is air heated?

A. By “convective currents.”

Q. What are meant by “convective currents?”

A. When a portion of air is heated, it rises upward in a current, carrying the heat with it: other colder air succeeds, and (being heated in a similar way) ascends also; and these are called convective currents.

(“Convective currents;” so called from the Latin words, cum-vectus (carried with) because the heat is “carried with” the current.)

Q. Is air heated by the rays of the sun?

A. No; air is not heated (in any sensible degree) by the action of the sun’s rays passing through it.

Q. Why then is the air hotter on a sunny day, than on a cloudy one?

A. On a fine day, the sun heats the surface of the earth, and the air (resting on the earth) is heated by contact; as soon as it is heated it ascends, and other air succeeding is heated in a similar way, till all is heated by convection.

Q. If air be a bad conductor, why does hot iron get cold, by being exposed to the air?

A. A piece of hot iron exposed to the air, is made cold—1st—By “convection;” and

2ndly—By “radiation.”

Q. How is hot iron (exposed to the air) made cold by convection?

A. The air around the iron (being intensely heated by contact) rapidly ascends, carrying some of its heat with it: other air succeeds, absorbs more heat, ascends, and gives place to that which is colder; till the hot iron is cooled completely down.

Q. How is hot iron cooled by radiation?

A. While the heat of the iron is being carried off by “convection,” it is throwing off heat (on all sides) by radiation.

Q. What is meant by radiation?

A. Heat emitted (in all directions) from any surface, by innumerable rays.

Q. Why is broth cooled by being left exposed to the air?

A. Hot broth throws off some heat by radiation; but it is mainly cooled down by convection.

Q. How is hot broth cooled down by convection?

A. The air resting on the hot broth (being heated) ascends; colder air succeeding absorbs more heat, and ascends also; and this process is repeated, till the broth is made cool.

Q. Why is hot tea and broth cooled faster, for being stirred about?

A. 1st—The agitation assists the liquor in bringing its hottest particles to the surface:

2ndly—The action of stirring agitates the air, and brings it quicker to the broth or tea: and

3rdly—As the hottest particles are more rapidly brought into contact with the air, therefore convection is more rapid.

Q. Why is hot tea, &c. cooled more rapidly by blowing it?

A. Because the heated air is blown more rapidly away; in consequence of which, cold air more rapidly succeeds to absorb heat from the surface of the tea or broth.

Q. If a shutter be closed in the day-time, the stream of light (piercing through the crevice) seems in constant agitation. Why is this?

A. The air (in the sun-beam piercing through the shutter-crevice) is more heated, than that in its neighbourhood; the convective current, therefore, is distinctly seen, where little motes and particles of dust are thrown into agitation by the violence of the current.

Q. Why is the gallery of a church or theatre hotter than the aisle or pit?

A. The hot air ascends from the bottom to the top of the room, and cold air (from the doors and windows) flies to the bottom to supply its place.

Q. Why does a crowded room produce head-ache?

A. Because we breathe air vitiated by the crowd.

Q. How does a crowd vitiate the air of a room?

A. Whenever we breathe, the elements of the air are separated in the lungs, some of the oxygen is absorbed by the blood, and some of it is converted into carbonic acid gas, and exhaled with the nitrogen.

Q. Is all the nitrogen rejected by the lungs?

A. Yes; all the nitrogen of the air is always exhaled.

Q. What is carbonic acid gas?

A. As carbon has a very great affinity for oxygen, therefore, whenever they are exposed to heat, they combine, and form carbonic acid gas (or what is vulgarly called fixed air).

Q. Is carbonic acid gas wholesome?

A. No; it is quite fatal to animal life; and whenever it is inhaled, it acts like a narcotic poison, (producing drowsiness which ends in death).

Q. Why is a crowded room unwholesome?

A. Because the oxygen of the air is either absorbed by the lungs, or substituted for carbonic acid gas, which is a noxious poison.

Q. Mention the historical circumstances, so well known in connection with the “Black Hole of Calcutta.”

A. In the reign of George II, the Raja (or Prince) of Bengal[12] marched suddenly to Calcutta to drive the English from the country; as the attack was unexpected, the English were obliged to submit, and 146 persons were taken prisoners.

[12] The Sur Raja, at Dowlat; a young man of violent passions, who had but just succeeded to the throne. A. D. 1756.

Q. What became of these prisoners?

A. They were driven into a place about 18 feet square, and 15 or 16 feet in height, with only two small grated windows. 123 of the prisoners died in one night; and (of the 23 who survived) the larger portion died of putrid fevers, after they were liberated in the morning.

Q. Why were 123 persons suffocated in a few hours, from confinement in this close hot prison-hole?

A. Because the oxygen of the air was soon consumed by so many lungs, and its place supplied by carbonic acid exhaled by the hot breath.

Q. Why do persons in a crowded church feel drowsy?

A. 1st—Because the crowded congregation inhale a large portion of the oxygen of the air, which alone can sustain vitality and healthy action: and

2ndly—The air of the church is impregnated with carbonic acid gas, which (being a strong narcotic) produces drowsiness in those who inhale it.

Q. Why did the captives in the black hole die sleeping?

A. 1st—Because the absence of oxygen quickly affects the vital functions, depresses the nervous energies, and produces a lassitude which ends in death: and

2ndly—The carbonic acid gas inhaled by the captives (being a narcotic poison) would also produce drowsiness and death.

Q. Why do persons, who are so much in the open air, enjoy the best health?

A. Because the air they inhale is much more pure.

Q. Why is country air more pure than the air in cities?

A. 1st—Because there are fewer inhabitants to vitiate the air:

2ndly—There are more trees to restore the equilibrium of the vitiated air: and

3rdly—The free circulation of air keeps it pure and wholesome (in the same way as running streams are pure and wholesome, while stagnant waters are the contrary).

Q. Why does the scantiness of a country population render the country air more pure?

A. Because the fewer the inhabitants, the less carbonic acid will be exhaled; and thus country people will inhale pure oxygen, instead of air impregnated with the narcotic poison, called carbonic acid gas.

Q. Why do trees and flowers help to make country air wholesome?

A. Because trees and flowers absorb the carbonic acid generated by the lungs of animals, putrid substances, and other noxious exhalations.

Q. Why is the air of cities less wholesome than country air?

A. 1st—Because there are more inhabitants to vitiate the air:

2ndly—The sewers, drains, bins, and filth of a city, very greatly vitiate the air:

3rdly—The streets and alleys prevent a free circulation: and

4thly—Besides all this, there are fewer trees to absorb the excess of carbonic acid gas, and restore the equilibrium.

Q. Why are persons who live in close rooms and crowded cities, generally sickly?

A. Because the air they breathe is not pure, but is both defective in oxygen, and impregnated with carbonic acid gas.

Q. Where does the carbonic acid of close rooms and cities come from?

A. From the lungs of the inhabitants, the sewers, drains, and so on: besides, trees and gardens are not numerous enough to absorb the noxious gas as fast as it is generated.

Q. What becomes of the carbonic acid of crowded cities?

A. Some of it is absorbed by vegetables, and the rest is blown away by the wind, and diffused through the whole volume of the air.

Q. Does not this constant diffusion of carbonic acid affect the purity of the whole air?

A. No; because after it is thus diffused, it is carried to various lands, and absorbed in its passage by the vegetable world.

Q. Why do persons who ascend in balloons feel intense pain in their eyes and ears?

A. Because the air of the upper regions is more rarefied than the air on the earth; and the air inside their bodies (seeking to become of the same rarity) bursts through their eyes and ears, producing an intense pain.

Q. Why is it often painful, and difficult to breathe, on a mountain top?

A. Because the pressure of air on the mountain top is not so great as on the plain; and the air inside our bodies (seeking to become of the same rarity) bursts through the pores of the body, and produces great pain.

Q. Why do we feel oppressed just previous to a storm?

A. Because the air is greatly rarefied by heat and vapour; and the air inside us (seeking to become of the same rarity) produces an oppressive and suffocating feeling.

Q. Why do divers suffer great pain in their eyes and ears under water?

A. Because the air at the bottom of the sea is more dense than the air on the surface; and while the air inside the diver’s body is settling into the same density, he feels oppressed with pain, especially in the ears.

Q. Why is this pain felt especially about the ears of a diver?

A. The ear is fitted with a small membrane called the drum (or tympanum), through which the dense air bursts, and the rupture very often produces incurable deafness.

Q. Why do our corns ache just previous to rain?

A. Previous to rain, the density of air is greatly lowered (as every one knows from the fall of the barometer); in consequence of an unequal pressure, our feet swell; but the hard corn, not being elastic, is painfully stretched and pressed.

(Some of this pain is due to electricity.)

Q. Why do cellars feel warm in winter?

A. As the external air has not free access into cellars, they remain at a pretty even temperature, which (in winter time) is about 10 degrees warmer than the external air.

Q. Why do cellars feel cold in summer time?

A. As the external air has not free access into cellars, they remain at a pretty even temperature, which (in summer time) is about 10 degrees colder than the external air.

Q. Why does lightning strike the oak-tree more frequently than any other tree?

A. 1st—Because the grain of the oak, being closer than that of any other tree, renders it a better conductor: and

2ndly—The sap of the oak contains a large quantity of iron in solution, which is a most admirable conductor of lightning.

Q. Why does air rust iron?

A. The oxygen of the air combines with the surface of the iron, and produces oxide of iron, which is generally called rust.

This rust is a species of combustion.

Q. Why does hot iron scale and peel off, when struck with a hammer?

A. The oxygen of the air very readily unites with the surface of the hot iron, and forms a metallic oxide (or rust) which scales off when struck with a hammer.

Q. Does iron rust in dry air?

A. No; iron undergoes no change in dry air.

Q. Why do stoves and fire-irons become rusty, in rooms which are not occupied?

A. Because the air is damp; and moist air oxidizes (or rusts) iron and steel.

Q. In what part of the year is it most difficult to keep stoves and fire-irons bright?

A. In autumn and winter; because the capacity of the air for holding water being on the decrease, its vapour is deposited on every-thing with which it comes in contact.

Q. Why does greasing iron prevent its becoming rusty?

A. Because grease prevents the humidity of air from coming in contact with the surface of the iron.

Q. Why do not stoves rust so frequently as pokers and tongs?

A. Because stoves are generally covered with plumbago, or black lead.

Q. What is plumbago, or black lead?

A. A mixture of charcoal and iron filings.

A most excellent varnish to prevent rust is made of 1 pint of fat oil varnish, mixed with 5 pints of highly rectified spirits of turpentine, rubbed on the iron or steel with a piece of sponge. This varnish may be applied to bright stoves and even mathematical instruments, without injuring their delicate polish.

Q. Why does ornamental steel (of a purple or lilac colour) rust more readily than polished white steel?

A. Because the lilac tinge is produced by partial oxidation; and the process which forms rust has, therefore, already commenced.

Q. How can lilac steel be kept free from rust?

A. By keeping it in a very dry place; for then no additional oxygen will come in contact with it, to increase its amount of rust.

Q. Do any other metals (besides iron) combine rapidly with oxygen?

A. Yes; copper, lead, mercury, and even silver to some extent.

Q. Why does copper tarnish?

A. The tarnish of copper is caused by its oxidation; that is, the oxygen of the air combines with the surface of the copper, and instead of rusting it, covers it with a dark tarnish.

Q. Why does lead lose its brightness, and become dull and of a darker hue, by being exposed to the air?

A. The vapour of the air combines with the lead, and oxidizes its surface; but instead of becoming rusty, the surface becomes dull, and of a darker colour.

Q. Why is it difficult to keep silver bright?

A. Because the vapour of the air oxidizes its surface, and tarnishes it.

Q. Why do silver tea-pots and spoons tarnish more quickly than silver ore or bullion?

A. Because alloy (of some baser metal) is used to make it more hard and lasting; and this alloy oxidizes more quickly than silver itself.

Q. Why does German silver turn a dingy yellow in a few hours?

A. German silver has a great affinity for oxygen, and shows its oxidation by a sickly yellow tarnish, instead of rust.

Q. If quicksilver (or mercury) is tarnished like copper and lead,—Why does it preserve its brilliancy in barometers and thermometers?

A. Because air is excluded from it, and no moisture comes in contact with it to oxidize (or tarnish it).

Q. Is gold affected by the atmosphere?

A. Not readily: gold will never combine with oxygen of itself, (or without aid).

Q. Which of the metals is capable of resisting oxidation altogether?

A. Plat’inum; in consequence of which, the graduated arcs of delicate instruments for observation are made of plat’inum instead of any other metal.

Q. Why is plat’inum used for the graduated arcs of delicate mathematical instruments, instead of any other metal?

A. Because it will never oxidize; but retain its bright surface in all weathers free from both rust and tarnish.

Q. Before plat’inum was discovered, which of the metals was employed for the same purpose?

A. Gold.

Platinum, (a white metal), so called from “plata,” the Spanish word for silver. It was first introduced into England by Mr. Wood, (A. D. 1749) from South America.

Q. For what other scientific purposes is plat’inum now used?

A. For crucibles in which acids are employed, and for galvanic batteries.

Q. Why are crucibles (in which acids are employed) made of plat’inum?

A. Because the acid would act upon other metals, or upon glass, and prevent the experimenter’s success.

Q. Which of the metals have the greatest affinity to oxygen?

A. Those called potassium and sodium.

Potassium and sodium derive their names from potash and soda. Potassa is the oxide of potassium; and soda is the oxide of sodium.

Q. How is the affinity of potassium and sodium for oxygen shewn?

A. They decompose water the moment they are brought into contact with it.

Q. What effect has potassium on water?

A. It catches fire the moment it is thrown into water, and burns with a vivid flame, which is still further increased by the combustion of hydrogen separated from the water.

(N.B. Water is composed of oxygen and hydrogen; and potassium separates the two gases.)

Q. What effect has sodium on water?

A. It does not take fire as potassium does, but undergoes very rapid oxidation.

Q. Is the furr of kettles an oxide?

A. No; the furr (or deposit of boiling water) is a precipitate of lime and mineral salt, separated from the water by the process of boiling.

Q. Is not this furr of boiling water often dangerous?

A. Yes; especially in tubular boilers, such as those employed in railways.

Q. Why is this furr especially troublesome in railway engines?

A. Because it is a bad conductor of heat; in consequence of which, it hinders the evaporating effect of the fire, and prevents the economy of fuel.

Q. Why is this furr especially dangerous in railway engines?

A. Because when it is deposited in the boilers, they are likely to become over-heated; and then explosion will take place from the sudden generation of highly elastic steam.

Q. Why cannot railway engines be fed with brackish water?

A. Because brackish water contains mineral salt, which makes a much larger deposit of furr, than that which contains only vegetable matter.

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CHAPTER XIX.
CARBONIC ACID GAS.

Q. What is choke damp?

A. Carbonic acid gas accumulated at the bottom of wells and pits, which renders them noxious, and often fatal.

Q. Why is not this carbonic acid taken up by the air, and diffused, as it is in cities?

A. Because (being heavier than common air) it cannot rise from the well or pit; and no wind can get to it to blow it away.

Q. Is carbonic acid wholesome?

A. No; it is fatal to animal life, when inhaled through the mouth; acting on the stomach, as a narcotic poison (i. e.. a poison which produces death from drowsiness).

Q. How can any one know, if a place be infested with carbonic acid gas?

A. If a pit or well contain carbonic acid, a candle (let down into it) will be instantly extinguished. The rule, therefore, is this—Where a candle will burn, a man can live; but what will extinguish a candle, will also destroy life.

Q. Why does a miner lower a candle into a mine, before he descends?

A. Because the candle will be extinguished, if the mine contains carbonic acid gas: but if the candle is not extinguished, the mine is safe, and the man may fearlessly descend.

Q. Why are persons sometimes killed, by leaning over beer vats?

A. Vats (where beer has been made) contain a large quantity of carbonic acid gas, produced by the “vinous fermentation” of the beer; and when a man incautiously leans over a beer vat, and inhales the carbonic acid, he is immediately killed thereby.

Q. Why are persons often killed, who enter beer vats to clean them?

A. Carbonic acid (being heavier than atmospheric air) often rests upon the bottom of a vat: when, therefore, a person enters the vat, and stoops to clean the bottom, he inhales the pernicious gas, which kills him.

Q. Why are the jungles of Jarva and Hindostan so fatal to life?

A. Because vast quantities of carbonic acid are thrown off by decaying vegetables; and (as the wind cannot penetrate the thick brushwood to blow it away) it settles there, and destroys animal life.

Q. Why are persons sometimes killed by having a charcoal fire in their bed-rooms?

A. When charcoal is burned, the carbon of the charcoal unites with the oxygen of the air, and forms carbonic acid gas, which is a narcotic poison.

Q. Why does the carbonic acid gas of a charcoal fire rise and disperse itself about the room; whereas the carbonic acid gas of a beer vat settles near the floor?

A. The carbonic acid gas of a charcoal fire is heated by the combustion of the fuel, and rises; but the carbonic acid gas of a beer vat is not heated, and, therefore, rests on the bottom of the vat.

Q. Why do persons throw lime into bins to prevent their offensive smell, in summer time?

A. Bins contain large quantities of carbonic acid gas, which readily combines with lime, and produces “carbonate of lime,” which is entirely free from all offensive odour.

Q. Why do persons throw lime into sewers in summer time?

A. Sewers (like bins) contain large quantities of carbonic acid, which readily combines with lime, and produces carbonate of lime; and thus the offensive gas of the sewer is neutralized.

Q. Can carbonic acid be removed in any way besides by lime?

A. Yes; water thrown into a pit will disperse the carbonic acid.

Q. What effect has water on carbonic acid gas?

A. Water (under pressure) absorbs carbonic acid gas; and parts with it (when the pressure is removed) in the form of effervescence.

Q. Why does aerated water effervesce, when the cork is removed?

A. While the cork was fastened down, the water absorbed the carbonic acid; but the moment the pressure is removed (by taking out the cork) the gas is given out with effervescence.

Q. Why does soda water effervesce?

A. Soda water contains 8 times its own bulk of carbonic acid gas, which makes its escape in effervescence, the moment that the cork is removed.

Q. Why does ginger pop fly about in froth, when the string of the cork is cut?

A. All vinous fermentation produces carbonic acid gas. While the cork is fast, the water of the liquor absorbs the carbonic acid; but the moment that the pressure is removed, the gas is given off in effervescence.

Q. Why does bottled ale froth, more than draught ale?

A. Because the pressure is greater in a bottle than in a tub which is perpetually tapped: and effervescence is always produced in proportion to the pressure.

Q. Why does bottled ale and porter become “lively” and frothy by being set before the fire?

A. The heat of the fire expands the air (between the liquid and the cork), and as this air expands, it presses the liquid down, which causes effervescence.

Q. What produces the froth of bottled porter?

A. The carbonic acid gas, produced by its vinous fermentation; which is absorbed by the liquor so long as the bottle is well corked, but is given off in froth as soon as the pressure of the cork is removed.

Q. What gives the pleasant acid taste to soda water, ginger beer, champagne, and cider?

A. The presence of carbonic acid, generated by fermentation, and liberated by effervescence when the pressure of the cork is removed.

Q. Why does fresh spring water sparkle, when poured from one vessel to another?

A. Because fresh spring and pump water contain carbonic acid; and it is the presence of this gas which makes the water sparkle.

Q. What is the fermentation of beer and wine?

A. The production of carbonic acid gas and al’cohol.

Q. How is carbonic acid gas produced by fermentation?

A. Malt and fruit both contain sugar; and sugar consists of carbon, oxygen, and hydrogen. In fermentation, a part of the carbon and oxygen of the sugar escape, in the form of carbonic acid gas.

Carbonic acid gas is a compound of carbon and oxygen, in the following proportions:—3 lbs. of carbon and 8 lbs. of oxygen will form 11 lbs. of carbonic acid gas. Now, 100 lbs. of white sugar contains 43 lbs. of carbon; 50 lbs. of oxygen; and 7 lbs. of hydrogen.

Q. How is al’cohol produced by fermentation?

A. The hydrogen of the sugar combines with the residue of the oxygen and carbon to form “al’cohol.”

Q. What is al’cohol?

A. Al’cohol is the spirit of wine or beer, obtained by fermentation.

(100 gallons of alcohol consist of 38 gallons of oxygen; 43-1/2 of carbon; 15 of hydrogen; and 3-1/2 of nitrogen.)

Q. Why is barley malted?

A. Because germination is produced by the artificial heat; and in germination the starch of the grain is converted into sugar.

Q. How is barley malted?

A. The barley is moistened with water, and heaped up; by which means, great heat is produced, which makes the barley sprout.

Q. Why is not the barley suffered to grow, as well as sprout?

A. Plants in the germ contain more sugar than in any other state; as soon as the germ puts forth shoots, the sugar of the plant is consumed, to support the shoot.

Q. How is barley prevented from shooting, in the process of malting?

A. The barley is put into a kiln as soon as it sprouts; and the heat of the kiln checks or destroys the young shoot.

Q. Why is yeast put into beer to make it work?

A. Yeast supplies the beer with nitrogen, which is one of the ingredients of alcohol.

Alcohol consists of oxygen, carbon, and hydrogen, (obtained from the sugar of malt), and nitrogen, (obtained from yeast).

Q. Why is it not needful to put yeast into wine?

A. Because fruit contains carbon, hydrogen, oxygen, and nitrogen, in the form of “gluten;” and, therefore, ferments spontaneously.

(Gluten is explained fully in the Appendix.—Turn to the word in the Index.)

Q. Does not malt contain carbon, hydrogen, oxygen, and nitrogen, as well as fruit?

A. No; the sugar of malt contains carbon, hydrogen, and oxygen, but no nitrogen; in consequence of which, yeast (which contains nitrogen) is added to the wort.

Q. Why do not grapes ferment while they hang on the vine?

A. 1st—Because the skin lets out the water of the pulp, which causes the grapes to shrivel and dry up: and

2ndly—The skin prevents the admission of oxygen into the pulp, from the air without.

Q. What is the froth or scum of fermented liquors?

A. Carbonic acid gas, which (being heavier than common air) settles on the top of the liquor, in the form of scum.

Q. Why does a small piece of raw meat, or a few raisins improve flat beer?

A. 1st—Because they supply it with nitrogen to form it into al’cohol.

2ndly—As the raw meat, &c. putrifies, it gives off carbonic acid gas into the beer, which gives it “life.”

Q. Why is beer flat, if the cask be open too long?

A. Because too much of the carbonic acid gas (produced by fermentation) is suffered to escape.

Q. How is the carbonic acid gas of beer generated?

A. The saccharine (or sugar) of the malt is converted by fermentation into carbonic acid gas and alcohol.

Q. Why does beer turn flat, if the vent peg be left out of the tub?

A. Because the carbonic acid gas escapes through the vent hole.

Q. Why will not beer run out of the tub, till the vent peg is taken out?

A. When the tap is turned, air rushes through the tap into the bottom of the tub, and holds the liquor in.

The upward pressure of air is illustrated by the

following simple experiment:—Fill a wine-glass with water; cover the top of the glass with a piece of writing paper; turn the glass topsy turvy, and the water will not run out. The paper is used merely to give the air a medium sufficiently dense to act against.

Q. Why does the beer run freely, immediately the vent peg is taken out?

A. As soon as the vent peg is taken out, air rushes through the vent hole at the top of the tub,—presses the liquor down, and forces it through the tap.

Q. Why does liquor flow reluctantly out of a bottle held upside down?

A. Because the upward pressure of the air prevents the liquor from flowing out.

Q. Why should a bottle be held obliquely, in order to be emptied of its liquor?

A. Because air will then flow into the bottle, and help the liquor out by balancing the upward pressure.

Q. Why does wine (poured from a bottle quickly) spirt about without going into the decanter?

A. The liquor fills the top of the decanter (like a cork), and leaves no room for the air inside to escape; therefore, the decanter (being full of air) refuses to admit the wine.

Q. Why is beer made stale, by being exposed to the air?

A. Because air absorbs its carbonic acid, which gave it “life.”

Q. Why is porter made stale, by being exposed to the air?

A. Because air absorbs its carbonic acid, which gave it “life.”

Q. Why does the effervescence of soda water and ginger beer so soon go off?

A. Because air absorbs the carbonic acid, which produced the effervescence.

Q. Why is boiled water flat and insipid?

A. Because the whole of the carbonic acid is expelled by boiling, and absorbed by the air.

Q. Why does water become flat and insipid, after it has been drawn some time?

A. Because air absorbs its carbonic acid; and when its carbonic acid is absorbed, the water is flat and insipid.

Q. Why should spring water (used for washing) be exposed to the air?

A. Spring water contains carbonic acid; but (by being exposed to the air) this carbonic acid is absorbed, and the water becomes more soft.

Q. Why does yeast make bread light?

A. Flour contains a small portion of saccharine matter (or sugar); and the yeast (mixing with this) produces fermentation, as it does in brewing.

Q. How does fermentation make the dough rise?

A. During fermentation, carbonic acid gas is evolved; but the sticky texture of the dough will not allow it to escape, so it forces up little bladders all over the dough.

Q. Why is dough placed before the fire?

A. 1st—Because the heat of the fire increases the fermentation: and

2ndly—It expands the gas which is confined in the little bladders; in consequence of which, the bladders are blown up larger, and the dough becomes lighter and more porous.

Q. Why is bread heavy, if the dough be removed from the fire?

A. Because the dough gets cold, and then the air in the bladders condenses,—the paste falls,—and the bread is close and heavy.

Q. Whence does the heat of fire arise?

A. The carbon of fuel (when heated) combines with the oxygen of the air, and produces carbonic acid gas: again, the hydrogen of the fuel combining with other portions of oxygen, condenses into water; by which chemical actions heat is evolved.

Q. Whence does the heat of our own body arise?

A. The carbon of the blood combines with the oxygen of the air inhaled, and produces carbonic acid gas; which produces heat in a way similar to burning fuel.

Q. Whence does the heat of a dunghill arise?

A. The straw, &c. of the dunghill undergoes fermentation as it decays: the fermentation produces carbonic acid gas, and heat is evolved by a species of combustion (as in the two former cases).

Q. What changes do vegetables undergo from putrefaction?

A. The hydrogen of the vegetables combines with the oxygen of the air, and forms water: again, the carbon of the vegetables combines with oxygen of the air, and forms carbonic acid gas. Putrefaction, therefore, is only another species of combustion.

Q. What changes do animal bodies undergo from putrefaction?

A. The same as vegetables, with this addition—they give out ammonia, sulphur, and phosphorus also; which causes the offensive smell of putrefying animal bodies.

Q. Why is lime heated by a kiln?

A. All marl and chalk abound in carbonic acid; and (when heated by a fire) the carbonic acid flies off in gas, producing great heat.

Q. What is mortar?

A. Lime mixed with sand and water.

Q. What is lime?

A. Lime-stone burnt produces lime.

Q. Why is the lime-stone burnt, in order to make it into lime?

A. The fire expels the carbonic acid, and converts the hard lime-stone into a loose powder.

Q. Why does mortar become hard, after a few days?

A. Because the lime re-imbibes the carbonic acid of the air, which was expelled by fire; and the loose powder again becomes as hard as the original lime-stone.

Q. Why is mortar adhesive?

A. When the carbonic acid is expelled, the hard lime-stone is converted into a loose powder, which (being mixed with sand and water) becomes a soft and sticky plaster; but, as soon as it is placed between bricks, it imbibes carbonic acid again, and hardens into lime-stone.

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CHAPTER XX.
CARBURETTED HYDROGEN GAS.

Q. What is choke-damp?

A. Carbonic acid gas accumulated at the bottom of wells and pits. It is called choke damp, because it chokes (or suffocates) every animal that inhales it. (see p. [264]).

Q. What is marsh-gas or fire-damp?

A. Carburetted hydrogen gas accumulated on marshes, in stagnant waters, and coal pits; it is frequently called “inflammable air.”

Q. What is carburetted hydrogen gas?

A. Carbon combined with hydrogen.

Q. How may carburetted hydrogen gas be procured on marshes?

A. By stirring the mud at the bottom of any stagnant pool, and collecting the gas (as it escapes upwards) in an inverted glass vessel.

Q. What is coal gas?

A. Carburetted hydrogen extracted from coals, by the heat of fire.

Q. Why is carburetted hydrogen gas called fire-damp, or inflammable air?

A. Because it very readily catches fire and explodes, when a light is introduced to it.

Q. Why is carburetted hydrogen gas frequently called marsh gas?

A. Because it is generated in meadows and marshes from putrefying vegetable substances. (See ignis fatuus, p. [285]).

Q. What gas is evolved by the wick of a burning candle?

A. Carburetted hydrogen gas: that is, the carbon and hydrogen of the tallow combine into a gas from the heat of the flame; and this gas is carburetted hydrogen, or inflammable air.

Q. Why do coal-mines so frequently explode?

A. Because the carburetted hydrogen gas (which is generated in these mines by the coals) explodes, when a light is incautiously introduced.

Q. How can miners see in the coal-pits, if they may never introduce a light?

A. Sir Humphrey Davy invented a lantern for the use of miners, called “the Safety Lamp,” which may be used without danger.

Q. Who was Sir Humphrey Davy?

A. A very clever chemist, born in Cornwall. (1778—1829).

Q. What kind of thing is the safety lamp?

A. It is a kind of lantern covered with a fine gauze wire, instead of glass or horn.

Q. How does this fine gauze wire prevent an explosion in the coal mine?

A. 1st—Because flame will never pass through fine gauze wire: and

2ndly—Though the wire get red-hot, it will not ignite the gas; for carburetted hydrogen gas can be ignited only by flame.

(N. B. The interstices of the gauze wire must not exceed the 7th of an inch in diameter.)

Q. Why will not flame pass through very fine wire-gauze?

A. Because the metal wire is a very rapid conductor of heat; and when the flame of burning gas in the lamp reaches the wire gauze, the heat (which is needful to produce flame) is conducted away by the wire, and the flame is extinguished.

Q. Does the gas of the coal-pit get through the wire gauze into the lantern?

A. Yes; but the inflammable gas ignites and burns inside the lamp: as soon, however, as this is the case, the miner is in danger, and should withdraw.

Q. Why is the miner in danger, if the gas ignites and burns in the inside of the safety-lamp?

A. Because the heat of the burning gas will soon destroy the wire gauze, and then the flame (being free) will set fire to the mine.

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CHAPTER XXI.
PHOSPHURETTED HYDROGEN GAS.

Q. From what does the very offensive effluvia of church-yards arise?

A. From a gas called phosphuretted hydrogen; which is phosphorus combined with hydrogen gas.

Q. What is phosphorus?

A. A pale amber-coloured substance, resembling wax in appearance. The word is derived from two Greek words, which mean “to produce or carry light.” (φῶς-φέρεινφῶς).

Q. How is phosphorus obtained?

A. By heating bones to a white heat; by which means the animal matter and charcoal are consumed, and what is left is called “phosphate of lime.”

Q. How is phosphate of lime converted into phosphorus?

A. It is reduced to powder, and mixed with sulphuric acid; which (being heated and filtered) is converted into phosphorus.

Q. Of what are lucifer matches made?

A. Of phosphorus; and above 250 thousand lbs. of phosphorus are used every year in London alone, merely for the manufacture of lucifer matches.

Q. Why does a putrefying dead body smell so offensively?

A. From the phosphuretted hydrogen gas, which always arises from putrefying animal substances.

The escape of ammonia and sulphur contributes also to this offensive effluvia.

Q. What is the cause of the ignis fatuus, Jack o’Lantern, or Will o’the Wisp?

A. This luminous appearance (which haunts meadows, bogs, and marshes) arises from the gas of putrefying animal and vegetable substances; especially decaying fish.

Q. What gases arise from these putrefying substances?

A. Phosphuretted hydrogen gas from putrefying animal substances: and

Carburetted hydrogen, (or inflammable gas) from fermenting vegetable matters.

Some persons erroneously think that the Aurora Borealis, or Northern Lights, may be attributed to the same gases, burning in the upper regions of the air.

Q. How are these gases ignited on bogs and meadows?

A. By the electricity of the air, the rays of the sun, some accidental spark, the lamp of some traveller, or in some similar way.

And sometimes from the spontaneous combustion of some dung-heaps, &c. in the locality.

Q. Why does an ignis fatuus or Will o’the Wisp fly from us when we run to meet it?

A. When we run towards an ignis fatuus, we produce a current of air, which drives the light gas forwards.

Q. Why does an ignis fatuus run after us, when we flee from it in fright?

A. When we run away from the ignis fatuus, we produce a current in the way we run, which attracts the light inflammable gas in the same course.

Q. Is not a kind of Jack o’Lantern sometimes produced by an insect?

A. Yes; a swarm of luminous insects sometimes passes over a meadow, and produces an appearance exactly like that of the ignis fatuus.

Q. May this meteoric appearance be attributed to any other cause, besides those mentioned?

A. Yes; if many horses, sheep, pigs, or oxen, are pastured on a meadow, the animal vapour arising from them (strongly electrified by the air) will ignite, and produce a luminous appearance.

Q. May not many ghost stories have risen from some ignis fatuus lurking about church-yards?

A. Perhaps all the ghost stories (which deserve any credit at all) have arisen from the ignited gas of church-yards lurking about the tombs, to which fear has added its own creations.

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CHAPTER XXII.
WIND.

Q. What is wind?

A. Wind is air in motion.

Q. What puts the air in motion, so as to produce wind?

A. The principal causes are the variations of heat and cold, produced by the succession of day and night, and the four seasons.

Q. What effect has heat upon the air?

A. Heat rarefies the air, and causes it to expand.

Q. How do you know that heat causes the air to expand?

A. If a bladder half full of air (tied tight round the neck), were laid before a fire, the heat of the fire would expand the air so much, that the bladder would soon be entirely inflated; (in this case, the air in the bladder is expanded to twice its original bulk, by the heat of the fire).

Q. What effect is produced upon air by rarefaction?

A. It causes the air to ascend through colder strata, as a cork (put at the bottom of a basin of water) would ascend through the water.

Q. How do you know that rarefied air ascends?

A. When a boy sets fire to the cotton of his balloon, the flame heats the air inside the balloon; and the air becomes so light, that it ascends, and carries the balloon with it.

Q. What effect is produced upon air by cold?

A. Air is condensed by cold, or squeezed into a smaller compass; in consequence of which, it becomes heavier, and descends towards the ground.

Q. How do you know that air is condensed by cold?

A. After the bladder is fully inflated, (by lying before the fire), if it be taken away from the fire, the bladder will collapse, and show that it is not half full.

Q. What is meant by the bladder “collapsing?”

A. The skin will become wrinkled, shrivelled, and flabby, because there is not sufficient air inside to fill it out.

Q. How do you know that condensed air will descend?

A. As soon as the cotton of the balloon is burnt out, the air inside becomes cold again, and the balloon falls to the earth.

Q. Does the sun heat the air as it does the earth?

A. No; the air is not heated by the rays of the sun, because air (like water) is a very bad conductor.

Q. How is the air heated?

A. By convection, thus:—The sun heats the earth, and the earth heats the air resting upon it; the air thus heated rises, and is succeeded by other air, which is heated in a similar way, till all is warmed by “convective currents.”

Q. What is meant by “convective currents of air?”

A. Streams of air heated by the earth, which rise upwards and carry heat with them, are called “convective currents” of hot air.

Q. Is the air in a room in perpetual motion, as the air abroad is?

A. Yes; there are always two currents of air in the room we occupy, one of hot air flowing out of the room, and another of colder air flowing into the room.

Q. How do you know, that there are these two currents of air in every occupied room?

A. If I hold a lighted candle near the crevice at the top of the door, the flame will be blown outward (towards the hall); but if I hold the candle at the bottom of the door, the flame will be blown inwards (into the room).

Q. Why would the flame be blown outwards (towards the hall), if the candle were held at the top of the door?

A. Because as the air of the room is warmed by the fire, &c., it ascends; and (floating about the upper part of the room) some of it escapes through the crevice at the top of the door, and thus produces a current of air outwards (into the hall).

Q. Why would the flame be blown inwards (into the room), if the candle were held at the bottom of the door?

A. Because after the warm air of the room has ascended to the ceiling, or made its escape into the hall, &c., a partial vacuum is made at the bottom of the room; and cold air (from the hall) rushes under the door to supply the void.

Q. What is meant by a “partial vacuum being made, at the bottom of the room?”

A. A vacuum means a place from which the air has been taken: and a “partial vacuum” means, a place from which a part of its air has been taken away. Thus when the air on the floor ascends to the ceiling, a partial vacuum is made on the floor.

Q. And how is the vacuum filled UP again?

A. It is filled up by colder air, which rushes (under the door, and through the window crevices) into the room.

Q. Give me an illustration.

A. If I dip a pail into a pond and fill it with water, a hole (or vacuum) is made in the pond as big as the pail; but the moment I draw the pail out, the hole is filled up by the water around.

Q. Show how this illustration applies.

A. The heated air which ascends from the bottom of a room, is as much taken away, as the water in the pail; and (as the void was instantly supplied by other water in the pond) so the void of air is supplied by a current from without.

Q. What is the cause of wind?

A. The sun heats the earth, and the earth heats the air resting upon it; as the warm air ascends, the void is filled up by a rush of cold air to the place, and this rush of air we call wind.

Q. Does the wind always blow?

A. Yes; there is always some motion in the air; but the violence of the motion is perpetually varying.

Q. Why is there always some motion in the air?

A. As the earth is always turning round, the vertical rays of the sun are always varying.

Q. What do you mean by “the vertical rays of the sun?”

A. The rays made at noon-day: when the sun is in a direct line above any place, his rays are said to be “vertical” to that place.

Q. How are the vertical rays of the sun always varying?

A. Suppose the brass meridian of a globe to represent the vertical rays of the sun; as you turn the globe round, different parts of it will pass under the brass rim, in constant succession.

Q. And is it noon-day to the place over which the sun is vertical?

A. Yes; as each place passes under the brass meridian, it is noon-day to one half, and mid-night to the other.

Q. Show how this rotation of the earth affects the air.

A. If we suppose the brass meridian to be the vertical sun, the whole column of air beneath will be heated by the noon-day rays; that part which the sun has left, will become gradually colder and colder; and that part to which the sun is approaching, will grow constantly warmer and warmer.

Q. Then there are three qualities of air about this spot?

A. Yes; the air over the place which has passed the meridian is cooling: the air under the vertical sun is the hottest; and the air which is over the place about to pass under the meridian, is increasing in heat.

Q. How does this variety in the heat of air produce wind?

A. The air always seeks to preserve an equilibrium; so the cold air rushes to the void, made by the upward current of the warmer air.

Q. Why does not the wind always blow one way, following the direction of the sun?

A. Because the direction of the wind is subject to perpetual interruptions from hills and valleys, deserts and seas.

Q. How can hills and mountains alter the course of the wind?

A. Suppose a wind, blowing from the north, comes to a mountain, as it cannot pass through it, it must either rush back again, or fly off at one side (as a marble when it strikes against a wall).

Q. Do mountains affect the wind in any other way?

A. Yes; many mountains are capped with snow, and the warm air is condensed as it comes in contact with them; but as soon as the temperature of the wind is changed, its direction may be changed also.

Suppose A B C to be three columns of air. A, the column of air which is cooling down; B, the column to which the sun is vertical; and C, the column which is to be heated next. In this case the cold air of A will rush towards B C, because the air of B and C is hotter than A. But suppose now C to be a snow-capped mountain. As the hot air of B reaches C, it is chilled; and (being now colder than the air behind) it rushes back again towards A, instead of following the sun.

Q. How can the ocean affect the direction of the wind?

A. When the ocean rolls beneath the vertical sun, the water is not made so hot as the land; and (as another change of temperature is produced) another obstacle is offered to the uniform direction of the wind.

Q. Why is not the water of the sea made so hot by the vertical sun, as the surface of the land?

A. 1st—Because the evaporation of the sea is greater than that of the land:

2ndly—The waters are never still: and

3rdly—The rays of the sun strike into the water, and are not reflected from its surface, as they are by land.

Q. Why does the evaporation of the sea prevent its surface from being heated by the vertical sun?

A. As water absorbs heat by being converted into vapour; the surface of the sea is continually losing heat by evaporation.

Q. How does the motion of the sea prevent its surface from being heated by the vertical sun?

A. As one portion is heated it rolls away, and is succeeded by another; and this constant motion prevents one part of the sea from being heated more than another.

Q. How is the wind affected by the sea?

A. When air from the hot earth reaches the sea, it is often condensed, and either rushes back again, or else its violence is very greatly abated.

Q. Do clouds affect the wind?

A. Yes. As passing clouds screen the direct heat of the sun from the earth, they diminish the rarefication of the air also: and this is another cause why neither the strength nor direction of the wind is uniform.

Q. Would the winds blow regularly from east to west, if these obstructions were removed?

A. Without doubt they would. If the whole earth were covered with water, the winds would always follow the sun, and blow from east to west. Their irregularity is owing to the interspersion of sea and land, and the irregularities of the earth’s surface.

Q. Do winds never blow regularly?

A. Yes; in those parts of the world, where these obstructions do not exist; as on the Atlantic and Pacific Ocean, the winds are pretty uniform.

Q. What are the winds, which blow over the Atlantic and Pacific Ocean, called?

A. They are called “Trade Winds.”

Q. Why are they called trade winds?

A. Because (as they blow uniformly in one direction) they are very convenient to those who carry on trade by means of these oceans.

Q. In what direction do the trade winds blow?

A. That in the northern hemisphere blows from the north-east: that in the southern hemisphere from the south-east.

Q. Why do they not blow from the full north and south?

A. Because the polar current, combining with the equatorial current, give the wind a new direction.

Q. What is the cause of the equatorial current?

A. The rotation of the earth upon its axis.

Q. What is the cause of the polar current?

A. As the heat in the torrid zone is always greatest, and at the poles the least, therefore a constant current of air rushes from the poles towards the equator.

Q. How does the combination of these two currents give a new direction to them both?

A. When these currents of air meet at the equator, they clash together, and fly off in a new direction.

Q. Do trade winds blow from the north-east and south-east all the year round?

A. Yes, in the open sea; that is, in the Atlantic and Pacific Oceans for about 30 deg^s. each side of the equator.

Q. Do the trade winds blow uniformly from north-east and south-east in the indian ocean?

A. No; nor yet in those parts of the Atlantic and Pacific which verge on the land.

Q. Why do not the trade winds blow uniformly from north-east and south-east in the indian ocean?

A. Because when Arabia, Persia, India, and China, are exposed to the enormous heat of their summer sun, the air is so rarefied, that the colder air from the south pole rushes towards these nations, and not to the equator; in consequence of which, a south-west wind is produced for six months of the year.

Q. How does it blow for the other 6 months?

A. When the sun has left the northern side of the equator for the southern, then the southern part of the torrid zone is most heated; and the cold air from the north (rushing towards the southern tropic) is diverted to the north-east, where it continues for the other six months of the year.

Q. What are the six-month trade winds called?

A. They are called monsoons; and blow from the north-east from September to April, and from the south-west for the other six months of the year.

Q. Have we any regular winds in England?

A. No; our island (having a continent on one side, and a sea on the other) has a most variable climate.

Q. Have the winds in England no general direction throughout the year?

A. We generally find that easterly winds prevail during the spring of the year, and westerly winds are most common in the summer and autumn.

S-West winds are most frequent in July and August. N-East winds in January, March, April, May, June; and most seldom in July, September, and December.

Q. When are the winds in England generally the highest?

A. The winds in December and January are generally the highest. Those in February and November the next; and those in August and September the least boisterous.

Q. Why are the winds of Europe generally highest in December and January?

A. Because the sun is furthest south in those months; and (as the heat in these northern regions rapidly decreases) the contrast between our temperature and that of the torrid zone is greater in December and January, than in any other two months throughout the year.

Q. Why does this contrast of heat increase the violence of the winds?

A. As the air always seeks to preserve an equilibrium, therefore the greater the contrast, the more violent will be the rush of air to equalize the two volumes.

Q. Why are the winds in Europe generally the most placid during the months of September and August?

A. August and September are our warmest months, when we approach nearer to the heat of the torrid zone than in any other two months; therefore, the air (to and from the equator) moves with less velocity in our northern hemisphere.

Q. Show the goodness and wisdom of God in the constant tendency of air to equilibrium.

A. If the cool air of the polar regions did not rush into the torrid zone, it would become so hot, that no human being could endure it. If (on the other hand) the hot air from the torrid zone did not modify the polar regions, they would soon become insufferably cold.

Q. Why are east winds in England generally dry?

A. Because, as they come over the vast continents of Asia and Europe, they absorb very little water.

Q. Why does their imbibing so little water make them dry winds?

A. Being thirsty when they reach our island, they readily imbibe moisture from the air and clouds; and, therefore, bring dry weather.

Q. Why is the north wind in England generally cold?

A. The north wind comes from the polar regions, over mountains of snow, and seas of ice; in consequence of which, it is very cold.

Q. Why are north winds in England generally dry and biting?

A. As they come from regions colder than our own, they are warmed by the heat of our island; and (as their temperature is raised) they absorb moisture from every thing they touch; in consequence of which, they are both dry and parching.

Q. Why is the south wind generally warm in England?

A. The south wind comes over the hot sandy deserts of Africa, and is heated by the land it traverses.

Q. Why does the south wind often bring us rain?

A. The south wind (being much heated by the hot sands of Africa) imbibes water very plentifully, as it passes over the Mediterranean Sea and British Channel.

Q. Why does the saturation of the south wind cause rain?

A. As soon as it reaches our cold climate, it is condensed, and its vapour is squeezed out (as water from a wet sponge).

Q. Why are west winds in England generally rainy?

A. The west winds come over the Atlantic Ocean, and are laden with vapour: if, therefore, they meet with the least chill, some of the vapour is squeezed out.

Q. Why is a fine clear day sometimes overcast in a few minutes?

A. Because some sudden change of temperature has condensed the vapour of the air into clouds.

Q. Why are clouds sometimes dissipated quite as suddenly?

A. Because some dry wind (blowing over the clouds) has imbibed their moisture, and carried it off in invisible vapour.

Q. Why does a south-west wind bring us rain?

A. As it comes from the torrid zone, and crosses the ocean, the hot wind is laden with vapour; and as some of the heat escapes (as soon as it reaches our northern island) the vapour is condensed, and precipitated as rain.

Q. Why does a north-east wind rarely bring rain?

A. As it comes from a climate colder than our own, its capacity for imbibing vapour is increased when it reaches our island; in consequence of which, it dries the air, dispels the clouds, and promotes evaporation.

Q. Why does wind sometimes bring rain, and sometimes fine weather?

A. If the wind be colder than the clouds, it will condense their vapour into rain: if the wind be warmer than the clouds, it will dissolve them, and cause them to disappear.

Q. Why are March winds dry?

A. Because they generally blow from the east or north-east; and, therefore, sweep over the continent of Europe.

Q. What is the use of March winds?

A. They dry the soil (which is saturated with the floods of February), break up the heavy clods, and fit the land for the seeds which are committed to it.

Q. Why does “March come in like a lion?”

A. Because it comes in with blustering east winds, which are essential to dry the soil, which would otherwise rot the seed committed to it.

Q. Why does “March go out like a lamb?”

A. Because the water (evaporated by the high winds) falls again in showers to fertilize the earth, and breaks the violence of the winds.

Q. Why is it said that “A bushel of March dust is worth the king’s ransom?”

A. Because it indicates that there has been a continuance of dry weather; and unless March be dry, the seed will rot in the wet soil.

Q. Why is it said “A dry cold March never begs bread?”

A. Because the dry cold winds of March prepare the soil for seeds, which germinate, and produce fruit in the autumn.

Q. Why is it said that “A wet March makes a sad autumn?”

A. Because, if March be wet, so much of the seed rots in the ground, that the autumn crops are spoiled.

Q. Why is it said that “March flowers make no summer bowers?”

A. Because, if the spring be very mild, vegetation gets too forward, and is pinched by the nightly frosts, so as to produce neither fruits nor flowers.

Q. Why is it said “A late spring makes a fruitful year?”

A. Because if the vegetation of spring be backward, the frosty nights will do no harm; for the fruits and flowers will not put forth their tender shoots, till the nights become too warm to injure them.

Q. Why is it said that “April showers bring May flowers?”

A. Before seeds can germinate, three things are essential:—Darkness, Heat, and Moisture. April showers supply the principal nourishment on which seeds depend for existence.

Q. Does rain-water possess any fertilizing properties besides that of mere moisture?

A. Yes; rain-water contains “ammonia,” to which much of its fertilizing power may be attributed.

(Ammonia is a compound of nitrogen and hydrogen. Common hartshorn is only ammonia and water.)

Q. Why has God made November a very rainy month?

A. Because the rain hastens the putrefaction of the fallen leaves, and this makes the earth fertile.

Q. Why is there more rain from September to March than from March to September?

A. From September to March, the temperature of the air is constantly decreasing; on which account, its capacity for holding vapour is on the decrease, and the vapour is precipitated as rain.

Q. Why is there less rain from March to September, than from September to March?

A. From March to September, the temperature of the air is constantly increasing; on which account, its capacity for holding vapour is on the increase, and very little is precipitated as rain.

Q. Why is the rising sun in summer accompanied with a breeze?

A. Because the heat of the rising sun stops the radiation of heat from the earth, and warms its surface.

Q. How does this warmth produce a breeze?

A. The air (resting on the earth’s surface) is warmed by contact, ascends upwards, and colder air rushes in to fill up the void, which is the cause of the morning breeze.

Q. Why is there often an evening breeze during the summer months?

A. The earth radiates heat at sun-set, and the air is cooled down quickly by contact: this condensation causes a motion in the air, which is the evening breeze.

Q. Why are tropical islands always subject to a sea-breeze every morning (i. e.. a breeze blowing from the sea to the land)?

A. The solar rays are unable to heat the surface of the sea as they do the earth; therefore, the air resting on the earth is more heated than the air resting on the sea; and the colder sea air blows inland to restore the equilibrium.

Q. Why is the land breeze unhealthy?

A. Because it is frequently loaded with exhalations from putrefying animal and vegetable substances.

Q. Why is the sea breeze fresh and healthy?

A. Because it passes over the fresh sea, and is not laden with noxious exhalations.

It is healthy, therefore, to walk on the sea-beach before ten o’clock in the morning; but unhealthy after sun-set.

Q. Why is there generally a fresh breeze from the sea (in English watering places) during the summer and autumn mornings?

A. As the land is more heated by the sun than the sea; therefore, air resting on the land is hotter than air resting on the sea; in consequence of which, cooler sea air glides inland, to restore the equilibrium.

Q. Why does the sea breeze feel cool?

A. As the sun cannot make the surface of the sea so hot as the surface of the land; therefore, the air which blows from the sea, feels cooler than the air of the land.

Q. Why are tropical islands subject to a land breeze every evening (i. e.. a breeze blowing from the land towards the sea)?

A. The surface of land cools down faster (after sun-set) than the surface of the sea: in consequence of which, the air of the cold land is condensed, sinks down, and spreads itself into the warmer sea air, causing the land breeze.

Q. Why is the land breeze cool?

A. As the surface of the land is cooled at sun-set quicker than the surface of the sea; therefore, the seaman feels the air from the land to be chill.

Q. Why is the temperature of islands more equable than that of continents?

A. Because the water around the island absorbs the extreme heat of summer, and gives out heat to mitigate the extreme cold of winter.

Q. Why does the sea round an island give out heat in winter?

A. Unless the sea be frozen (which is rarely the case), it is warmer than the frozen land; and, therefore, the warmth of the sea air (mixing with the cold land air) helps to mitigate the intense cold.

Q. Why are there waves in the sea?

A. The wind (acting on the surface of the sea) piles up ridges of water, which leave behind an indentation: as the water on all sides rushes to fill up this indentation, the disturbance spreads on all sides, and billow rolls after billow.

Q. Why does wind in England generally feel cold?

A. Because a constantly changing surface comes in contact with our body, to draw off its heat.

Q. Why is a room (even without a fire) generally warmer than the open air?

A. As the air in a room is not subject to much change, it soon becomes of the same temperature as our skin, and no longer feels cold.

Q. Why do we generally feel colder out-of-doors?

A. Because the air (which surrounds us) is always changing; and as fast as one portion of air has become warmer by contact with our body, another colder portion surrounds us to absorb more heat.

Q. Why are hot foods made cool by blowing them?

A. Blowing causes the air (which covers the hot food) to change more rapidly; in consequence of which, the hot air is quickly blown away, and gives place to fresh cold air.

Q. Why do ladies fan themselves in hot weather?

A. By the action of the fan, fresh particles of air are perpetually brought in contact with the face, and every fresh particle of air absorbs some heat from the skin.

Q. Does the fan cool the air?

A. No; it makes the air hotter, by imparting to it the heat out of our face: but it cools the face blown upon, by transferring its heat to the air.

Q. Is the air in summer time ever so hot as our bodies?

A. No, not in England. In the hottest day in summer, the air of England is 15 or 20 degrees cooler than the human body.

Q. How fast does wind travel?

A. A gentle breeze goes at about the rate of 5 miles an hour. A high wind from 20 to 60. A hurricane from 80 to 100 miles an hour.

Q. How is the velocity of winds ascertained?

A. By observing the velocity of the clouds, and by an instrument for the purpose.

This instrument is called an Anemometer.

Q. How is the velocity of the clouds ascertained?

A. By observing the speed of their shadow along the ground; which is found in a high wind to vary from 20 to 60 miles an hour.

Q. Why is there always a strong draught through the keyhole of a door?

A. As the air of the room we occupy is warmer than the air in the hall, therefore the cold hall air rushes through the keyhole into the room, and causes a draught.

Q. Why is there always a strong draught under the door, and through the crevice on each side?

A. The cold air rushes from the hall under the door, &c. into the room, to supply the void caused in the room (by the escape of warm air up the chimney, &c.)

Q. Why is there always a draught through the window crevices?

A. The external air (being colder than the air of the room we occupy) rushes through the window crevices to supply the deficiency, caused by the escape of air up the chimney, &c.

Q. Why is there more draught if you open the lower sash of a window, than if you open the UPPER sash?

A. If the lower sash be open, the cold external air will rush more freely into the room; but if the upper sash be open the heated air of the room will rush out; and (of course) there will be less draught.

Q. By which means is the room better ventilated, by opening the lower or the upper sash?

A. A room is better ventilated by opening the upper sash; because the hot vitiated air (which always ascends towards the ceiling) can better escape.

Q. By which means is a hot room more quickly cooled—By opening the upper or the lower sash?

A. A hot room is cooled more quickly by opening the lower sash; because the cold air can enter more freely by an under current, than by one higher up.

Q. Why does wind dry damp linen?

A. Because dry wind (like a dry sponge) imbibes the particles of vapour from the surface of the linen, as fast as they are formed.

Q. Which is the hottest place in a church, chapel, or theatre?

A. The gallery.

Q. Why is the gallery of all public places hotter than the lower parts of the building?

A. Because the heated air of the room ascends, and all the cold air (which can enter through the doors and windows) keeps to the floor, till it has become heated.

Q. Why do plants often grow out of walls and towers?

A. Because sometimes the wind blows the seed there with the dust; and sometimes birds (flying over) drop the seed which they had formerly eaten.

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CHAPTER XXIII.
BAROMETER.

Q. What is a barometer?

A. A weather-glass, or instrument to show the changes of the weather, by marking the variations in the weight of air.

Q. What is a thermometer?

A. An instrument to show how hot or cold anything is.

Q. What is the difference between a thermometer and a barometer?

A. In a thermometer the mercury is sealed up from the air:

In a barometer the mercury is left exposed (or open) to the air.

Q. If the mercury of the thermometer be sealed up from the air, how can the air affect it?

A. The heat of the air passing through the glass tube into the mercury, causes it to expand more or less, and rise in the tube accordingly.

Q. Why is the tube of a barometer left open?

A. That the air may press upon it freely; and as this pressure is more or less, the mercury rises or falls in the tube.

Q. How can weather be affected by the weight of the air?

A. When air is warm or moist, it is lighter than usual:

When it is cold or dry, it is heavier: and as a barometer marks whether the air be light or heavy, it indicates these changes.

Q. How can you tell (by looking at a barometer) what kind of weather it will be?

A. Because the mercury in the tube rises and falls, as the air becomes lighter or heavier: and we can generally tell by the weight of the air, what kind of weather to expect.

Q. Does the weight of the air vary much?

A. Yes; the atmosphere in England varies as much as one-tenth part more or less.

Q. What is the chief use of a barometer?

A. To warn sailors how to regulate their ships, before squalls come on.

Q. How can a barometer warn sailors to regulate their ships?

A. As the barometer will tell when wind, rain, or storm is at hand, the sailor can make his ship trim before it overtakes him.

Q. Are there any rules which can be depended on?

A. Yes; there are ten special rules to direct us how to know the changes of weather, by marking the mercury of a barometer.

Q. What is the 1ST SPECIAL RULE in regard to the barometer?

A. The barometer is highest of all during a long frost; and it generally rises with a north-east wind.

Q. Why is the barometer highest of all during a long frost?

A. Because long frost condenses the air very greatly; and the more air is condensed, the greater is its pressure on the mercury of the barometer.

Q. Why does the barometer generally rise with a north-east wind?

A. Because north-east winds make the air both cold and dry: the air, therefore, is both condensed, and without vapour.

Q. What is the 2ND SPECIAL RULE in regard to the barometer?

A. The barometer is lowest of all during a thaw which follows a long frost: it generally falls with south and western winds.

Q. Why does the barometer fall lowest of all at the breaking up of a long frost?

A. 1st—Because the air (which had been much dried by the frost) absorbs the moisture of the fresh warm current of wind from the south or south-west: and

2ndly—The air (which had been much condensed by the frost) is suddenly expanded by the warm wind which is introduced.

Q. Why does the barometer fall very low with south and west winds?

A. Because south and west winds come heavily laden with vapour; and vaporized air is lighter than dry air.

Q. What effect has wind on the mercury?

A. All winds make the barometer drop, except eastern winds: those winds which blow from the south, and south-west make it drop the lowest.

Q. Why do winds generally make the mercury of a barometer drop?

A. Wind is caused by a partial vacuum in some parts of the globe; and as the air rushes in to supply this deficiency, its general pressure is lessened, and the barometer falls.

Q. What is the 3RD SPECIAL RULE in regard to the barometer?

A. While the barometer stands above 30°, the air must be very dry or very cold, or perhaps both, and no rain may be expected.

Q. Why will there be no rain if the air be very dry?

A. If the air be very dry it will absorb moisture, and not part with what it has in rain.

Q. Why will there be no rain if the air be very cold?

A. If the air be very cold it is so much condensed, that it has already parted with as much moisture as it can spare.

Q. What is the 4TH SPECIAL RULE in regard to the barometer?

A. When the barometer stands very low indeed, there is never much rain, although a fine day will seldom occur at such times.

Q. What kind of weather will it be when the barometer is unusually low?

A. There will be short heavy showers, with sudden squalls of wind from the west.

Q. Why will there be very little rain if the barometer be unusually low?

A. Because the air must be very warm, or very moist, or perhaps both.

Q. Why will there be little or no rain, if the air be very warm?

A. If the air be very warm it will have a tendency to imbibe more moisture, and not to part with what it has.

Q. Why will there be little or no rain if the air be moist, and the barometer remains very low?

A. If the air be ever so moist, rain will never fall till cold air has been introduced to condense the vapour; and the moment that the cold air is introduced, the barometer will rise.

Q. What is the 5TH SPECIAL RULE in regard to the barometer?

A. In summer-time (after a long continuance of fair weather) the barometer will fall gradually for 2 or 3 days before rain comes; but if the fall of the mercury be very sudden, a thunder-storm is at hand.

Q. What is the 6TH SPECIAL RULE in regard to the barometer?

A. When the sky is cloudless, and seems to promise fair weather, if the barometer be low, the face of the sky will soon be suddenly overcast.

Q. What is the 7TH SPECIAL RULE in regard to the barometer?

A. Dark dense clouds will pass over without rain, when the barometer is high; but if the barometer be low, it will often rain without any gathering of clouds.

Q. What is the 8TH SPECIAL RULE in regard to the barometer?

A. The higher the barometer, the greater is the probability of fair weather.

Q. Why is the barometer high in fine weather?

A. Because the air contains but very little vapour. The drier the air, the higher does the mercury of the barometer rise.

Q. What is the 9TH SPECIAL RULE in regard to the barometer?

A. When the mercury is in a rising state, fine weather is at hand; but when the mercury is in a sinking state, foul weather is near.

Q. Why does the mercury rise at the approach of fine weather?

A. Because the air is becoming more dry, and therefore its pressure is greater.

Q. Why does the mercury sink at the approach of foul weather?

A. Because the air is laden with vapour, or disturbed by wind.

Q. Why does vapour in the air make the mercury sink?

A. Because vaporized air is lighter than dry air, and therefore its pressure is less on the mercury of the barometer.

Q. What is the 10TH SPECIAL RULE in regard to the barometer?

A. If (in frosty weather) it begins to snow, the barometer generally rises to 32°, where it remains as long as the snow continues to fall; if, after this, the weather clear up, you may expect very severe cold.

Q. How can you know if the mercury of the barometer be rising?

A. If it be convex (i. e. higher in the middle than at the sides;) it is in a rising state.

Q. How can you tell if the mercury of the barometer be about to fall?

A. If it be concave (i. e. hollow in the middle) it is in a falling state.

Q. Why is the mercury convex when it is rising?

A. The sides of the mercury rub against the glass tube, and are delayed by it, so that the middle part rises faster than the sides.

Q. Why is the mercury concave when it is falling?

A. The sides of the mercury rub against the glass tube, and are delayed by it, so that the middle part sinks faster than the sides.

Q. What effect does a thunder-storm produce on the weather?

A. Thunder is generally preceded by hot weather, and followed by cold and showery weather.

Q. What effect does a sudden change produce on the weather?

A. A great and sudden change (either from hot to cold, or from cold to hot) is generally followed by rain within 24 hours.

Q. Why is a sudden change from hot to cold followed by rain?

A. The cold condenses the air and its vapour; which, being condensed and squeezed out, falls in rain.

Q. Why is a sudden change from cold to hot followed by rain?

A. Because the air is quickly saturated with moisture; and as soon as night comes on, the temperature is lowered again, and some of the abundant moisture falls in rain.

Q. Why is the air quickly saturated with moisture, when heat succeeds rapidly from cold?

A. Because the evaporation (which was checked by the cold) is carried on very rapidly, in consequence of the diminished pressure of the air.

(N. B. The less the pressure of the air, the more rapidly it evaporates moisture.)

Q. When does the barometer vary most?

A. In winter time.

Q. Why does the barometer vary more in winter than in summer time?

A. Because the difference of temperature between the torrid and temperate zones is so great, that the state of the air is perpetually disturbed by their mixing together.

Q. When does the barometer vary least?

A. In summer time.

Q. Why does the barometer vary less in summer than in winter time?

A. Because the temperature of our island is so nearly equal to that of the torrid zone, that its state is not much disturbed by interchange of currents.

Q. What effect has wind on the barometer?

A. North and east winds make the mercury rise; all other winds make it sink; but south and west winds make it sink lower than any other winds.

Q. Have heat and cold any effect on the barometer?

A. No, not of themselves; but because cold weather is generally either dry, or rough with north-east winds, therefore the mercury rises in cold weather; and because warm weather is often moist or fanned by south-west winds, therefore, the mercury sinks.

Q. Why is the mercury of a barometer lower in the torrid than in the frigid zones?

A. Because the warm air of the torrid zone contains much more vapour than the condensed air of the frigid zone; and the moister the air, the less is its pressure.

Q. In what months is the barometer highest?

A. In May and August; next to these, in June, March, September, and April.

Q. In what months is the barometer lowest?

A. In November and February; then in October, July, December, and January.

Q. What are the driest months?

A. March and June; then May and August; then April and November.

Q. What are the wettest months?

A. October and February; then July and September; then January and December.

Q. Why is there less wet from March to August, than there is from August to March?

A. Because the heat is constantly increasing; and the capacity of the air to absorb and retain moisture increases likewise.

Q. Why is there more wet from August to March, than there is from March to August?

A. Because the heat is constantly decreasing, and the capacity of the air to retain moisture decreases also; so that (although it often rains) yet the air is always on the point of saturation.

Q. Why does the mercury of a barometer rise in a frost?

A. Because frost condenses the air; and condensed air is heavier than rarefied air.

Q. Why does the mercury of a barometer fall in a thaw?

A. Because the air is both warmer (or more rarefied), and also filled with vapour.

Q. What does a sudden rise or fall of the barometer indicate?

A. If the rise be sudden, fine weather will not continue long:

If the fall be sudden, foul weather will not continue long.

Q. What sort of weather may we expect if the barometer be very fluctuating?

A. If the mercury fluctuates much, the weather will be very changeable and unsettled.

The fall of the barometer.

In very hot weather, the fall of the mercury denotes thunder.

Except in very hot weather, the sudden falling of the barometer denotes high wind.

In frosty weather, the fall of the barometer denotes thaw.

If wet weather happens soon after the fall of the barometer, expect but little of it.

In wet weather if the barometer falls, expect much wet.

In fair weather, if the barometer falls much and remains low, expect much wet in a few days, and probably wind.

N. B. The barometer sinks lowest of all for wind and rain together, next to that for wind (except it be an east or north-east wind).

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The rise of the barometer.

In winter the rise of the barometer presages frost.

In frosty weather, the rise of the barometer presages snow.

If fair weather happens soon after the rise of the barometer, expect but little of it.

In wet weather, if the mercury rises high and remains so, expect continued fine weather in a day or two.

In wet weather, if the mercury rises suddenly very high, fine weather will not last long.

N. B. The barometer rises highest of all for north and east winds; for all other winds it sinks.

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If the barometer be unsettled.

If the motion of the mercury be unsettled, expect unsettled weather.

If it stand at “much rain” and rise to “changeable,” expects fair weather of short continuance.

If it stand at “fair” and fall to “changeable,” expect foul weather.

N. B. Its motion upwards indicates the approach of fine weather: its motion downwards indicates the approach of foul weather.

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CHAPTER XXIV.
SNOW. HAIL. RAIN.

Q. What is snow?

A. The condensed vapour of the air frozen, and precipitated to the earth.

Q. What is the cause of snow?

A. When the air is nearly saturated with vapour, and condensed by a current of air below freezing point, some of the vapour is squeezed out, and frozen into snow.

A few years ago, some fishermen (who wintered at Nova-Zembla), after they had been shut up in a hut for several days, opened the window, and the cold external air rushing in, instantly condensed the air of the hut, and the vapour (which was squeezed out) fell on the floor in a shower of snow.

Q. Why does snow fall in winter time?

A. Because the sun’s rays are too oblique to heat the surface of the earth; and (as the earth has no heat to radiate into the air) the air is very cold.

Q. What is sleet?

A. When flakes of snow (in their descent) pass through a bed of air above freezing point, they melt; and fall to the earth as half-melted snow or sleet.

Q. What is the use of snow?

A. To keep the earth warm, and to nourish it.

Q. How can snow keep the earth warm?

A. Because it is a very bad conductor; in consequence of which, the earth which is covered with snow, very rarely descends below freezing point, even when the air is 15 or 20 degrees colder.

Q. Why is snow a bad conductor of heat and cold?

A. Because air is confined and entangled between the crystals, and air is a very bad conductor; when, therefore, the earth is covered with snow, it cannot throw off its heat by radiation.

Q. Tell me the words of the psalmist (cxlvii. 16.) respecting snow, and explain what he means.

A. The Psalmist says—“The Lord giveth snow like wool:” and he means not only that snow is as white as wool, but that it is also as warm as wool.

Q. Why is wool warm?

A. Because air is entangled between the fibres of the wool, and air is a bad conductor.

Q. Why is snow warm?

A. Because air is entangled between the crystals of the snow, and air is a bad conductor.

Q. Why does snow nourish the earth?

A. Because it supplies it with moisture for a considerable time; which penetrates slowly into the soil, and insinuates itself through every clod, ridge, and furrow.

Q. Why is there no snow in summer time?

A. No snow reaches the general surface of the earth in summer time, because the heat of the earth melts it in its descent.

Q. Why are some mountains always covered with snow?

A. 1st—Because the air is more rarefied; and rarefied air abstracts heat which it holds in a latent state:

2ndly—As the mountain top is not surrounded by earth to radiate heat into the air; therefore, the snow is not melted in its descent, but falls on the mountain, and lies there.

Q. Why is snow white?

A. Snow is formed of an infinite number of very minute crystals and prisms, which reflect all the colours of the rays of light; and these colours uniting before they meet the eye, cause snow to appear white.

Q. What is hail?

A. Rain, which has passed in its descent through some cold bed of air, and has been frozen into drops of ice.

Q. Why is one bed of air colder than another?

A. This is frequently caused by electricity in the air, unequally distributed.

Q. Why is hail frequently accompanied with thunder and lightning?

A. 1st—Because the congelation of water into hail disturbs the electricity of the air: and

2ndly—The friction (produced by the fall of hail) excites it still more.

Q. Why does hail fall generally in summer and autumn?

A. 1st—Because the air is more highly electrified in summer and autumn: and

2ndly—The vapours (being rarefied) ascend to the more elevated regions, where the cold is greater than it is nearer the earth.

Q. What two things are essential to cause HAIL?

A. Two strata of clouds having opposite electricities, and two currents of wind. The lower cloud (being negative) is the one precipitated.

Q. What is rain?

A. The vapour of the clouds or air condensed, and precipitated to the earth.

Q. Why is the vapour of the air or clouds precipitated?

A. When the air is saturated with vapour, if a cold current condenses it, it is no longer able to hold all its vapour in solution, and some of it is squeezed out, and falls as rain.

Q. Why does rain fall in drops?

A. The vapoury particles in their descent attract each other; and those which are sufficiently near, unite and form into a drop.

Q. Why does not the cold of night always cause rain?

A. When the air is not near saturation (although condensed by the chill of evening), it will still be able to hold its vapour in solution.

Q. Why does a passing cloud often drop rain?

A. Because the cloud (travelling about on the wind) comes into contact with something that chills it; and its vapour being squeezed out, falls to the earth as rain.

Q. Why are rain-drops sometimes much larger than at other times?

A. When the rain-cloud is floating near the earth, the drops are large, because such a cloud is much more dense than one which is more elevated.

The size of the rain-drop is increased according to the rapidity with which the vapours are condensed.

Q. Does not wind sometimes increase the size of rain-drops?

A. Yes; by blowing two or more drops into one.

Q. Why do clouds fall in rainy weather?

A. 1st—Because the clouds are heavy with abundant vapour: and

2ndly—As the density of the air is diminished, it is less able to buoy the clouds up.

Q. How do you know that the density of the air is diminished in rainy weather?

A. Because the mercury of a barometer falls.

Q. Why is rain-water more fertilizing than pump-water?

A. Because it contains a compound of hydrogen and nitrogen (called ammonia), which is a very excellent food for young plants.

Q. Why is November made by God to be a rainy month?

A. Because rain hastens the putrefaction of the fallen leaves by causing fermentation.

Q. Why does rain purify the air?

A. 1st—Because it beats down the noxious exhalations collected in the air, and dissolves them:

2ndly—It mixes the air of the upper regions with that of the lower regions: and

3rdly—It washes the earth, and sets in motion the stagnant sewers and ditches.

Q. Why are mountainous countries more rainy than flat ones?

A. The air (striking against the side of the mountains) is carried up the inclined plane, and brought in contact with the cold air of the higher regions, by which it is condensed, and its vapour squeezed out.

Q. Why does a sponge swell when it is wetted?

A. Because the water penetrates the pores of the sponge, and drives the particles of the sponge further from each other; in consequence of which, the bulk of the sponge is greatly increased.

Q. Why do fiddle-strings snap in wet weather?

A. Because the moisture of the air (penetrating the string) causes it to swell; and (as the cord thickens) its tension is increased, and the string snaps.

Q. Why does paper pucker when it is wetted?

A. Because the moisture (penetrating the paper) drives its particles further apart; and (as the moisture is absorbed unequally by the paper) some parts are more enlarged than others; in consequence of which, the paper blisters or puckers.

Q. Why do the weather toys called capu’chins lift the cowl over the figures in wet weather, and remove it in dry?

A. The cowl of the capu’chin is fastened to a piece of cat-gut. When the weather is wet, the moisture swells the cat-gut and it is shortened, by which means the cowl is pulled up; but in dry weather, the string is loosened, and the cowl falls down.

Q. In another weather toy, the man comes out in wet weather, and the lady in fine:—Why is this?

A. The two figures are attached to a piece of cat-gut in such a manner, that when the cat-gut is shortened by moisture, it pulls the man out; but when it is loose, the woman falls out by her own weight.

Q. Why are wet stockings difficult to pull on?

A. The moisture (by penetrating the threads of the stockings) causes them to shrink in size.

Q. What is the most rainy spot in England?

A. Keswick (in Cumberland); and then Kendal (a market town in Westmoreland).

(In Keswick, about 63 inches of rain fall in a year. In Kendal, 58; Manchester, 38; Liverpool, 34; Dublin and Cambridge, 25; Lincoln, 24; London, 21; and in Paris, only 18.)

Q. In which part of the day does the most rain fall?

A. More rain falls by night than by day; because the cold night condenses the air, and diminishes its capacity for holding vapour in solution.

Q. Does more rain fall in summer or in winter time?

A. There are more rainy days from September to March; but heavier rains between March and September.

Q. Why are there more rainy days from September to March, than from March to September?

A. Because the temperature of the air is constantly decreasing, and its capacity for vapour decreases also; in consequence of which, it is perpetually obliged to part with some of its vapour in rain.

Q. In what part of the world does rain fall most abundantly?

A. Near the equator; and the quantity of rain decreases as we approach the poles.

Q. Why does more rain fall at the equator than at the poles?

A. Because the contrast between the night and day is very great. The hot air absorbs moisture very abundantly during the day; and when the cold night condenses the air, it is unable to retain the moisture imbibed, and some of it falls in rain.

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CHAPTER XXV.
WATER.

Q. What is water?

A. Water is composed of two gases, oxygen and hydrogen.

(In 9 lbs. of water, 8 are oxygen, and 1 is hydrogen.)

Q. Why is water fluid?

A. Because its particles are kept separate by latent heat; but when a certain quantity of this latent heat is driven out, water becomes solid, and is called ice.

Q. How can water be converted into a gas?

A. By increasing its latent heat, the particles, of water are again subdivided into invisible steam.

Q. Why is pump water called hard water?

A. Because it is laden with foreign matters, and will not readily dissolve substances immersed in it.

Q. What makes pump-water hard?

A. Because when it filters through the earth, it becomes impregnated with sulphate of lime, and many other impurities from the earths and minerals with which it comes in contact.

Q. Why is it difficult to wash our hands clean with hard water?

A. Because the soda of the soap combines with the sulphuric acid of the hard water, and the oil of the soap with the lime, and float in flakes on the top of the water.

N.B. Sulphate of lime consists of sulphuric acid and lime.

Q. Why is it difficult to wash in salt water?

A. Because salt water contains muriatic acid; and the soda of soap combines with the muriatic acid of the salt water, and produces a cloudiness.

Q. Why does a black hat turn red at the sea side?

A. The muriatic acid of the sea-water disturbs the gallic acid of the black dye, and turns it red.

Q. Of what is soap made?

A. Of kelp (or the ashes of sea-weed dried and burnt in a pit) mixed with oil or fat.

Yellow Soap is made of whale-oil, soda, and resin. Soft soap is made of oil and potash. Hard soap of oil and soda.

Q. Why does water clean dirty linen?

A. Because the oxygen of the water attaches itself to the stains of the linen, and dissolves them; as oxalic acid dissolves ink spots.

Q. Why does soap greatly increase the cleansing power of water?

A. 1st—Because soap increases the oxygen of the water: and

2ndly—It neutralizes the grease of the things washed.

Q. Why is rain water soft?

A. Because it has not come in contact with earths and minerals.

Q. Why is it more easy to wash with soft water than with hard?

A. Because it unites freely with the soap, dissolving it instead of decomposing it, as hard water does.

Q. Why do wood ashes make hard water soft?

A. 1st—Because the carbonic acid of the wood ashes combines with the sulphate of lime in the hard water, and converts it into chalk: and

2ndly—The sulphuric acid of the water combines with the potash of the wood ashes, and prevents it from neutralizing the oily matter of the soap.

Q. Why has rain water such an unpleasant smell, when it is collected in a rain water tub or tank?

A. Because it is impregnated with decomposed organic matter, washed from roofs, trees, or the casks in which it is collected.

Q. Why does water melt sugar?

A. Because very minute particles of water insinuate themselves into the pores of the sugar, and force the crystals apart from each other.

Q. Why does water melt salt?

A. Because very minute particles of water insinuate themselves into the pores of the salt, and force the crystals apart from each other.

Q. Why does melted sugar or salt give a flavour to the water?

A. Because the sugar or salt (being disunited into very minute pieces) floats about the water, and mixes with every part.

Q. Why does hot water melt sugar and salt quicker than cold water?

A. 1st—Because the heat of the water entering the pores of the sugar or salt, opens a passage for the water: and

2ndly—The particles of hot water being smaller than those of cold, can more readily penetrate the pores of salt or sugar.

Q. Why is sea-water salt?

A. 1st—Because it contains mines of salt at the bottom of its bed:

2ndly—It is impregnated with bituminous matter, which is brackish: and

3rdly—It contains many putrid substances, which increase its brackishness.

Q. Why is not rain-water salt, although most of it is evaporated from the sea?

A. Because salt will not evaporate; and, therefore, when sea-water is turned to vapour, its salt is left behind.

Q. Why does stagnant water putrefy?

A. Because leaves, plants, insects, &c. are decomposed in it.

Q. Why is stagnant water full of worms, eels, &c.?

A. Because numberless insects lay their eggs in the leaves and plants which float on the surface; these eggs are soon hatched, and produce swarms of worms, eels, and insects.

Q. Why are flowing waters free from these impurities?

A. 1st—Because the motion of running water prevents its fermentation:

2ndly—It dissolves the putrid substances which happen to fall into it: and

3rdly—It casts on the bank (by its current) such substances as it cannot dissolve.

Q. Why does running water oscillate and whirl in its current?

A. 1st—Because it impinges against its banks, and is perpetually diverted from its forward motion: and

2ndly—Because the centre of a river flows faster than its sides.

Q. Why do the sides of a river flow more tardily than its centre?

A. Because they rub against the banks, and are delayed in their current thereby.

Q. Why does soapy water bubble?

A. Because the soap makes the water tenacious, and prevents the bubbles from bursting as soon as they are formed.

Q. Why will not water bubble without soap?

A. Because it is not tenacious enough to hold together the bubbles that are formed.

Q. When soap bubbles are blown from a pipe, why do they ascend?

A. Because they are filled with warm breath, which is lighter than air.

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CHAPTER XXVI.
ICE.

Q. What is ice?

A. Frozen Water. When the air is reduced to 32 degrees of heat, water will no longer remain in a fluid state.

Q. Why is solid ice lighter than water?

A. Because water expands by freezing; and as the bulk is increased, the gravity must be less.

Nine cubic inches of water become ten when frozen.

Q. Why do ewers break in a frosty night?

A. Because the water in them freezes; and as the water is expanded by frost, it bursts the ewers to make room for its increased volume.

Q. Why does it not expand upwards (like boiling water), and run over?

A. Because the surface is first frozen, and the frozen surface acts as a plug, which is more difficult to burst than the earthen ewer itself.

Q. Why do tiles, stones, and rocks often split in winter?

A. Because the moisture (which they imbibed) freezes, and by its expansion splits the solid mass.

Q. In winter time, foot-marks and wheel-ruts are often covered with an icy net-work, through the interstices of which the soil is clearly seen,—Why does the water freeze in net-work?

A. The water in these hollows froze first at the sides of the foot-prints: other crystals gradually shot across the water, and would have covered the whole surface, had not the earth absorbed the water before it had time to freeze.

Q. In winter time these foot-marks and wheel-ruts are sometimes covered with a perfect sheet of ice, and not an icy net-work,—Why is this?

A. The air being colder and the earth harder (than in the former case), the entire surface of the foot-print is frozen over, before the earth can draw the water in.

Q. Why is not the ice solid in these ruts?—why is there only a very thin film or net-work of ice?

A. Because the earth absorbs the water, and leaves the icy film behind.

Q. Does not water expand by heat as well as cold?

A. Yes; it expands as soon as it is more than 42 degrees till it boils, and then it flies off in steam.

Here A B measures the bulk of a portion of water at 42 degrees.
It goes on increasing in bulk to C D, when it boils. It also goes on increasing in bulk to E F, when it freezes.

Q. Why do water-pipes frequently burst in frosty weather?

A. Because the water in them freezes; and as the water expands by frost, it bursts the pipes to make room for its increased volume.

Q. When does water begin to expand from cold?

A. Water (which is wisely ordained by God to be an exception to a very general rule) contracts till it is reduced to 42 degrees, and then it expands till it freezes.

(Water freezes at 32°.)

Q. Why does water expand when it freezes?

A. Because it is converted into solid crystals, which do not fit close, like the particles of water.

Q. Why is the water at the bottom of a river never frozen?

A. Because when water is colder than 42 degrees, it instantly ascends to the surface; and (if it freezes) floats there till it is melted.

(When a river is frozen, the water below the surface is never less than 42°.)

Q. Show the wisdom of God in this wonderful exception to a general law.

A. If ice were heavier than water, it would sink; and a river would soon become a solid block of ice, which could never be dissolved.

Q. Why does not the cold ice on the surface of a river chill the water beneath, and make it freeze?

A. 1st—Water is a very bad conductor, and is heated or chilled by convection only:

2ndly—If the ice on the surface were to communicate its coldness to the water beneath, the water beneath must communicate its heat to the ice, and the ice would instantly melt: and

3rdly—The ice on the surface acts as a shield to prevent the cold air from penetrating the river to freeze it below the mere crust.

Q. Why does water freeze at the surface first?

A. Because the surface is in contact with the air, and the air carries away its heat.

Q. Why does the coat of ice grow thicker and thicker, if the frost continues?

A. Because the heat of the water (immediately below the frozen surface) passes through the pores of the ice into the cold air.

Q. Why then are not whole rivers frozen (layer by layer) till they become solid ice?

A. Because water is so slow a conductor, that our frosts never continue long enough to convert a whole river into a solid mass of ice.

Q. Why does not running water freeze so fast as still water?

A. 1st—Because the motion of the current dissolves the crystals as fast as they are formed; and

2ndly—The heat of the under surface is more freely distributed to the upper surface by the rolling water.

Q. When running water is frozen, why is the ice generally very rough?

A. Because little flakes of ice are first formed and carried down the stream, till they meet some obstacle to stop them; other flakes of ice (impinging against them) are arrested in like manner; and the edges of the different flakes overlapping each other, make the surface rough.

Q. Why do some parts of a river freeze less than others?

A. Because springs issue from the bottom, and (as they bubble upwards) thaw the ice, or make it thin.

Q. When persons fall into a river in winter time, why does the water feel remarkably warm?

A. Because the frosty air is at least 10 or 12 degrees colder than the water.

(The water below the surface is at least 42°; but the air 32°, or even less.)

Q. Why is shallow water frozen quicker than deep water?

A. Because (as the whole volume of water must be cooled to 42 degrees before the surface can be frozen) it will take a longer time to cool down a deep bed of water than a shallow one.

Q. Why is sea-water rarely frozen?

A. 1st—Because the mass of water is so great that it requires a very long time to cool the whole volume down to 42 degrees:

2ndly—The ebb and flow of the sea interfere with the cooling influence of the air: and

3rdly—Salt never freezes till the surface is cooled down at least 25 degrees below the freezing point.

Q. Why do some lakes rarely if ever freeze?

A. 1st—Because they are very deep:

2ndly—Because their water is supplied by springs, which bubble from the bottom.

Q. Why does the depth of the water retard its freezing?

A. As the whole volume of water must be reduced to 42 degrees before the surface will freeze, the deeper the water, the longer it will be before the whole volume is thus reduced.

Q. Why do springs at the bottom of a lake prevent its freezing?

A. Because they keep continually sending forth fresh water, which prevents the lake from being reduced to the necessary degree of coldness.

Q. Why is it colder in a thaw than in a frost?

A. When frozen water is thawed, it absorbs heat from the air and objects around to melt its ice, in consequence of which the cold is greatly increased.

Q. Why is it warmer in a frost than in a thaw?

A. When water freezes it gives out its latent heat, in order that it may be converted into solid ice; and as much heat is liberated from the water into the air, we feel warmer.

Q. Why does salt dissolve ice?

A. Water freezes at 32°, but salt and water will not freeze till the air is 25° colder: if, therefore, salt be added to frozen water it becomes liquid, unless the thermometer stands below 7°, (which it never does in our island).

Q. Will any thing do instead of salt?

A. Yes; any acid, such as sulphuric, nitric, &c.

Q. Why are salt and snow mixed together, colder than snow?

A. When salt is mixed with snow, it dissolves the crystals into a fluid; and whenever a solid is converted to a liquid, heat is absorbed, and the cold made more intense.

Q. Why does frost make the earth crack?

A. During the warm weather the earth absorbed abundance of moisture, which the winter freezes: and (as water expands by frost) the expanding water thrusts the particles of earth apart from each other, and leaves a chink or crack behind.

Q. Show the wisdom of God in this arrangement.

A. These cracks in the earth let in the air, the dew and rain, and many gases favourable to vegetation.

Q. Why does the earth crumble in spring?

A. In spring the ice of the clods dissolves, and the particles of earth (which had been held apart by the expanded ice) are left unsupported, and tumble into minute parts (because their cement is dissolved).

Q. Why does mortar crumble away in frost?

A. If the mortar was not dried in the warm weather, its moisture freezes, expands, and thrusts the particles of the mortar away from each other; but (as soon as the frost goes) the water condenses and leaves the mortar full of cracks and chinks.

Q. Why does stucco peel from a wall in frosty weather?

A. If the stucco was not dried in the warm weather, its moisture freezes, expands, and thrusts its particles away from the wall; but as soon as the water condenses again by the thaw, the stucco (being unsupported) falls by its own weight.

Q. Why cannot bricklayers and plasterers work in frosty weather?

A. Because the bricks and plaster would start from their position as soon as the frost came and expanded the mortar.

Q. Why do bricklayers cover their work with straw in spring and autumn?

A. Because straw is a non-conductor, and prevents the mortar of their new work from freezing during the cold nights of spring and autumn.

Q. Why are water-pipes often covered with stall-litter in winter time?

A. Because straw (being a non-conductor) prevents the water of the pipes from freezing, and the pipes from bursting.

Q. Why are delicate trees covered with straw in WINTER?

A. Because straw (being a non-conductor) prevents the sap of the tree from being frozen.

Q. Can water be frozen in any way besides by frosty weather?

A. Yes; in very many ways. For example—a bottle of water wrapped in cotton, and frequently wetted with ether, will soon freeze.

Q. Why would water freeze if the bottle were kept constantly wetted with ether?

A. Because evaporation would carry off the heat of the water, and reduce it to freezing point.

Q. Why does ether freeze under the receiver of an air-pump, when the air is exhausted?

A. Because evaporation is very greatly increased by the diminution of atmospheric pressure; and the ether freezes by evaporation.

FREEZING MIXTURES.

1. If nitre be dissolved in water, the heat of the liquid will be reduced 16 degrees.

2. If 5 oz. of nitre, and 5 of sal-ammoniac (both finely powdered) be dissolved in 19 oz. of water, the heat of the liquid will be reduced 40 degrees.

3. If 3 lbs. of snow be added to 1 lb. of salt, the mixture will fall to 0° (or 32 degrees below freezing point).

The two following are the coldest mixtures yet known:—

1. Mix 3 lbs. of muriate of lime with 1 lb. of snow.

2. Mix 5 lbs. of diluted sulphuric acid with 4 lbs. of snow.

Q. Why is it more easy to swim in the sea than in a river?

A. Because the specific gravity of salt water is greater than that of fresh, and therefore it buoys up the swimmer better.

Q. How do cooks ascertain if their brine be salt enough for pickling?

A. They put an egg into their brine. If the egg sinks the brine is not strong enough, if the egg floats it is.

Q. Why will the egg sink if the brine be not strong enough for pickling?

A. As an egg is heavier than water, it will sink if immersed therein; but if as much salt be added as the water can dissolve, the egg will float.

Q. Why will the egg float in strong brine?

A. Because the specific gravity of salt and water is greater than that of water only.

Q. Why do persons sink in water when they are unskilful swimmers?

A. 1st—Because (in their floundering about) they take in water at their nose and mouth, which makes them heavier:

2ndly—Fear contracts the body; and as the body is compressed by fear into a smaller compass, it becomes heavier: and

3rdly—The water and fear take away the breath; and when the breath is taken from the body, its bulk is reduced, and it becomes heavier.

Q. Why can quadrupeds swim more easily than man?

A. 1st—Because the trunk of a quadruped is lighter than water, and this is the greatest part of them:

2ndly—The position of a beast in water is a natural one.

Q. Why is it more difficult for a man to swim than for a beast?

A. Because the head and limbs of a man (like those of a beast) are heavier than water, and these compose more than half his body:

2ndly—The position of a man in water is unnatural to him.

Q. Why can fat men swim more easily than spare men?

A. Fat is lighter than water; and the fatter a man is, the more buoyant will he be.

Q. How are fishes able to ascend to the surface of water?

A. Fishes have an air-bladder near their abdomen: when this bladder is filled with air, the fish increases in size; and (being lighter) ascends through the water to its surface.

Q. How are fishes able to dive in a minute to the bottom of a stream?

A. They expel the air from their air-bladder; in consequence of which, their size is diminished, and they sink instantly.

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CHAPTER XXVII.
LIGHT.

Q. What is light?

A. Rapid undulations of a fluid called ether, striking on the optic nerve of the eye. (See p.[46].)

The heat of fire or of the sun sets the atoms of matter in motion; and these atoms, striking against the fluid ether, cause it to undulate.

Q. How fast does light travel?

A. Light travels so fast, that it would go eight times round the earth, while a person counts “one.”

Q. Does all light travel equally fast?

A. Yes; the light of the sun, or the light of a candle, or the light from houses, trees, and fields.

Q. Where does the light of houses, trees, and fields come from?

A. The light of the sun (or of some lamp or candle) is reflected from their surfaces.

Q. Why are some surfaces brilliant like glass and steel, and others dull like lead?

A. Those surfaces which reflect the most light, are the most brilliant; and those which absorb light are dull.

Q. What is meant by reflecting light?

A. Throwing the rays of light back again, from the surface on which they light.

Q. What is meant by absorbing light?

A. Letting the rays of light sink below the surface which they touch, so as not to be seen.

Q. Why can a thousand persons see the same object at the same time?

A. Because it throws off from its surface an infinite number of rays in all directions; and one person sees one portion of these rays, and another person another.

Q. Why is the eye pained by a sudden light?

A. Because the pupil of the eye is burdened with rays, before it has had time to contract.

Q. Why does it give us pain, if a candle be brought suddenly towards our bed at night time?

A. In the dark the pupils of the eyes dilate very much, in order to admit more rays. When a candle is brought before them, the enlarged pupil is overladen with rays, and feels pained.

Q. Why can we bear the candle-light after a few moments?

A, Because the pupil contracts again almost instantly, and adjusts itself to the quantity of light which falls upon it.

Q. Why can we see nothing, when we leave a well-lighted room, and go into the dark road or street?

A. Because the pupil (which contracted in the bright room) does not dilate instantaneously; and the contracted pupil is not able to collect rays enough (from the dark road or street) to enable us to see before us.

Q. Why do we see better, when we get used to the dark?

A. Because the pupil dilates again, and is able to gather together more rays; in consequence of which, we see more distinctly.

Q. If we look at the sun for a few moments, why do all other things appear dark?

A. Because the pupil of the eye (which was very much contracted by looking at the sun) is too small to collect sufficient rays from other objects, to enable us to distinguish their colours. (See “accidental colours.”)

Q. If we watch a bright fire for a few moments, why does the room seem dark?

A. Because the pupil of the eye (which was very much contracted by looking at the fire) is too small to collect sufficient rays from the objects around, to enable us to distinguish their colours.

Q. Why can we see the proper colour of every object again, after a few minutes?

A. Because the pupil dilates again, and accommodates itself to the light around.

Q. Why can tigers, cats, and owls see in the dark?

A. Because they have the power of enlarging the pupil of their eyes, so as to collect several scattered rays of light; in consequence of which, they can see distinctly when it is not light enough for us to see any thing at all.

Q. Why do cats and owls sleep almost all day?

A. As the pupil of their eyes is very broad, daylight fatigues them; so they close their eyes for relief.

Q. Why do cats keep winking, when they sit before a fire?

A. As the pupil of their eyes is very broad, the light of the fire pains them; and they keep shutting their eyes to relieve the sensation of too much light.

Q. Why do tigers, cats, owls, &c. prowl by night for prey?

A. As these animals cannot see distinctly in strong daylight, they sleep during the day: and as they can see clearly in the dark, they prowl then for prey.

Q. Why do glow-worms glisten by night only?

A. Because the light of day is so much stronger, that it eclipses the feeble light of a glow-worm; in consequence of which, glow-worms are invisible by day.

Q. Why can we not see the stars in the day-time?

A. Because the light of day is so powerful, that it eclipses the feeble light of the stars: in consequence of which, they are invisible by day.

Q. Why can we see the stars even at mid-day, from the bottom of a deep well?

A. As the rays of the sun never come directly over a well, but the rays of the stars do; therefore the light from those stars (in such a situation) is more clear than the light of the sun.

Q. What is the use of two eyes, since they present only one image of any object?

A. The use of two eyes is to increase the light, or take in more rays of light from the object looked at, in order that it may appear more distinct.

Q. Why do we not see things double, with two eyes?

A. 1st—Because the axis of both eyes is turned to one object; and, therefore, the same impression is made on the ret´ina of each eye.

2ndly—The nerves (which receive the impression) have one point of union, before they reach the brain.

Q. Why do we see ourselves in a glass?

A. The rays of light from our face strike against the surface of the glass, and (instead of being absorbed) are reflected, or sent back again to our eye.

Q. Why are the rays of light reflected by a mirror?

A. Because they cannot pass through the impenetrable metal with which the back of the glass is covered; so they rebound back, just as a marble would do if it struck against a wall.

Q. When a marble is rolled towards a wall, what is that path through which it runs called?

A. The line of the angle of incidence.

Q. When a marble rebounds back again, what is the path it then describes called?

A. The line of the angle of reflection.

Q. When the light of our face goes to the glass, what is the path through which it goes called?

A. The line of the angle of incidence.

Q. When the light of our face is reflected back again from the mirror, what is this returning path called?

A. The line of the angle of reflection.

Q. Why does our reflection in a mirror seem to approach us as we walk towards it, and to retire from us as we retire?

A. Because the line of the angle of incidence is always equal to the line and angle of reflection.

Here CA, EA and DB, FB are the lines of the angle of incidence; and GA, KA and HB, LB are the lines of the angle of reflection. When the arrow is at CD, its shadow will appear at GH, because the line CA=GA and the angle CAB=angle GAB, &c.; and the same may be said about the point D.

Q. Why can a man see his whole person reflected in a little mirror not 6 inches in length?

A. Because the line of the angle of incidence is always equal to the line and angle of reflection.

Take the last figure—CD is much larger than the mirror AB; but the head of the arrow C is reflected obliquely behind the mirror to G; and the barb D appears at H.—Why? Because the line CA=AG and the angle CAB=angle GAB, &c. The same may be said of the point D.

Q. Why does a shadow in water always appear topsy-turvy?

A. Because the line of the angle of incidence is always equal to the line and angle of reflection.

Here the arrow-head A strikes the water at F, and is reflected to D; and the barb B strikes the water at E, and is reflected to C.
If a spectator stands at G, he will see the reflected lines CE and DF, produced as far as G.
It is very plain that the more elevated object A will strike the water, and be projected from it more perpendicularly than the point B, and therefore the shadow will seem inverted.

Q. When we see our shadow in water, why do we seem to stand on our head?

A. Because the line of the angle of incidence is always equal to the line and angle of reflection.

Suppose our head to be at A, and our feet at B; then the shadow of our head will be seen at D, and the shadow of our feet at C. (See last figure.)

Q. Why do windows seem to blaze at sun-rise and sun-set?

A. Because glass is a good reflector of light; and the rays of the sun (striking against the window glass) are reflected, or thrown back.

Q. Why do not windows reflect the noon-day rays also?

A. They do, but the reflection is not seen.

Q. Why is the reflection of the rising and setting sun seen in the window, and not that of the noon-day sun?

A. As the angle of incidence always equals the angle of reflection, therefore the rays of the noon-day sun enter the glass too perpendicularly for their reflection to be seen.

Here AB represents a ray of the noon-day sun striking the window at B; its reflection will be at C:
But DB (a ray of the rising or setting sun) will be reflected to E (the eye of the spectator).

Q. Why can we not see the reflection of the sun in a well, during the day-time?

A. Because the rays of the sun fall so obliquely, that they never reach the surface of the water at all, but strike against the brick sides.

Let BDEC be the well, and DE the water.
The ray AB strikes against the brick-work inside the well; and
The ray AC strikes against the brick-work outside the well.
None will ever touch the water DE.

Q. Why do we see the moon reflected in a well very often?

A. As the rays of the moon are not so oblique as those of the sun, they will often reach the water. (See next figure.)

Q. Why are the stars reflected in a well, although the sun is not?

A. As the rays of the stars are not so oblique as those of the sun, they will often reach the water.

Here the moon's rays AB, AC, both strike the water DE, and are reflected by it.

Q. In a sheet of water at noon, the sun appears to shine upon only one spot, and all the rest of the water seems dark,—Why is this?

A. Because the rays (which fall at various degrees of obliquity on the water) are reflected at similar angles; but as only those which meet the eye of the spectator are visible, all the sea will appear dark but that one spot.

Here of the rays SA, SB, and SC, only the ray SC meets the eye of the spectator D.
The spot C, therefore, will appear luminous to the spectator D, but no other spot of the water ABC.

Q. At night the moon seems to be reflected from only one spot of a lake of water, while all the rest seems dark,—Why is this?

A. Because the rays (which fall at various degrees of obliquity on the lake) are reflected at similar angles; but as only those which enter the eye of the spectator will be visible, all the water will appear dark but that one spot. (See last figure.)

Q. Why are more stars visible from a mountain, than from a plain?

A. As the air absorbs and diminishes light, the higher we ascend, the less light will be absorbed.

Q. Why does the sun seem larger at his rise and set, than it does at noon?

A. Because the earth is surrounded by air, which acts like a magnifying glass; and when the sun is near the horizon (as its rays pass through more of this air), it is more magnified.

Here SC represents a ray of the sun at noon, and MC a ray of the sun near the horizon. DEG represents the air or atmosphere around the earth.
Because EC is longer than DC, therefore the rays of the sun at M pass through more air than the rays of the sun at S, and the sun is more magnified.

Q. Why does the rising and setting moon appear so much larger, than after it is risen higher above our heads?

A. Because the earth is surrounded by air, which acts like a magnifying glass; and when the moon is near the horizon (as its rays pass through more of this air) it is more magnified. (See last figure.)

Q. When candles are lighted, we cannot see into the street or road,—Why is this?

A. 1st—Because glass is a reflector, and throws the candle-light back into the room again; and

2ndly—The pupil of the eye (which has become contracted by the light of the room) is too small to collect rays enough from the dark street, to enable us to see into it.

Q. Why can’t persons in the street see into a well-lighted room?

A. Because the pupil of their eyes is much dilated by the dark, and cannot collect from the window sufficient rays to enable them to see into the room.

Q. Why do we often see the fire reflected in our parlour window in winter time?

A. Because glass is a good reflector; and the rays of the fire (striking against the window-glass) are reflected back into the room again.

Q. Why do we often see the shadow of our candles in the window, while we are sitting in our parlour?

A. Because the rays of the candle (striking against the glass) are reflected back into the room: and the darker the night, the clearer the reflection.

Q. Why is this reflection more clear, if the external air be dark?

A. Because the reflection is not then eclipsed by the brighter rays of the sun striking on the other side of the window.

Q. Why is the shadow of an object (thrown on the wall) larger and larger, the closer any object be held to the candle?

A. Because the rays of light diverge (from the flame of a candle) in straight lines, like lines drawn from the centre of a circle.

Here the arrow A held close to the candle, will cast the shadow BF on the wall: while the same arrow held at C, would cast only the little shadow D E.

Q. When we enter a long avenue of trees, why does the avenue seem to get narrower and narrower till it appears to meet?

A. Because the further the trees are off, the more acute will be the angle that any two will make with our eye.

Here the width between the trees A and B will seem to be as great as the line AB: But the width between the trees C and D will seem to be no more than EF.

Q. In a long straight street, why do the houses seem to approach nearer and nearer as they are more distant?

A. Because the more distant the houses are, the more acute will be the angle which any two make with our eye.

Thus in the last figure—

If A and B were two houses at the top of the street, the street would seem to be as wide as the line A B:

And if C and D were two houses at the bottom of the street, the street at the bottom would seem to be no wider than E F.

Q. In an avenue of trees, why do they seem to be smaller as their distance increases?

A. Because the further the trees are off, the more acute will be the angle made by their perpendicular height with our eye.

Here the first tree A B will appear the height of the line A B; but the last tree C D will appear only as high as the line E F.

Q. In a long straight street, why do the houses seem to be smaller and smaller the further they are off?

A. Because the further any house is off, the more acute will be the angle made by its perpendicular height with our eye.

Thus in the last figure—

If A B be a house at the top of the street, its perpendicular height will be that of the line A B.

If C D be a house at the bottom of the street, its perpendicular height will appear to be that of E F.

Q. Why does a man on the top of a mountain or church spire seem to be no bigger than a crow?

A. Because the angle made by the perpendicular height of the man (at that distance) with our eye, is no bigger than the perpendicular height of a crow close by.

Let AB be a man on a distant mountain or spire, and CD a crow close by:
The man will appear only as high as the line CD, which is the height of the crow.

Q. Why does the moon appear to us so much bigger than the stars, though in fact it is a great deal smaller?

A. Because the moon is very much nearer to us than any of the stars.

Let AB represent a fixed star, and CD the moon.
AB, though much the larger body, will appear no bigger than EF; whereas the moon (CD) will appear as big as the line CD to the spectator G.
The moon is 240,000 miles from the earth, not quite a quarter of a million of miles. The nearest fixed stars are 20,000,000,000,000. (i. e.. 20 billions.)
If a ball went 500 miles an hour, it would reach the moon in twenty days: but it would not reach the nearest fixed star in 4,500,000 years. Had it begun, therefore, when Adam was created, it would be no further on its journey than a coach (which has to go from the bottom of Cornwall to the top of Scotland) after it has past about three-quarters of a mile.

Q. Why does the moon (which is a sphere) appear to be a flat surface?

A. It is so far off, that we cannot distinguish any difference between the length of the rays which issue from the edge, and those which issue from the centre.

The rays AD and CD appear to be no longer than the ray BD; but if all the rays seem of the same length, the part B will not seem to be nearer to us than A and C, and therefore ABC will look like a flat or straight line.
The rays AD and CD are 240,000 miles long.
The ray BD is 238,910 miles long.

Q. Why do the sun and stars (which are spheres) appear to be flat surfaces?

A. Because they are such an immense way off, that we can discern no difference of length between the rays which issue from the edge, and those which issue from the centre of these bodies.

The rays AD and CD appear no longer than BD; and as B appears to be no nearer than A or C, therefore ABC must all seem equally distant; and ABC will seem a flat or straight line. (See last figure.)

Q. Why does distance make an object invisible?

A. Because the angle (made by the perpendicular height of the distant object with our eye) is so very acute, that one line of the angle merges in the other.

Here the tree AD would not be visible to the spectator C, even if he were to approach as far as B; because no visible perpendicular can be inserted between the two lines AC, DC, till after the point B is past; when the tree will appear like a very little speck.

Q. Why do telescopes enable us to see objects invisible to the naked eye?

A. Because they concentrate several rays within the tube of the telescope, and bend them upon the mirror or lens, which acts as a magnifying glass.

Q. When a ship (out at sea) is approaching the shore, why do we see the small masts before we see the bulky hull?

A. Because the earth is round, and the curve of the sea hides the hull from our eyes, after the tall masts have become visible.

Here only that part of the ship above the line AC can be seen by the spectator A; the rest of the ship is hidden by the swell of the curve DE.

Q. What is meant by refraction?

A. The bending of a ray of light, as it passes from one medium to another.

Q. How is a ray of light bent, as it passes from one medium to another?

A. When a ray of light passes into a denser medium, it is bent towards the perpendicular. When it passes into a rarer medium, it is bent from the perpendicular.

Suppose DE to be a perpendicular line.
If AB (a ray of light,) enters the water, it will be bent towards the perpendicular to C.
If (on the other hand) CB (a ray of light) emerges from the water, it would be bent away from the perpendicular towards A.

Q. Why does a spoon (in a glass of water) always appear bent?

A. Because as the light of the spoon emerges from the water, it is refracted.

And the spoon looks like ABC. (See the last figure.)

Q. Why does a river always appear more shallow than it really is?

A. Because the light of the bottom of the river is refracted as it emerges out of the water: and (as a stick is not so long when it is bent, as it is when it is straight) so the river seems less deep than it really is.

Q. How much deeper is a river than it seems to be?

A. One-third. If, therefore, a river seems only 4 feet deep, it is really 6 feet deep.

N. B. Many boys get out of their depth in bathing, in consequence of this deception. Remember, a river is always one-third deeper than it appears to be:—thus, if a river seems to be 4 feet deep, it is in reality 6 feet deep, and so on.

Q. Why do fishes always seem to be nearer the surface of a river than they really are?

A. Because the rays of light from the fish are refracted as they emerge from the eye: and (as a bent stick is not so far from end to end as a straight one) so the fishes appear nearer our eye than they really are.

Q. Why are some persons near-sighted?

A. Because the COR´NEA of their eye is so prominent, that the image of distant objects is reflected before it reaches the ret’ina; and, therefore, is not distinctly seen.

N.B. The cor´nea shields the crystalline lens, and is more or less convex according to the lens which it covers.

Q. What is meant by the “cor’nea of the eye?”

A. All the outside of the visible part of the eye-ball.

The curve A B C is called the cor'nea.
If this curve be too prominent (or convex), the eye is near-sighted.
If too flat (or concave), the eye is far-sighted.

Q. What is meant by the “ret’ina of the eye?”

A. The net-work which lines the back of the eye, is so called.

The net-work ABC is called the ret'ina, and the projecting part DEF is called the cor'nea.

Q. What sort of glasses do near-sighted persons wear?

A. If the cor’nea be too convex (or projecting), the person must wear double concave glasses, to counteract it.

Q. What is meant by “double concave glasses?”

A. Glasses hollowed in on both sides.

The figure A is double concave, or concave on both sides.

Q. What is meant by the “image of objects being reflected before it reaches the ret’ina?”

A. If the cor’nea be too convex, the image of a distant object is reflected (on the vitreous humours of the eye) before it reaches the ret’ina.

Thus the image is reflected at DE, instead of on ABC (the ret'ina).

Q. What is the use of double concave spectacle glasses?

A. Near-sighted spectacles cast the reflection further back; and the image (being thrown upon the ret’ina) becomes visible.

Q. Why are old people far-sighted?

A. Because the humours of their eyes are dried up by age, and the cor’nea sinks in, or becomes flattened.

Q. Why does the flattening of the cor’nea prevent persons seeing objects which are near?

A. As the cor’nea is too flat, the image of any near object is formed behind the ret’ina of the eye, and is not seen at all.

The reflection is made at DE, instead of at ABC (the retina).

Q. What sort of glasses do old people wear?

A. As their cor’nea is not sufficiently convex, they must use double convex glasses, to enable them to see objects near at hand.

Q. What sort of glasses are double convex spectacle-glasses?

A. Glasses which curve outwards on both sides.

The figure A is double convex, or convex on both sides.

Q. What is the use of double convex spectacle-glasses?

A. As the image of near objects is reflected behind the ret’ina, these double convex glasses shorten the focus of the eye, and bring the image into the eye (upon the ret’ina).

Q. Why do near-sighted persons bring objects close to the eye, in order to see them?

A. As the distance between the front and back of their eye is too great, distant objects are reflected before they reach the ret’ina; therefore, near-sighted persons bring the objects closer, in order that the reflection may be cast further back, (to reach the ret’ina).

Q. Why do old people hold objects further off, in order to see them better?

A. As the distance between the front and back of their eye is not great enough, the reflection of near objects is thrown beyond the ret’ina; therefore, they hold objects a long way off, in order to bring their images forward (so as to cast it on the ret’ina).

Q. Why are hawks able to see such an immense way off?

A. Because they have a muscle in the eye which enables them to flatten their cor’nea, by drawing back the crystalline lens.

This muscle is called the “marsupium.”

Q. Why can hawks not only see such a long way off, but also objects within half-an-inch of their eye?

A. Because their eyes are furnished with a broad circular rim which confines the action of this muscle, and throws the cor’nea forward.

Q. Into how many parts may a ray of light be divided?

A. Into three parts: Blue, Yellow, and Red.

N.B. These 3 colours, by combination, make seven. 1.—Red. 2.—Red and yellow form orange. 3.—Yellow. 4.—Yellow and blue make green. 5.—Blue. 6 and 7.—Shades of blue called indigo and violet.

Q. How is it known, that a ray of light consists of several different colours?

A. Because, if a ray of light be cast upon a triangular piece of glass (called a prism), it will be distinctly divided into seven colours: 1.—Red; 2.—Orange; 3.—Yellow; 4.—Green; 5.—Blue; 6.—Indigo; and 7.—Violet.

Q. Why does a prism divide a ray of light into various colours?

A. Because all these colours have different refractive powers. Red is refracted least, and blue the most; therefore, the blue colour of the ray will be bent to the top of the prism, and the red will remain at the bottom.

Here the ray AB received on a prism, would have the blue part bent up to C; the yellow part to D; and the red part no further than E.

Q. What is meant by the refraction of a ray?

A. Bending it from its straight line.

Thus the ray AB of the last figure is refracted at B into three courses, C, D, and E.

Q. What is the cause of a rainbow?

A. When the clouds opposite the sun are very dark, and rain is still falling from them, the rays of the bright sun are divided by the rain-drops, as they would be by a prism.

Let A, B, and C be three drops of rain; SA, SB, and SC three rays of the sun. SA is divided into the 3 colours; the blue and yellow are bent above the eye D, and the red enters it.
br /> The ray SB is divided into the three colours; the blue is bent above the eye, and the red falls below the eye D; but the yellow enters it.
The ray SC is also divided into the three colours. The blue (which is bent most) enters the eye; and the other two fall below it. Thus the eye sees the blue of C, and all drops in the position of C; the yellow of B, and of all drops in the position of B; and the red of A, &c.; and thus it sees a rainbow.

Q. Does every person see the same colours from the same drops?

A. No; no two persons see the same rainbow.

To another spectator the rays from SB might be red instead of yellow; the ray from SC, yellow; and the blue might be reflected from some drop below C. To a third person the red may issue from a drop above A, and then A would reflect the yellow, and B the blue, and so on.

Q. Why are there often two rainbows at one and the same time?

A. In one rainbow we see the rays of the sun entering the rain-drops at the top, and reflected to the eye from the bottom.

In the other rainbow, we see the rays of the sun entering the rain-drops at the bottom, and reflected to the top, whence they reach the eye.

Here the ray SA strikes the drop at A,—is refracted or bent to B,—is then reflected to C, where it is refracted again, and reaches the eye of the spectator.

Here the ray SB strikes the drop at B,—is refracted to A,—is then reflected to C,—is again reflected to D, when it is again refracted or bent till it reaches the eye of the spectator.

Q. Why are the colours of the second bow all reversed?

A. Because in one bow we see the rays which enter at the top of the raindrops, refracted from the bottom:

But in the other bow we see the rays which enter at the bottom of the raindrops (after two reflections), refracted from the top.

Here A, B, C, represent three drops of rain in the primary (or inner) rainbow.
The least refracted line is red, and blue the most.
So the red (or least refracted rays) of all the drops in the position of A,—the yellow of those in the position of B,—and the blue (or the most refracted rays) of the lowest drops, all meet the eye D, and form a rainbow to the spectator.
The reason why the primary bow exhibits the stronger colours is this—because the colours are seen after one reflection and two refractions; but the colours of the secondary (or upper) rainbow undergo two reflections and three refractions.

Here also the least refracted ray is red, and the most refracted blue (as in the former case); but the position of each is reversed.

Q. Why does a soap bubble exhibit such variety of colours?

A. The changing colour of the bubble depends upon the changing thickness of the film through which the ray passes.

Q. How does the thickness of the film affect the colour of the soap bubble?

A. Because different degrees of thickness produce different angles of refraction, and, therefore, different colours reach the eye.

Q. Why is the soap bubble so constantly changing its thickness?

A. As the bubble is suspended, the water keeps running down from the top to the bottom of the bubble, till the crown becomes so thin as to burst.

Q. Why are the late evening clouds red?

A. Because red rays (being the least refrangible) are the last to disappear.

Here it will be seen that the red ray PA, being reflected on the horizon at A, will be visible to us; but the yellow and blue rays will be hidden by the curve of the earth.

Q. Why are the early morning clouds red?

A. Because red rays (being the least refrangible) are the first to appear.

See last figure.—It is evident that PA (the red rays) will be reflected on the horizon before either the yellow or blue ones.

Q. What becomes of the blue and yellow rays?

A. They are refracted below the horizon, and are soon made invisible by the curve of the earth. (See last figure.)

Q. Why are the edges of clouds more luminous than their centres?

A. Because the body of vapour is thinnest at the edges of the clouds.

Q. What is the cause of morning and evening twilight?

A. When the sun is below the horizon, the rays (which strike upon the atmosphere or clouds) are bent down towards the earth, and produce a little light called twilight.

See figure on p. [399].—Here the rays of PA will give some light.

Q. Why is a ray of light composed of various colours?

A. If solar light were of one colour only, all objects would appear of that one colour (or else black.)

Q. Why are some things of one colour, and some of another?

A. As every ray of light is composed of all the colours of the rainbow, some things reflect one of these colours, and some another.

Q. Why do some things reflect one colour, and some another?

A. Because the surface of things is so differently constructed, both physically and chemically; and, therefore, some things reflect one ray; some two rays; some all the rays; and some none.

Q. What mainly determines the colour of any object?

A. The fluid or gas either in the body, or on its surface.

N. B. Nitrogen gives green,—Oxygen gives red,—Hydrogen gives blue colours.

Q. Why does dying a silk, &c. change its colour?

A. Because the materials used in dyeing alter the chemical construction of the substance dyed.

Q. Why is a rose red?

A. Because the surface of a rose absorbs the blue and yellow rays of light, and reflects only the red ones.

Q. Why does a rose absorb the yellow and blue rays, and reflect the red?

A. Because the action of the sun’s rays on the oxygen (accumulated in the petals) produces an acid which turns them red.

The leaves which compose a flower, are called petals.

Q. Why is a violet blue?

A. Because the surface of the violet absorbs the red and yellow rays of the sun, and reflects the blue only.

Q. Why do violets absorb the red and yellow rays, and reflect the blue?

A. Because the petals of the violet contain an alkali, which gives them a purple tinge.

Q. Why is a primrose yellow?

A. Because the surface of the primrose absorbs the blue and red rays of solar light, and reflects the yellow ones.

All plants which have much alkali in their ash, have blue or yellow flowers.

Those which have acid in their ash, have orange, pink, or red flowers.

N. B. Anti-acids (like soda) are called alkalis.

Q. Why are some things black?

A. Because they absorb all the rays of light, and reflect none.

Q. Why are some things white?

A. Because they absorb none of the rays of light, but reflect them all.

Q. Why are coals black?

A. Because they absorb all the rays of the sun which impinge upon them, and stifle their reflection.

Q. Why is snow white?

A. Snow consists of a vast number of crystals (or small prisms), which separate the rays into their elemental colours; but as these crystals are very numerous, the colours unite again before they meet the eye, and appear white.

N. B. The combination of all colours makes white.

Q. Why is sugar white?

A. Sugar consists of a vast number of small crystals, which separate the rays into their elemental colours; but as these crystals are very numerous, the colours unite again before they meet the eye, and appear white.

Q. Why is salt white?

A. Salt consists of a vast number of small crystals, which reflect the various rays of light from different points of the salt; and as these colours unite before they meet the eye, the salt appears to be white.

N. B. The combination of all colours makes white.

Q. Why are the leaves of plants green?

A. Because the carbon of the leaves is a bluish olive, and the sap and tissue of the cells, yellow; when, therefore, the yellow sap flows into the blue carbon, it produces a green leaf.

Q. Why are leaves a light green in spring?

A. Because the young leaves of spring have more sap than carbon; and, therefore, the yellow of the green prevails.

Q. Why are leaves a yellowish brown in autumn?

A. Because the carbon of the leaves is dying away, and the yellow tinge of the tissue and falling sap prevails over the blue.

Q. Why are plants a pale yellow when kept in the dark?

A. Solar light is essential for the production of carbon; and as plants kept in the dark lose their carbon, they lose the blue colour which should convert their yellow sap to green.

Q. Why are potatoes yellow?

A. Potatoes are grown underground, and, therefore, contain very little carbon (or blue colour); hence the yellow sap of the potato is not converted to green by carbon.

Q. Why are potatoes (which grow exposed to the air and light) green?

A. Because the sun-light increases their carbon; which (mingling with the yellow sap) turns the potato green.

Q. Why is it dangerous to sleep in a room which contains living plants?

A. Because they exhale carbon in the dark in the form of carbonic acid gas, which is destructive to animal life.

Q. Why are some things (like glass) transparent?

A. In transparent bodies (like glass) all the rays of light emerge on the opposite side.

Q. Why are some things shining and splendid?

A. Those objects which reflect the most rays are the most splendid; and those which absorb them most, are dull.

Q. Why are deserts so dazzling in summer time?

A. Because each separate grain of sand reflects the rays of the sun like a mirror.

Q. If you move a stick (burnt at one end) round pretty briskly, it seems to make a circle of fire,—Why is this?

A. Because the eye retains the image of any bright object, after the object itself is withdrawn; and as the spark of the stick returns before the image has faded from the eye, therefore, it seems to form a complete circle.

Q. If separate figures (as a man and a horse) be drawn on separate sides of a card, and the card twisted quickly, the man seems to be seated on the horse,—Why is this?

A. Because the image of the horse remains upon the eye till the man appears.

The Thaumatrope is constructed on this principle.

Q. Why do the stars twinkle?

A. Fixed stars are so far off, that their rays of light do not strike upon the eye in a continuous flow, but at intervals: when their rays reach the eye, the star becomes visible, and then is obscured till the next batch of rays arrive; and this perpetually occurring, makes a kind of twinkling.

Q. If we look at a red-hot fire for a few minutes, why does every thing seem tinged with a bluish green colour?

A. Because bluish green is the “accidental colour” of red: and if we fix our eye upon any colour whatsoever, when we turn aside, we see every object tinged with its accidental colour.

Q. If we wear blue glasses, (when we take them off,) every thing appears tinged with orange,—Why is this?

A. Because orange is the “accidental colour” of blue: and if we look through blue glasses, we shall see its “accidental colour,” when we lay our glasses aside.

Q. If we look at the sun for a few moments, every thing seems tinged with a violet colour,—Why is this?

A. Because violet is the “accidental colour” of yellow light; and as the sun is yellow, we shall see its “accidental colour” blue, when we turn from gazing at it.

Q. Does not the dark shadow (which seems to hang over every thing after we turn from looking at the sun) arise from our eyes being dazzled?

A. Partly so: the pupil of the eye is very much contracted by the brilliant light of the sun, and does not adjust itself immediately to the feebler light of terrestrial objects; but, independent of this, the “accidental colour” of the sun being dark violet, would tend to throw a shadow upon all things. (See p. [366].)

Q. Why is black glass for spectacles the best for wear?

A. Because white is the accidental colour of black; and if we wear black glasses, every thing will appear in white light, when we take them off.

Q. Why does every thing seem shadowed with a black mist, when we take off our common spectacles?

A. Because the glasses are white, and black being its “accidental colour,” every thing appears in a black shade, when we lay our glasses down.

The accidental colour of red is bluish green.
The accidental colour of orange is blue.
The accidental colour of violet is yellow.
The accidental colour of of black is white.

And the converse of this is true:—

The accidental colour of bluish green is red.
The accidental colour of of blue is orange.
The accidental colour of of yellow is violet.
The accidental colour of of white is black.

(The law of an accidental colour is this—The accidental colour is always half the spectrum. Thus, if we take half the length of the spectrum by a pair of compasses, and fix one leg in any colour, the other leg will hit upon its accidental colour.)

N. B. The spectrum means the seven colours—Red, orange, yellow, green, blue, indigo, and violet, divided into seven equal bands, and placed side by side in the order just mentioned.

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CHAPTER XXVIII.
SOUND.

Q. What is sound?

A. The vibration of some sonorous substance produces motion in the air called sound waves, which strike upon the drum of the ear, and give the sensation of sound.

Q. What are musical sounds?

A. Regular and uniform successions of vibrations, which are always pleasing to the ear.

Q. How fast does sound travel?

A. About 13 miles in a minute, or 1142 feet in a second of time.

Q. How fast does light travel?

A. Light would go 8 times round the whole earth, while sound is going its 13 miles.

Q. Why are some things sonorous, and others not?

A. The sonorous quality of any substance depends upon its hardness and elasticity.

Q. Why are copper and iron sonorous, and not lead?

A. Copper and iron are hard and elastic; but as lead is neither hard nor yet elastic, it is not sonorous.

Q. Of what is bell-metal made?

A. Of copper and tin in the following proportions:—In every 5 pounds of bell-metal, there should be 1 lb. of tin, and 4 lbs. of copper.

Q. Why is this mixture of tin and copper used for bell-metal?

A. Because it is much harder and more elastic than either of the pure metals.

Q. Why is the sound of a bell stopped by touching the bell with our finger?

A. The weight of the finger stops the vibrations of the bell; and as soon as the bell ceases to vibrate, it ceases to make sound-waves in the air.

Q. Why does a split bell make a hoarse disagreeable sound?

A. The split of the bell causes a double vibration; and as the sound-waves clash and jar, they impede each other’s motion, and produce discordant sounds.

Q. Why does a fiddle-string give a musical sound?

A. The bow drawn across the string causes it to vibrate, and this vibration of the string sets in motion the sound-waves of the air, and produces musical notes.

Q. Why does a drum sound?

A. The parchment head of the drum vibrates from the blow of the drum-stick, and sets in motion the sound-waves of the air.

Q. Why do musical glasses give sounds?

A. Because the glasses vibrate as soon as they are struck, and set in motion the sound-waves of the air.

Q. Why do flutes, &c. produce musical sounds?

A. The breath of the performer causes the air in the flute to vibrate, and sets in motion the sound-waves of the air.

Q. Why do piano-fortes produce musical sounds?

A. The keys of the piano (being struck with the finger) lift up a little hammer which knocks against a string; and the vibration thus produced, sets in motion the sound-waves of the air.

Q. Why are some notes bass and some treble?

A. Slow vibrations produce bass or deep sounds; whereas, quick vibrations produce shrill or treble sounds.

Q. Why is an instrument flat when the strings are unstrung?

A. Because the vibrations are too slow; in consequence of which, the sounds produced are not shrill or sharp enough.

Q. Why can persons living a mile or two from

a town hear the bells of the town-church some times, and not at others?

A. Fogs, rain, and snow, obstruct the passage of sound; but when the air is cold and clear, sound is propagated more easily.

Q. Why can we not hear sounds (as distant church bells) in rainy weather, so well as in fine weather?

A. Because the falling rain interferes with the undulations of the sound-waves, and breaks them up.

Q. Why can we not hear sounds (as distant church bells) in snowy weather, so well as in fine weather?

A. Because the falling snow interferes with the undulations of the sound-waves, and stops their progress.

Q. Why can we hear distant clocks most distinctly in clear cold weather?

A. Because the air is most uniform then: there are not two currents of air (one up and one down) to interrupt the sound-waves.

Q. Why can persons hear the voices of men in conversation for a mile distant, near the poles, in winter time?

A. Because the air is very cold and very clear; in consequence of which, there are not two currents of air (one up and one down) to interrupt the sound-waves.

Captain Ross heard the voices of his men in conversation, a mile and a half from the spot where they stood.

Q. Why are not sounds (such as distant church bells) heard so distinctly on a hot day as in frosty weather?

A. Because there are two currents of air; the current of hot air ascending from the earth, and the current of colder air falling towards the earth; and these two currents break up the sound-waves.

Q. Why can we not hear sounds (such as distant clocks) so distinctly in a thick mist or haze, as in a clear night?

A. Because the mist diminishes the velocity of the sound-waves, and (by overburdening them with vapour) limits their length.

Q. Why do we hear sounds better by night than by day?

A. 1st—Night air is more uniform, because the ascending currents of air (raised by the action of the sun’s rays) cease as the evening advances; and

2ndly—Night is more still from the suspension of business, and the cessation of the hum of men.

Q. How should partition walls be made to prevent the voices in adjoining rooms from being heard?

A. The space between the laths (or canvass) should be filled with shavings or saw-dust; and then no sound would ever pass from one room to another.

Q. Why would shavings or saw-dust prevent the transmission of sound from room to room?

A. Because there would be several different media for the sound to pass through: 1st—the air;

2ndly—the laths and paper;

3rdly—the saw-dust or shavings;

4thly—the air again: and every variety diminishes the strength of the sound-waves.

Q. Why can deaf people hear through an ear trumpet?

A. The ear trumpet restrains the spread of the voice, and limits the diameter of the sound-waves; in consequence of which, their strength is increased.

Q. Why are mountains so noiseless and quiet?

A. Because the air of mountains is very rarefied; and as the air becomes rarefied, sound becomes less intense.

Q. How do you know that the rarety of air diminishes the intensity of sound?

A. If a bell be rung in the receiver of an air-pump, the sound becomes fainter and fainter as the air is exhausted, till at last it is quite inaudible.

Q. What is the cause of echo?

A. Whenever a sound-wave strikes against any obstacle (such as a wall or hill), it is reflected (or thrown back); and this reflected sound is called an echo.

The same laws govern echo as light. (See p. [370].)

Q. What places are most famous for echoes?

A. Caverns, grottoes, and ruined abbeys; the areas of antique halls; the windings of long passages; the aisles of cathedral churches; mountains, and ice-bergs.

Q. Why are caverns, grottoes, and ruined abbeys famous for echoes?

A. 1st—Because the sound-waves cannot pass beyond the cavern or grotto, and must flow back:

2ndly—The return waves (being entangled by the cavern) are detained for a short time, and come deliberately to the ear.

Q. Why are antique halls, winding passages, and cathedral aisles famous for echoes?

A. Because the sound-waves cannot flow freely forward, but strike against the winding walls perpetually, and are beaten back.

Q. Why are mountains and ice-bergs famous for echoes?

A. Because they present a barrier to the sound-waves which they cannot pass; and are sufficiently elastic to throw them back.

Q. Why do not the walls of a room or church produce echo?

A. Because sound travels with such velocity, that the echo is blended with the original sound, and produce but one impression on the ear.

Sound travels 13 miles in a minute.

Q. Why do very large buildings (as cathedrals), often reverberate the voice of the speaker?

A. Because the walls are so far off from the speaker, that the echo does not get back in time to blend with the original sound; and, therefore, each is heard separately.

Q. Why do some echoes repeat only one syllable?

A. The further the echoing body is distant, the more sound it will reflect. If, therefore, the echoing body be near, it will repeat but one syllable.

Q. Why does an echo sometimes repeat two or more syllables?

A. Because the echoing body is far off; and, therefore, there is time for one reflection to pass away before another reaches the ear.

Q. Why do windows rattle when carts pass by a house?

A. 1st—Glass is sonorous; and the air communicates its vibrations to the glass, which echoes the same sound: and

2ndly—The window-frame is shaken by the sound-waves impinging against the window, and contributes to the noise.

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CHAPTER XXIX.
MISCELLANEOUS.

Q. Why do the bubbles in a cup of tea range round the sides of the cup?

A. Because the cup attracts them.

Q. Why do all the little bubbles tend towards the large ones?

A. Because the large bubbles (being the superior masses) attract them.

Q. Why do the bubbles of a cup of tea follow a tea-spoon?

A. Because the tea-spoon attracts them.

Q. Why are the sides of a pond covered with leaves, while the middle of the pond is quite clear?

A. Because the shore attracts the leaves to itself.

Q. Why do all fruits, &c. (when severed from the tree) fall to the earth?

A. Because the earth attracts them.

Q. Why do persons (who water plants) very

often pour the water into the saucer, and not over the plants?

A. Because the water in the saucer is supped up by the mould (through the hole at the bottom of the flower-pot), and is transferred to the stem and leaves of the plant by capillary attraction, (See p. [84]).

Q. Why is vegetation on the margin of a river more luxuriant than in an open field?

A. Because the porous earth on the bank sups up water to the roots of the plants by capillary attraction.

Q. Why is a lump of sugar (left at the bottom of a cup) so long in melting?

A. Because as it melts, it makes the tea above it heavier; and (so long as it remains at the bottom) is surrounded by tea fully saturated with sugar; in consequence of which, the same portions of liquid will hold no more sugar in solution.

Q. Why does the lump of sugar melt more quickly when stirred about?

A. Because fresh portions of unsaturated tea keep coming in contact with the lump, and soon dissolve it.

Q. Why does a piece of sugar (held in a spoon at the top of our tea) melt very rapidly?

A. Because as the tea becomes sweetened, it descends to the bottom of the cup by its own gravity; and fresh portions of unsweetened tea are brought constantly into contact with the sugar, till the lump is entirely dissolved.

Q. How can a sick room be kept free from unhealthy effluvia?

A. Vinegar boiled with myrrh, or camphor, sprinkled in a sick room, will entirely correct putridity.

Q. Why does lime destroy the offensive smells of bins, sewers, &c.?

A. Because it combines with the carbonic acid of these places, and converts it into carbonate of lime, which is entirely free from smell.

Q. Why does chloride of lime fumigate a sick room?

A. Because the chlorine absorbs the hydrogen of the stale air; and by this means removes both the offensive smell and the infection of a sick room.

Q. How can the taint of meat be removed?

A. Either by washing with pyroligneous acid,—covering it for a few hours with common charcoal,—or by putting a few lumps of charcoal into the water in which it is boiled.

Q. Why do these things destroy the taint of meat?

A. Because they combine with the putrescent particles, and neutralize their offensive taste and smell.

Q. Why should bed-rooms, cottages, hospitals, and stables, be washed occasionally with lime-white?

A. Because the lime is very caustic, and removes all organic matters adhering to the walls.

Q. How can mouldiness be prevented?

A. The perfume of any essential oil will prevent mouldiness from ink, paste, preserves, &c.

Alum, salt of amber, borax, nitre, salt, camphor, charcoal, and pyroligneous acid, are all excellent antiseptics.

Salt, corrosive sublimates, copperas, and alum, all arrest the decay of timber. (See p. [426].)

Q. Why will strong Souchong tea poison flies?

A. Because it produces prussic acid, which destroys their nervous system.

Q. Why is strong green tea unwholesome?

A. Because it contains prussic acid, which destroys the nervous system.

Q. Why is a dead man taller than a living man?

A. Because at death the cartilages are relaxed. So, also, after a night’s rest, a man is taller than when he went to bed.

Q. What is sleep?

A. Sleep is the rest of the brain and nervous system.

Q. Why can we not see, when we are asleep with our eyes open?

A. Because the “RET´INA of the eye” is inactive and at rest.

Q. Why can we not hear in sleep?

A. Because the drum or “tympanum of the ear” is placid and at rest.

Q. Why can we not taste when we are asleep?

A. Because the nerves at the end of the tongue (called papillæ) are inactive and at rest.

Q. Why can we not feel when we are asleep?

A. Because the ends of the nerves (called papillæ), situated in the skin, are inactive and at rest.

Q. Why have persons in sleep no will of their own, but may be moved at the will of any one?

A. Because the “cerebellum” (or posterior part of the brain) is inactive and at rest.

Q. Why have dreamers no power of judgment or reason?

A. Because the “cerebrum” (or front of the brain) is inactive and at rest.

Q. Why are dreams such foolish and inconsistent things?

A. Because the “pineal gland” is acting without the brain; and the faculty of thinking exists in the “pineal gland,” but the faculty of judgment in the “cerebrum of the brain.”

The cerebrum of the brain occupies the top and front of the skull. The pineal gland is a small conical gland (about the size of a pea) in the brain.

Q. Why do some persons lose all power of sensation?

A. Because the “cerebrum” (or front of their brain) has been injured.

Q. Why are many persons idiots?

A. Because the “cerebellum of the brain” has been removed by some accident, or injured by some disease.

The cerebellum is all the posterior part of the brain.

Q. Why does a person feel when he is touched?

A. The ends of certain nerves (called PAPILLÆ) situated in the skin erect themselves when touched, and produce a nervous sensation called feeling.

Q. Why are persons able to taste different flavours?

A. Because the “PAPILLÆ” of the tongue and palate erect themselves when food touches them, and produce a nervous sensation called taste.

Q. Why do very old people lose the power of volition, sensation, and thought?

A. Because their brain ossifies; and as the “cerebrum” (or front of the brain) goes, they lose the power of sensation and reason; and as the “cerebellum” (or posterior part of the brain) goes, they lose the power of volition.

Q. Why are old people unable to walk?

A. Because their muscles become rigid.

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