CHAPTER III.
Q. What is the third chief source of heat?
A. Chemical Action.
Q. What is meant by chemical action being the source of heat?
A. Many things, when their chemical constitution is changed, (either by the abstraction of some of their gases, or by the combination of others not before united,) evolve heat, while the change is going on.
Q. Explain by illustration what you mean.
A. Water is cold, and sulphuric acid is cold; but if these two cold liquids be mixed together, they will produce boiling heat.
Q. Why will cold water, mixed with sulphuric acid, produce heat?
A. Because water (being lighter than sulphuric acid), is condensed by the heavier liquid; and its heat is squeezed out, as water from a sponge.
Q. Why does cold water, poured on lime, make it intensely hot?
A. The heat is evolved by the chemical action, produced by the cold water combining with the lime.
Q. Where does the heat come from?
A. It was in the water and lime before; but was in a latent state.
Q. Was there heat in the cold water and lime, before they were mixed together?
A. Yes. All bodies contain heat; the coldest ice, as well as the hottest fire.
Q. Is there heat even in ice?
A. Yes. But it is latent, (i. e. not perceptible to our senses).[4]
[4] Latent, from the Latin word, Lateo, (to lie hid.)
Q. How do you know there is heat, if you cannot perceive it?
A. Thus:—Ice is 32° by the thermometer; but if ice be melted over a fire, (though 140° of heat are thus absorbed,) it will feel no hotter than it was before. (i. e. it will be only 32°, and not 172°)[5].
[5] 32°, i. e. 32 degrees; 140°, i. e. 140 degrees, &c.
Q. What becomes of the 140°, which went into the ice to melt it?
A. It is hidden in the water; or (to speak more scientifically) it is stored up in a latent state.
Q. How much heat may be thus secreted or made latent?
A. All things contain a vast quantity of latent heat; but, as much as 1140° of heat may remain latent in water.
Q. How can 1140° of heat be added to water, without being perceptible to our feelings?
A. 1st—140° of heat are hidden in the water, when ice is melted by the sun or fire.
2ndly—1000° more of heat are secreted, when water is converted into steam. Thus, before ice is converted into steam, 1140° of heat become latent.[6]
[6] Thus, one pint of boiling water, (212° according to the thermometer,) will make 1800 pints of steam; but the steam is no hotter to the touch than boiling water, both are 212°: therefore, when water is converted into steam, 1000° of heat become latent. Hence, before ice is converted to steam, it must contain 1140° of latent heat.
Q. Can we be made to feel the heat of ice or snow?
A. Yes. Into a pint of snow put half as much salt; then plunge your hand into the liquid; and it will feel so intensely cold, that the snow itself will seem quite warm in comparison to it.
Q. Is salt and snow really colder than snow?
A. Yes, many degrees; and by dipping your hand into the mixture first, and into snow afterwards, the mere snow will seem to be comparatively warm.
Q. What is fire?
A. Combustion is another instance of heat, arising from chemical action.
Q. What two things are essential to produce combustion?
A. Fuel and air.
Q. What are the elements of fuel?
A. As bread is a compound of flour, yeast, and salt; so fuel is a compound of hydrogen and carbon.
Q. What are the elements of atmospheric air?
A. The air is a compound of oxygen and nitrogen mixed together; in the proportion of five gallons of nitrogen, to one of oxygen.
Q. What is carbon?
A. The solid part of fuel. It abounds also in all animal bodies, earths, and minerals.
Q. Mention some different species of carbon.
A. Common charcoal, lamp-black, coke, black lead, and the diamond, are all varieties of carbon.
Q. What is hydrogen?
A. An inflammable gas. The gas used in our streets, is only the hydrogen gas driven out of coals by heat.
Q. What are the peculiar characteristics of hydrogen gas?
A. Though this gas itself will burn, yet a candle will not burn when immersed in it; nor can an animal live in it. Hydrogen gas is the lightest of all known substances.[7]
[7] Hydrogen gas may be made thus:—Put some pieces of zinc or iron filings into a glass: pour over them a little sulphuric acid (vitriol), diluted with twice the quantity of water; then cover the glass over for a few minutes, and hydrogen gas will be given off.
Exp. If a flame be put into the glass, an explosion will be made.
If the experiment be tried in a phial, which has a piece of tobacco-pipe run through the cork; and a light held a few moments to the top of the pipe, a flame will be made.
If a balloon be held over the phial, (so that the gas can inflate it,) the balloon will ascend in a very few minutes.
Q. What is oxygen?
A. A gas, much heavier than hydrogen; which gives brilliancy to flame, and is essential to animal life.[8]
[8] Oxygen gas is much more troublesome to make than hydrogen. The cheapest plan is to put a few ounces of manganese (called the black oxide of manganese) into an iron bottle, furnished with a bent tube; set the bottle on a fire till it becomes red hot, and put the end of the tube into a pan of water. In a few minutes, bubbles will rise through the water; these bubbles are oxygen gas.
These bubbles may be collected thus:—Fill a common bottle with water; hold it topsy-turvy over the bubbles which rise through the pan, but be sure the mouth of the bottle be held in the water. As the bubbles rise into the bottle, the water will run out; and when all the water has run out, the bottle is full of gas. Cork the bottle while the mouth remains under water; set the bottle on its base; cover the cork with lard or wax, and the gas will keep till it be wanted.
N. B. The quickest way of making oxygen gas, is to rub together in a mortar half an ounce of oxide of copper, and half an ounce of chlorate of potassa. Put the mixture into a common oil flask, furnished with a cork which has a bent tube thrust through it. Heat the bottom of the flask over a candle or lamp; and when the mixture is red hot, oxygen gas will be given off. Note—the tube must be immersed in a pan of water, and the gas collected as before.
(Chlorate of potassa may be bought at any chemist’s; and oxide of copper may be procured by heating a sheet of copper red hot, and when cool, striking it with a hammer: the scales that peel off, are oxide of copper.)
Exp. Put a piece of red hot charcoal, (fixed to a bit of wire,) into your bottle of oxygen gas; and it will throw out most dazzling sparks of light.
Blow a candle out; and while the wick is still red, hold the candle (by a piece of wire,) in the bottle of oxygen gas; the wick will instantly ignite, and burn brilliantly.
(Burning sulphur emits a blue flame, when immersed in oxygen gas.)
Q. What is nitrogen?
A. Nitrogen is another invisible gas. It will not burn, like hydrogen; and an animal cannot live in it: it abounds in animal and vegetable substances, and is the chief ingredient of the common air.[9]
[9] Nitrogen gas may easily be obtained thus:—Put a piece of burning phosphorus on a little stand, in a plate of water; and cover a bell glass over. (Be sure the edge of the glass stands in the water.) In a few minutes the air will be decomposed, and nitrogen alone remain in the bell glass.
(N.B. The white fume which will arise and be absorbed by the water in this experiment, is phosphoric acid; i. e. phosphorus combined with oxygen of the air.)
Q. Why is there so much nitrogen in the air?
A. In order to dilute the oxygen. If the oxygen were not thus diluted, fires would burn out, and life would be exhausted too quickly.
Q. What three elements are necessary to produce combustion?
A. Hydrogen gas, carbon, and oxygen gas; the two former in the fuel, and the last in the air which surrounds the fuel.
Q. What causes the combustion of the fuel?
A. The hydrogen gas of the fuel being set free, and excited by a piece of lighted paper, instantly unites with the oxygen of the air, and makes a yellow flame: this flame heats the carbon of the fuel, which also unites with the oxygen of the air, and produces carbonic acid gas.
Q. What is carbonic acid gas?
A. Only carbon (or charcoal) combined with oxygen gas.
Q. Why does fire produce heat?
A. 1st—By liberating latent heat from the air and fuel: and
2ndly—By throwing into rapid motion the atoms of matter.
Q. How is latent heat liberated by combustion?
A. When the oxygen of the air combines with the hydrogen of the fuel, the two gases condense into water; and latent heat is squeezed out, as water from a sponge.
Q. How are the atoms of matter disturbed by combustion?
A. 1st—When hydrogen of fuel and oxygen of air condense into water, a vacuum is made; and the air is disturbed, as a pond would be, if a pail of water were taken out of it: and
2ndly—When the carbon of fuel and oxygen of air expand into carbonic acid gas, the air is again disturbed, as it would be by the explosion of gunpowder.
Q. How does fire condense hydrogen and oxygen into water?
A. The hydrogen of fuel and oxygen of air (liberated by combustion) combining together, condense into water.
Q. How does fire expand carbon into carbonic acid gas?
A. The carbon of fuel and oxygen of air (combining together in combustion) expand into a gas, called carbonic acid.
Q. Why is a fire (after it has been long burning) red hot?
A. When coals are heated throughout, the carbon is so completely mixed with the oxygen of the air, that the whole surface is in a state of combustion, and therefore red hot.
Q. In a blazing fire, why is the upper surface of the coals black, and the lower surface red?
A. Carbon (being very solid) requires a great degree of heat to make it unite with the oxygen of the air. When fresh coals are put on, their under surface is heated before the upper surface; and one is red (or in a state of combustion), while the other is black.
Q. Which burns the quicker, a blazing fire, or a red hot one?
A. A blazing fire burns out the fuel quickest.
Q. Why do blazing coals burn quicker than red hot ones?
A. In red hot coals, only the mere surface is in a state of combustion, because the carbon is solid; but in a blazing fire, (where the gases are escaping), the whole volume of the coal throughout is in a state of decomposition.
Q. What is smoke?
A. Unconsumed parts of fuel (principally carbon), separated from the solid mass, and carried up the chimney by the current of hot air.
Q. Why is there more smoke when coals are fresh added, than when they are red hot?
A. Carbon (being solid), requires a great degree of heat to make it unite with oxygen, (or, in other words, to bring it into a state of perfect combustion): when coals are fresh laid on, more carbon is separated than can be reduced to combustion; and so it flies off in smoke.
Q. Why is there so little smoke with a red hot fire?
A. When a fire is red hot, the entire surface of the coals is in a state of combustion; so a very little flies off unconsumed, as smoke.
Q. Why are there dark and bright spots in a clear cinder fire?
A. Because the intensity of the combustion is greater in some parts of the fire, than it is in others.
Q. Why is the intensity of the combustion so unequal?
A. Because the air flies to the fire in various and unequal currents.
Q. Why do we see all sorts of grotesque figures in hot coals?
A. Because the intensity of combustion is so unequal, (owing to the gusty manner in which the air flies to the fuel; and the various shades of red, yellow, and white heat mingling with the black of the unburnt coal), produce strange and fanciful resemblances.
Q. Why does paper burn more readily than wood?
A. Merely because it is of a more fragile texture; and, therefore, its component parts are more easily heated.
Q. Why does wood burn more readily than coal?
A. Because it is not so solid; and, therefore, its elemental parts are more easily separated, and made hot.
Q. When a fire is lighted, why is paper laid at the bottom, against the grate?
A. Because paper (in consequence of its fragile texture), so very readily catches fire.
Q. Why is wood laid on the top of the paper?
A. Because wood, (being more substantial), burns longer than paper; and, therefore, affords a longer contact of flame to heat the coals.
Q. Why would not paper do without wood?
A. Because paper burns out so rapidly, that it would not afford sufficient contact of flame to heat the coals to combustion.
Q. Why would not wood do without shavings, straw, or paper?
A. Because wood is too substantial to be heated into combustion, by the flame issuing from a mere match.
Q. Why would not the paper do as well, if placed on the top of the coals?
A. As every blaze tends upwards, if the paper were placed on the top of the fire, its blaze would afford no contact of flame to fuel lying below.
Q. Why should coal be placed above the wood?
A. As every flame tends upwards, if the wood were above the coal, the flame would not rise through the coal to heat it.
Q. Why is a fire kindled at the lowest bar of a grate?
A. As every flame tends upwards; when a flame is made at the bottom of a fire, it ascends through the fuel and heats it: whereas, if the fire were lighted from the top, the flame would not come into contact with the fuel piled below.
Q. Why does coal make such excellent fuel?
A. Because it is so very hard and compact, that it burns away very slowly.
Q. Why will cinders become red hot, quicker than coals?
A. Because they are more porous and less solid; and are, therefore, sooner reduced to a state of combustion.
Q. Why will not iron cinders burn?
A. Iron cinders are cinders saturated with oxygen; they are unfit for fuel, because they can imbibe no more oxygen, being saturated already.
Q. Why are cinders lighter than coals?
A. Because their vapour, gases, and volatile parts, have been driven off by previous combustion.
Q. Why will not stones do for fuel, as well as coals?
A. Because they contain no hydrogen (or inflammable gas) like coals.
Q. Why will not wet kindling light a fire?
A. 1st—Because the moisture of the wet kindling prevents the oxygen of the air from getting to the fuel to form it into carbonic acid gas: and
2ndly—The heat of the fire is perpetually drawn off, by the conversion of water into steam.
Q. Why does dry wood burn better than green?
A. 1st—Because no heat is carried away, by the conversion of water into steam: and
2ndly—The pores of dry wood are filled with air, which supply the fire with oxygen.
Q. Why do two pieces of wood burn better than one?
A. 1st—Because they help to entangle the heat of the passing smoke, and throw it on the fuel: and
2ndly—They help to entangle the air that passes over the fire, and create a kind of eddy or draught.
Q. Why does salt crackle when thrown into a fire?
A. Salt contains water; and the cracking of the salt is owing to the sudden conversion of the water into steam.
Q. Why will not wood or paper burn, if they are steeped in a solution of potash, phosphate of lime, or ammonia (hartshorn)?
A. Because any “al’kali” (such as potash) will arrest the hydrogen (as it escapes from the fuel), and prevent its combination with the oxygen of air.
Q. What is an al’kali?
A. The con’verse of an acid; as bitter is the con’verse of sweet, or insipid the con’verse of pungent.
Q. Why does a jet of flame sometimes burst into the room through the bars of a stove?
A. The iron bars conduct heat to the interior of some lump of coal: and its volatile gas (bursting through the weakest part) is kindled by the glowing coals over which it passes.
Q. Why is this jet sometimes of a greenish yellow colour?
A. When a lump of coals lies over the hot bars, or the coals below it are not red hot, the gas which bursts from the lump escapes unburnt, and is of a greenish colour.
Q. Why does the gas escape unburnt?
A. Because neither the bars nor coals (over which it passes) are red-hot.
Q. Why does a bluish flame sometimes flicker on the surface of hot cinders?
A. Gas from the hot coals at the bottom of the grate mixing with the carbon of the coals above, produces an inflammable gas (called carbonic oxide), which burns with a blue flame.
Q. Why is the flame of a good fire yellow?
A. Because both the hydrogen and carbon of the fuel are in a state of perfect combustion. It is the white heat of the carbon, which gives the pale yellow tinge to the flaming hydrogen.
Q. What is light?
A. Rapid undulations of a fluid called ether, striking on the eye.
Q. How does combustion make these undulations of light?
A. The atoms of matter (set in motion by heat) striking against this ether, produce undulations in it; as a stone thrown into a stream, would produce undulations in the water.
Q. How can undulations of ether produce light?
A. As sound is produced by undulations of air striking on the ear; so light is produced by undulations of ether striking on the eye.
Q. What is ether?
A. A very subtile fluid, which pervades and surrounds every thing we see.
Q. Mention a simple experiment to prove that light is produced by rapid motion.
A. When a fiddle-string is jerked suddenly, its rapid vibration produces a grey light; and when a carriage wheel revolves very quickly, it sends forth a similar light.
Q. Does heat always produce light?
A. No: the heat of a stack of hay, or reeking dunghill, though very great, is not sufficient to produce light.
Q. Why is a yellow flame brighter than a red hot coal?
A. Because yellow rays always produce the greatest amount of light; though red rays produce the greatest amount of heat.
Q. Why is the light of a fire more intense sometimes than at others?
A. The intensity of fire-light depends upon the whiteness to which the carbon is reduced, by combustion. If the carbon be white hot, its combustion is perfect, and the light intense; if not, the light is obscured by smoke.
Q. Why will not cinders blaze, as well as fresh coals?
A. The flame of coals is made chiefly by hydrogen gas. As soon as this gas is consumed, the hot cinders produce only an invisible gas, called carbonic acid.
Q. Where does the hydrogen gas of a fire come from?
A. The fuel is decomposed (by combustion) into its simple elements, carbon and hydrogen gas. (see p. [33])
Q. Why does not a fire blaze on a frosty night, so long as it does upon another night?
A. The air (being very cold) rushes to the fire so rapidly, that the coals burn out faster, and the inflammable gas is sooner consumed.
Q. Why does a fire burn clearest on a frosty night?
A. Because the volatile gases are quickly consumed; and the solid carbon plentifully supplied with air, to make it burn bright and intensely.
Q. Why does a fire burn more intensely in winter than in summer time?
A. Because the air is colder in winter, than in summer-time.
Q. How does the coldness of the air increase the heat of a fire?
A. For two reasons: 1st—Because cold air being more condensed than hot air, contains a greater body: and
2ndly—Cold air rushes more quickly to the fire, and supplies more oxygen.
Q. Why does the sun, shining on a fire, make it dull, and often put it out?
A. 1st—When the sun shines, the air is rarefied; and, therefore, flows more slowly to the fire.
2ndly—As the air is rarefied, even that which reaches the fire, affords less nourishment.
Q. Why does the air flow to the fire more tardily for being rarefied?
A. The greater the contrast (between the external air, and that which has been heated by the fire) the more rapid will be the current of air towards that fire.
Q. Why does rarefied air afford less nourishment to fire, than cold air?
A. Because it is spread out, (like a piece of gold beaten into leaf); and as a square inch of gold leaf will not contain so much gold as a square inch of bullion—so, a square inch of rarefied air has less body, than a square inch of cold air.
Q. Why does a fire burn more fiercely in the open air?
A. 1st—Because the air out-of-doors is more dense, than the air in-doors: and
2ndly—Because air is more freely supplied to a fire out-of-doors.
Q. Why is the air out-of-doors more dense than that in-doors?
A. Because the circulation is more free; and as soon as any portion has been rarefied, it instantly escapes, and is supplied by colder currents.
Q. Why does not a fire burn so freely in a thaw, as in a frost?
A. During a thaw, the air is filled with vapour; and, both moves too slowly, and is too much diluted to nourish the fire.
Q. Why does a fire burn so fiercely in windy weather?
A. In windy weather the air is rapidly changed, and affords plentiful nourishment to the fire.
Q. Why do a pair of bellows get a fire up?
A. A pair of bellows, (like the wind), drives the air more rapidly to the fire; and the plentiful supply of oxygen soon makes the fire burn intensely.
Q. Why is a candle blown out by the breath, and not made more intense, like a fire?
A. As the flame of a candle is confined to a very small wick, it is severed from it by the breath; and (being unsupported) must go out.
Q. Why is a smouldering wick sometimes rekindled by blowing it?
A. The breath carries the air to it with great rapidity; and the oxygen of the air kindles the red hot wick, as it kindles charred wood.
Q. Why is not the red hot wick kindled by the air around it, without blowing it?
A. Because oxygen is not supplied with sufficient freedom, unless it be blown to the wick.
Q. When is this experiment most likely to succeed?
A. In frosty weather; because the air contains more oxygen then, being condensed by the cold.
Q. Why does a poker, laid across a dull fire, revive it?
A. For two reasons. 1st—Because the poker concentrates the heat, and therefore increases it: and
2ndly—Because the poker arrests the air which passes over the fire, and produces a draught.
Q. Why do several pieces of wood or coal burn better than one?
A. When there are two or three pieces of wood on a fire, the air (circulating round them) produces an eddy or draught, which draws up the fire.
Q. Why are stoves fixed on the floor of a room?
A. In order that the air, on the lower part of the room, may be heated by the fire.
Q. Would not the air of the lower part of a room be heated equally well, if the stoves were fixed higher up?
A. No; the heat of a fire has a very little effect upon the air below the level of the grate; and, therefore, every grate should be as near to the floor as possible.
Q. Why are our feet so cold when we sit close by a good fire?
A. As the fire consumes the air which passes over it, cold air rushes through the crevices of the doors and windows along the bottom of the room to supply the deficiency; and these currents of cold air, rushing constantly over our feet, deprive them of their warmth.
Q. If a piece of paper be laid flat on a clear fire, it will not blaze, but char. Why so?
A. The carbon of a clear fire, being sufficiently hot to unite with the oxygen of the air, produces carbonic acid gas, which soon envelops the paper laid flat upon the cinders: but carbonic acid gas will not blaze.
Q. If you blow the paper, it will blaze immediately. Why so?
A. By blowing, or opening the door suddenly, the carbonic acid is dissipated, and the paper is instantly fanned into flame.
Q. Why does water extinguish a fire?
1st—Because the water forms a coating over the fuel, and keeps it from the air:
2ndly—The conversion of water into steam, draws off the heat of the burning fuel.
Q. Why does a little water make a fire fiercer, while a larger quantity of water puts it out?
A. Water is composed of oxygen and hydrogen; when, therefore, the fire can decompose the water into its simple elements, it serves for fuel to the flame.
Q. How can water serve for fuel to fire?
A. The hydrogen of the water will burn with a flame; and the oxygen of the water will increase the intensity of that flame.
Q. If a house be on fire, is too little water worse than no water at all?
A. Certainly. Unless the water be supplied so plentifully as to quench the fire, it will increase the intensity, like fuel.
Q. When will water extinguish fire?
A. When the supply is so rapid and abundant, that the fire cannot convert it into steam.
Q. Does not a very little water slacken the heat of fire?
A. Yes, till it is converted into steam; but then it increases the intensity of fire, and acts like fuel.
Q. Why does the wick of a candle (when the flame has been blown out) catch fire so readily?
A. As the wick is already very hot, a little extra heat will throw it into flame.
Q. Why does the extra heat revive the flame?
A. Because it again liberates the hydrogen of the tallow, and ignites it.
Q. Cannot wood be made to blaze without actual contact with fire?
A. Yes; if a piece of wood be held near the fire for a little time it will blaze, even though it does not touch the fire.
Q. Why will wood blaze, even if it does not touch the fire?
A. The heat of the fire drives out the hydrogen gas of the wood; which is inflamed by contact with the red-hot coals.
Q. Why will a neighbour’s house sometimes catch fire, though no flame of the burning house ever touches it?
A. The heat of the burning house sets at liberty the hydrogen gas of the neighbouring wood-work, which is ignited by the flames or red-hot bricks of the house on fire.
Q. What is coke?
A. Coal freed from its volatile gases, by the action of artificial heat.
Q. Why do arnott’s stoves sometimes smell so strong of sulphur?
A. The fire is made of coke, which contains sulphur; and, whenever the draught is not rapid enough to drive the sulphur up the flue, it is emitted into the room.
Q. What is meant by spontaneous combustion?
A. Ignition produced by the action of one uninflamed body on another.
Q. Give an example of spontaneous combustion.
A. Goods packed in a warehouse will often catch fire of themselves; especially such goods as cotton, flax, hemp, rags, &c.
Q. Why do such goods sometimes catch fire of themselves?
A. Because they are piled together in very great masses in a damp state or place.
Q. Why does this produce spontaneous combustion?
A. The damp produces decay or the decomposition of the goods, and the great heat of the piled-up mass makes the decaying goods ferment.
Q. How does this fermentation produce combustion?
A. During fermentation, carbonic acid gas is given off by the goods,—a slow combustion ensues,—till at length the whole pile bursts into flame.
Q. Why is the heat of a large mass of goods greater than that of a smaller quantity?
A. Because compression squeezes out heat, as water is squeezed from a sponge; and as the goods of a large pile are greatly compressed, much of their latent heat is squeezed out.
Q. Why do hay-stacks sometimes catch fire of themselves?
A. Either because the hay was got up damp, or because rain has penetrated the stack.
Q. Why will a hay-stack catch fire if the hay be damp?
A. Damp hay soon decays, and undergoes a state of fermentation; during which, carbonic acid gas is given off, and the stack catches fire.
Q. Why does roasted coffee sometimes catch fire spontaneously?
A. The heat of coffee is greatly increased by being roasted; and the carbon of the coffee uniting with the oxygen of the air, produces carbonic acid gas, and bursts into flame.
Q. Why do old rags, used for cleaning lamps and candles, sometimes set a house on FIRE?
A. Because they very readily ferment, and (during fermentation) throw off exceedingly inflammable gases.
(N.B. Lamp-black mixed with linseed oil is more liable to spontaneous combustion, than anything that servants handle.)
CHAPTER IV.
SMOKE.
Q. Why does smoke ascend the chimney?
A. As the air of the room passes over the fire, it becomes heated; and (being thus made lighter,) ascends the chimney, carrying the smoke with it.
Q. What is smoke?
A. Small particles of carbon, separated by combustion from the fuel, but not consumed.
Q. Why do smoke and steam curl, as they ascend?
A. Because they are moved in a right line, and then pushed on all sides; and this forces them into a circular motion.
Q. What are blacks?
A. When the hot air of the chimney has been cooled by the external air, it can no longer buoy up the solid smoke; so it falls to the earth in condensed flakes, called “blacks.”
Q. Why are there no blacks in the smoke of a railway engine?
A. The smoke of a railway engine consists chiefly of watery vapour, which dissolves in air, as sugar does in water; but the smoke of a common chimney consists of small fragments of unburnt fuel.
Q. Why does a “COPPER HOLE” DRAW up more fiercely than an open stove?
A. As the air, which supplies the copper hole, must pass through the furnace, it becomes exceedingly heated, and rushes up the chimney with great violence.
Q. What produces the roaring noise made by a copper-hole fire?
A. Air rushing rapidly through the crevices of the iron door, and up the chimney flue.
Q. Why is the roar less, if the copper-hole door be thrown open?
A. Because fresh air gets access to the fire more easily; and as the air is not so intensely heated, its motion is not so violent.
Q. Why do some chimneys smoke?
A. If fresh air is not admitted into a room, as fast as it is consumed by the fire, a current of air will rush down the chimney to supply the deficiency, and bring the smoke along with it.
Q. What prevents air being supplied, as fast as it is consumed by the fire?
A. Leather and curtains round the doors; sand-bags at the threshhold and on the window-frames; and other contrivances to keep out the draught.
Q. Why is it needful for cold fresh air to be so constantly supplied?
A. If water be taken with a pail out of a river, other water will rush towards the hole, as soon as the pail is lifted out; and if air be taken from a room, (as it is, when some of it goes up the chimney) other air will rush towards the void to fill it up.
Q. Why will it come down the chimney?
A. Because if doors and windows are all made air-tight, it can get to the room in no other way.
Q. What is the best remedy in such a case?
A. The speediest remedy is to open the door or window: but by far the best remedy is to carry a small tube from the hearth into the external air.
Q. Why is that the best remedy?
A. Because the fire will be plentifully supplied with air by the tube; the doors and windows may all remain air-tight; and we may enjoy a warm fireside, without the inconvenience of draughts and cold feet.
Q. Why is a chimney raised so high above the roof?
A. If it were not so, it would smoke; as all funnels do which are too short.
Q. What is meant by the funnel, or flue of a chimney?
A. That part of a chimney through which the smoke passes, is called the funnel, or flue.
Q. Why does a chimney smoke, if the funnel be very short?
A. Because the draught of a short flue is too slack to carry the smoke up the chimney.
Q. Why is the draught of a short flue more slack that that of a long one?
A. For many reasons. 1st—The fire is always dull and sluggish if the chimney be too short.
2ndly—The smoke rolls out of the chimney, before it has acquired its full velocity.
3rdly—The wind, rain, and air, have more influence over a short funnel, than over a long one.
Q. Why is the fire always dull and sluggish if the chimney-flue be very short?
A. Because the draught is so bad: and as the rarefied air passes up the chimney very tardily, fresh air flows as tardily towards the fire, to supply it with oxygen.
Q. On what does the intensity of fire depend?
A. The intensity of fire is always in proportion to the quantity of oxygen with which it is supplied.
Q. Why does not smoke acquire its full velocity in a short funnel?
A. Because the higher smoke ascends in a flue, (provided it be clear and hot) the faster it goes; (as a stone falls faster and faster the lower it descends): if, therefore, a funnel be very short, the smoke never acquires its full velocity.
Q. Does the draught of a chimney depend on the speed of the smoke through the flue?
A. Yes. The more quickly hot air flies up the chimney, the more quickly cold air will rush towards the fire to supply the place; and, therefore, the longer the flue, the greater the draught.
Q. Why is the draught of a long flue greater than that of a short one?
A. Because the higher smoke ascends, the faster it goes; (as a stone falls faster and faster, the nearer it approaches to the earth): if, therefore, a funnel be long, the smoke acquires great velocity, and the draught is great.
Q. If a chimney be too short, and cannot be lengthened, what is the best remedy to prevent smoking?
A. To contract the opening of the chimney contiguous to the stove.
Q. Why will a smaller opening against the stove prevent the smoking?
A. As all the air (which enters the chimney) must pass near the fire, it will become greatly heated, and rise rapidly through the funnel; and this increase of heat will compensate for the shortness of the flue.
Q. Why will a room smoke, if there be two fires in it?
A. Because the fiercer fire will exhaust the most air; and draw from the smaller one, to supply its demand.
Q. Why will a chimney smoke if there be a fire in two rooms communicating with each other?
A. Whenever the door between the two rooms is opened, air will rush from the chimney of the inferior fire, to supply the other; and both rooms will be filled with smoke.
Q. What is the remedy in this case?
A. Let a tube be carried from the hearth of each stove, into the external air; and then each fire will be so well supplied, that neither will need to borrow from the other.
Q. Why do vestry chimneys so often smoke?
A. Because the wind (striking against the steeple) is reflected back; and tumbles down the vestry chimney, forcing the smoke into the room.
Q. what winds make vestry chimneys smoke?
A. Those from the north-east or south-east; according to the position of the vestry.
Q. Why will the eastern winds make vestries smoke, more than those from the west?
A. Because they strike against the steeple, and bound back to the vestry chimney: but western winds cannot rebound over the roof of a church.
(N. B. The steeple of a church is always due west, and the other end of the church due east; if, therefore, a western wind rebound, it would rebound to the west, or away from the church, and not towards it.)
Q. Why does a house in a valley very often smoke?
A. Because the wind (striking against the surrounding hills) rebounds back again upon the chimney, and destroys its draught.
Q. What is the common remedy in both these cases?
A. To fix a cowl on the chimney top, to turn like a weather-cock, and present its back to the wind.
Q. Why will not a cowl always prevent a chimney smoking?
A. If the wind be strong, it will keep the opening of the cowl towards the steeple or hill; and then the reflected wind will blow into the cowl, and down the chimney.
Q. As a cowl is such a poor remedy, can any other be devised?
A. If the chimney flue can be carried higher than the steeple or hills, no wind can enter the flue.
Q. Why cannot the wind enter a chimney flue, if it be carried up higher than the steeple or hills?
A. Because the reflected wind would strike against the sides of the chimney-flue, and not pass over the opening at all.
Q. In what other cases will a chimney smoke?
A. If both door and chimney be placed on the same side of a room, the chimney will often smoke.
Q. Why will a chimney smoke, if the door and stove are both on the same side?
A. Because when the door is opened, a current of air will blow into the chimney-place, and drive the smoke into the room.
Q. What remedy can be applied to this evil?
A. The door must be set opposite to the chimney, or nearly so; and then the draught from the door will blow the smoke up the chimney, and not into the room.
Q. Why will a chimney smoke if it needs sweeping?
A. Because the obstruction in the chimney (presented by the loose soot, to the free passage of the smoke) delays its current, and prevents the draught.
Q. Why will a chimney smoke, if out of repair?
A. 1st—Because the loose mortar and bricks obstruct the smoke: and
2ndly—The cold air (oozing through the chinks) chills the air in the chimney, and prevents its ascent.
Q. Why will an arnott’s stove smoke, if the joints of the flue do not fit air-tight?
A. Because the cold air (which gets through the joints) chills the air in the flue, and prevents its ascent.
Q. Why does an old fashioned farm chimney-place so often smoke?
A. Because the opening is so very large, that much of the air which goes up the chimney, has never passed near the fire; and this cold air mixing with the other, so reduces its temperature, that it ascends very slowly, and the draught is destroyed.
Q. Why does a chimney smoke, if the draught be slack?
A. Because, unless the current of air up the chimney be very powerful, it cannot buoy the smoke up through the flue.
Q. If the opening of a chimney be too large, what remedy can be applied?
A. The chimney-place must be contracted.
Q. Why will contracting the chimney-place prevent its smoking?
A. As the air will then pass nearer the fire, it will be more heated, and fly up the chimney much faster.
Q. Why do almost all chimneys smoke in gusty weather?
A. The gust (blowing the air away from the top of the chimney) removes (for a time) all resistance to the smoke: but when the wind lulls again, the resistance of the air suddenly returns—the draught is checked—and a puff of smoke rushes into the room.
Q. What is the use of a chimney-pot?
A. When the opening of a chimney is large, the top must be contracted by a chimney-pot, in order to increase the draught.
Q. How does a chimney-pot increase the draught of a chimney?
A. As the same quantity of hot air has to escape through a much smaller opening, it must pass through more quickly.
Q. Why do tin blowers help to get a fire up?
A. Because they compel the air to go through the fire, and not over it; therefore the fire is well supplied with oxygen, and the draught greatly increased.
Q. Why does a tin blower increase the draught?
A. As all the air which enters the chimney has to pass through the fire, it is much hotter, and ascends the chimney very fast; and the faster the air flies up the chimney, the faster it rushes towards the fire also.
Q. Why does a parlour often smell disagreeably of soot in summer-time?
A. The air in the chimney (being colder than the air in the parlour) descends into the room, and leaves a disagreeable smell of soot behind.
Q. Why are the ceilings of public offices so black and filthy?
A. The heated air ascending, carries the dust and fine soot to the ceiling; where the hot air escapes through the plaster, and leaves the soot and dust behind.
Q. Why are some parts of the ceiling blacker and more filthy than others?
A. As the air cannot penetrate the thick joists of the ceiling, it passes by those parts, and deposits its soot and dust on those which are more penetrable.
Q. What is charcoal?
A. Wood which has been exposed to a red heat, till it has been deprived of all its gases and volatile parts.
Q. Why is a charcoal fire hotter than a wood fire?
A. Because so large a quantity of water has been abstracted from the fuel, by the red heat to which it has been already exposed.
Q. Why does charcoal remove the taint of meat?
A. Because it absorbs all odoriferous effluvia, whether they arise from putrefying animal or vegetable matter.
Q. Why is water purified by being filtered through charcoal?
A. Charcoal absorbs the impurities of the water, and removes all disagreeable tastes and smells, whether they arise from animal or vegetable matter.
Q. Why are water and wine casks charred inside?
A. Charring the inside of the cask reduces it to a kind of charcoal; and charcoal (by absorbing animal and vegetable impurities) keeps the liquor sweet and good.
Q. Why does a piece of burnt bread, steeped in impure water, make it fit to drink?
A. The surface of the bread is reduced to charcoal by being burnt; and the charcoal surface of the bread abstracts all the impurities of the water, and makes it palatable.
Q. Why should the toast and water, placed by the side of the sick, be made of burnt bread?
A. The surface of the bread being reduced to charcoal by being burnt, prevents the water from being affected by the impurities of the sick room.
Q. Why are timbers, which are to be exposed to damp, charred?
A. Charcoal undergoes no change by exposure to air and water; therefore timber will resist weather much longer, after it has been charred.
CHAPTER V.
LAMPS AND CANDLES.
Q. Of what are oil, tallow, and wax composed?
A. Principally of carbon and hydrogen gas. The solid part is carbon, the volatile part is gas.
Q. What is carbon?
A. A solid substance, generally of a black colour; well known under the forms of charcoal, lamp-black, coke, black-lead, &c.
Q. What is hydrogen gas?
A. The principal ingredient of water. It is well known in the form of common coal gas: it burns so readily that it used to be called “inflammable air.”[10]
[10] To make hydrogen gas, see p. [34>].
Q. Why does a candle burn when lighted?
A. The heat of the lighted wick decomposes the tallow into its elementary parts of carbon and hydrogen; and the hydrogen of the tallow, combining with the oxygen of the air, produces flame.
Q. Why is the flame of a candle hot?
A. 1st—Because the flame liberates latent heat from the air and tallow: and
2ndly—It throws into rapid motion the atoms of matter.
Q. How is latent heat liberated by the flame of a candle?
A. When the hydrogen of the tallow and oxygen of the air combine, they condense into water; and much of their latent heat is squeezed out.
Q. How are the atoms of matter disturbed by the flame of a candle?
A. 1st—When the hydrogen of the tallow and oxygen of the air condense into water, a vacuum is made; and the air is disturbed, as a pond would be, if a pail of water were taken out.
2ndly—When the carbon of tallow and oxygen of the air expand into carbonic acid gas, the air is again disturbed; in a similar way as by the explosion of gunpowder.
Q. Why does the flame of a candle produce light?
A. The chemical changes made by combustion, excite undulations of ether, which (striking the eye) produce light. (see p. [46].)
Q. Why is the flame of a candle yellow?
A. Only the outer coat of the flame is yellow; the lower part of the flame is violet; and the inside of the flame is hollow.
Q. Why is the outside of the flame yellow?
A. Because the carbon of the tallow (being in a state of perfect combustion) is made white-hot.
Q. Why is the bottom part purple of the flame of a candle?
A. The bottom part of the flame is overladen with hydrogen, raised from the tallow by the burning wick; and this half-burnt gas gives a purple tinge to the flame.
Q. Why is the inside of the flame of a candle hollow?
A. Because it is filled with vapour, raised from the candle by the heat of the wick.
Q. Describe the different parts of the flame of a common candle.
A. The flame consists of three cones. The innermost cone is hollow; the intermediate cone of a dingy purple hue; and the outside cone is yellow.
Q. Why is the intermediate cone of a flame purple, as well as the bottom of the flame.
A. Because the gases are not in a state of perfect combustion; but contain an excess of hydrogen, which gives this cone a purple tinge.
Q. Why is not the middle cone in a state of perfect combustion, as well as the outer cone?
A. Because the outer cone prevents the oxygen of the air from getting freely to the middle of the cone; and without the free access of oxygen gas, there is no such thing as complete combustion.
Q. Why does the flame of a candle point upwards?
A. The flame heats the surrounding air, which (being hot) rapidly ascends, and drives the flame upwards at the same time.
Q. Why is the flame of a candle pointed at the top, like a cone?
A. The upper part of a flame is more volatile than the lower parts; and as it affords less resistance to the air, is reduced to a mere point.
Q. Why is the upper part of a flame more volatile than the lower parts?
A. The lower parts of the flame are laden with unconsumed gas and watery vapour; which present considerable resistance to the air.
Q. Why is the flame of a candle blown out by a puff of breath?
A. As the flame of a candle is attached to a very small wick, a puff of breath severs the flame from the wick; and it goes out for want of support.
Q. Why does the flame of a candle make a glass damp, which is held over it?
A. The hydrogen of the tallow combining with the oxygen of the air, produce a “watery vapour,” which is condensed by the cold glass held above the flame.
Q. Why does our hand, held above a candle, suffer from the heat of the flame so much more, than when it is placed below the flame, or on one side of it?
A. Because the hot gases and air (in their ascent) come in contact with the hand placed above the flame: but when the hand is placed below the flame, or on one side, it only feels heat from radiation.
Q. Why is a rush light extinguished so much more quickly than a cotton-wicked candle?
A. As the rush wick is smooth and hard, the mere motion of the air (produced by carrying the candle from one place to another,) is sufficient to sever the flame from the rush.
Q. Why is it more difficult to blow out a cotton wick?
A. The cotton wick is quite full of small threads or filaments, which help to hold the flame on the wick, like the roots of a tree.
Q. Why does an extinguisher put a candle out?
A. Because the air in the extinguisher is soon exhausted of its oxygen by the flame: and when there is no oxygen to support it, the flame goes out.
Q. Why does not a candle set fire to a piece of paper twisted into an extinguisher, and used as such?
A. 1st—Because the flame very soon exhausts the little oxygen contained in the paper extinguisher: and
2ndly—The flame invests the inside of the paper extinguisher with carbonic acid gas, which prevents it from blazing.
Q. Why is a long wick never upright?
A. Because it is bent by its own weight.
Q. Why is a long wick covered with an efflorescence at the top?
A. The knotty or flowery appearance of the top of a wick arises from an accumulation of particles partly separated, but still loosely hanging to the wick.
Q. Why is not the end of a long wick burnt off, as it hangs over the flames?
A. Because the length of the wick so diminishes the heat of the flame, that it is not hot enough to burn it off.
Q. Why do palmer’s metallic wicks never need snuffing?
A. The wick is divided into two parts, each of which bends outward to the outside of the flame; where the end is intensely heated, and separated from the wick by the current of air up the candle.
Q. Why do common candles require to be snuffed?
A. Because the heat of the flame is not sufficient to consume the wick; and the longer the wick grows, the less heat the flame produces.
Q. Why do wax candles never need snuffing?
A. The wick of wax candles is made of very fine thread, which the heat of the flame is sufficient to consume: but the wick of tallow candles is made of coarse cotton, which is too substantial to be consumed by the heat of the flame, and must be cut off by snuffers.
Q. Why does a pin, stuck in a rush-light, extinguish it?
A. Because a pin (being a good conductor), carries away the heat of the flame from the wick, and prevents the combustion of the tallow.
Q. What is the smoke of a candle?
A. Solid particles of carbon separated from the wick and tallow, but not consumed.
Q. Why are some particles consumed and not others?
A. The combustion of the carbon depends upon its combining with the oxygen of the air: but as the outer surface of the flame prevents the access of air to the interior parts, therefore much of the carbon of those parts passes off in smoke.
Q. Why do lamps smoke?
A. Either because the wick is cut unevenly, or else because it is turned up too high.
Q. Why does a lamp smoke when the wick is cut unevenly?
A. 1st—Because the points of the jagged edge (being very easily separated from the wick,) load the flame with more carbon than it can consume: and
2ndly—As the heat of the flame is greatly diminished by these bits of wick, it is unable to consume even the usual quantity of smoke.
Q. Why does a lamp smoke when the wick is turned up too high?
A. Because more carbon is separated from the wick than can be consumed by the flame.
Q. Why do not “Argand burners” smoke?
A. Because a current of air passes through the middle of the flame; and therefore the carbon of the interior is consumed, as well as that in the outer coating of the flame.
Q. Why does a lamp-glass diminish the smoke of a lamp?
A. Because it both concentrates and reflects the heat of the flame; in consequence of which, the heat is so greatly increased, that very little carbon escapes unconsumed.
CHAPTER VI.
ANIMAL HEAT.
Q. What is the cause of animal heat?
A. Animal heat is produced by the combustion of hydrogen and carbon in the capillary veins.
Q. What are capillary veins?
A. Veins as small as hairs running all over the body; so called from the Latin word “capilla’ris” (like a hair).
Q. Do these capillary veins run all over the human body?
A. Yes. Whenever blood flows from a wound, some vein must be divided; and as you cannot insert a needle into any part of the body without bringing blood, therefore these little veins must run through every part of the human frame.
Q. How do hydrogen gas and carbon get into these very little veins?
A. The food we eat is converted into blood, and blood contains both hydrogen and carbon.
Q. How does combustion take place in the veins?
A. The carbon of the blood combines with the oxygen of the air we breathe, and forms into carbonic acid gas.
Q. What becomes of this carbonic acid gas formed in the human blood?
A. Some of it is thrown off by the breath; and the rest of it is absorbed by the blood, to keep up the animal heat.
Q. What is the cause of the combustion of fire?
A. The carbon of fuel unites with the oxygen of the air, and forms carbonic acid gas.
Q. What is the cause of the combustion of a candle or lamp?
A. The carbon of the oil or tallow unites with the oxygen of the air, and forms carbonic acid gas.
Q. What is the cause of spontaneous combustion?
A. The piled-up goods ferment from heat and damp; and (during fermentation) carbonic acid gas is formed, as in the two former cases.
Q. Does the heat of the human body arise from the same cause as the heat of fire?
A. Yes, precisely. The carbon of the blood, combining with the oxygen of air inhaled, produces carbonic acid gas, which is attended with combustion.
Q. If animal heat is produced by combustion, why does not the human body burn up like a coal or candle?
A. It actually does so. Every muscle, nerve, and organ of the body, actually wastes away like a burning candle; and (being reduced to air and ashes) is rejected from the system as useless.
Q. If every bone, muscle, nerve, and organ, is thus consumed by combustion, why is not the body entirely consumed?
A. It would be so, unless the parts destroyed were perpetually renewed: but as a lamp will not go out, so long as it is supplied with fresh oil; neither will the body be consumed, so long as it is supplied with sufficient food.
Q. When a man is starved, what parts of the body go first?
A. First the fat, because it is the most combustible; then the muscles; last of all the brain; and then the man dies, like a candle which is burnt out.
Q. Why does want of sufficient nourishment often produce madness?
A. After the fat and muscles of the body have been consumed by animal combustion, the brain is next attacked; and (unless the patient dies) madness must ensue from starvation.
Q. Why does a man shrink when starved?
A. A starved man shrinks just as a fire does, unless it be supplied with sufficient fuel.
Q. What is the fuel of the body?
A. Food is the fuel of the body; and the carbon of the food mixing with the oxygen of the air, evolves heat in the same way that a fire or candle does.
Q. Why is every part of the body warm?
A. As the capillary veins run through every part of the human body, and the combustion of blood takes place in the capillary veins, therefore every part of the body is warm.
Q. Why does running make us warm?
A. When we run, we inhale air more rapidly; and the rapidity with which we inhale air fans the combustion of our body, as a pair of bellows quickens the flame of a common fire.
Q. How does inhaling air rapidly make the body feel warm?
A. As the combustion of the blood is more rapid, (in consequence of the introduction of more oxygen from theair), therefore the blood is more heated, and every part of the body is warmer also.
Q. Why does hard work produce hunger?
A. Because it produces quicker respiration; by which means a larger amount of oxygen is introduced into the lungs, and the capillary combustion increased. Hunger is the notice (given by our body) to remind us, that our food-fuel must be replenished.
Q. Why does singing make us hungry?
A. Singing increases respiration; and as more oxygen is introduced into the lungs, our food-fuel is more rapidly consumed.
Q. Why does reading aloud make us feel hungry?
A. Reading aloud increases respiration; and as more oxygen is introduced into the lungs, our food-fuel is more rapidly consumed.
Q. Why do we feel more hungry in the day-time than in the night-time?
A. As we breathe more slowly during sleep, therefore, less oxygen is introduced into the lungs to consume our food-fuel.
Q. Why do we need warmer clothing by night than by day?
A. 1st—Because the night is generally colder than the day.
2ndly—As our respiration is slower, our animal combustion is slower also; in consequence of which, our bodies are more cold.
Q. Why do we perspire when very hot?
A. The pores of the body are like the safety valves of a steam-engine; when the heat of the body is too great, the combustible gas and grease flow out in perspiration, instead of burning in the blood.
Q. Why do persons feel lazy and averse to exercise, when they are half-starved or ill-fed?
A. Animal food contains great nourishment, and produces a desire for active occupations; but when the body is not supplied with strong food, this desire for muscular action ceases, and the person grows slothful.
Q. Why have persons, who follow hard out-of-doors occupations, more appetite than those who are engaged in sedentary pursuits?
A. Hard bodily labour in the open air causes much oxygen to be conveyed into the lungs by inspiration; the combustion of the food is carried on quickly; animal heat increased; and need for nutritious food more quickly indicated by craving hunger.
Q. Why have persons who follow sedentary pursuits less appetite than ploughmen and masons?
A. 1st—The air they inhale is not so pure, because its oxygen is partly exhausted: and
2ndly—Their respiration is neither so quick nor strong, and therefore the combustion of their food is carried on more slowly.
Q. Why do we like strong meat and greasy food when the weather is very cold?
A. Strong meat and grease contain large portions of hydrogen, which (when burned in the blood) produce a larger amount of heat than any other kind of food.
Q. Why do persons eat more food in cold weather, than in hot?
A. In cold weather the body requires more fuel to keep up the same amount of animal heat; and as we put more coals on a fire on a cold day to keep our room warm, so we eat more food on a cold day to keep our body warm.
Q. Why does cold produce hunger?
A. 1st—The air contains more oxygen in cold weather; and as fires burn fiercer, so animal combustion is more rapid: and
2ndly—We are more active in cold weather; and increased respiration acts like a pair of bellows on the capillary combustion.
Q. Why does rapid digestion produce a craving appetite?
A. This is a wise providence to keep our bodies in health; in order that the body itself may not be consumed, it gives notice (by hunger) that the capillary fires need replenishing.
Q. Why do we feel a desire for activity in cold weather?
A. 1st—Because activity increases the warmth of the body, by fanning the combustion of the blood: and
2ndly—The strong food we eat creates a desire for muscular exertion.
Q. Why are the Esquimeaux so passionately fond of train oil and whale blubber?
A. Oil and blubber contain a very large amount of hydrogen, which is exceedingly combustible; and as these people live in climates of intense cold, the heat of their bodies is increased by the greasy nature of their food.
Q. Why do we feel a dislike to strong meat and greasy foods in very hot weather?
A. Strong meat and grease contain so much hydrogen, that they would make us intensely hot; and therefore we refuse them in hot weather.
Q. Why do we like fruits and vegetables so very much in hot weather?
A. Fruits and vegetables contain less carbon than meat, and therefore produce less blood: instead of blood, they combine into water as they are digested, and keep the body cool.
Q. Why do people say that fruits and vegetables cool the blood?
A. 1st—Because they deprive the blood of carbon, which is the chief cause of animal heat: and
2ndly—These gases coalesce into water, which greatly tempers the animal heat.
Q. Why do we feel lazy and averse to activity in very hot weather?
A. 1st—Because muscular activity would increase the heat of the body, by quickening the respiration: and
2ndly—The food we eat in hot weather, not being greasy, naturally abates our desire for bodily activity.
Q. Why do the inhabitants of tropical countries live chiefly upon rice and fruit?
A. Rice and fruit by digestion are mainly converted into water, and (by cooling the blood) prevent the tropical heat from feeling so oppressive.
Q. Why are poor people generally averse to cleanliness?
A. 1st—Cleanliness increases hunger; and as poor people are generally ill-fed, they are averse to cleanliness.
2ndly—Dirt is warm, (thus pigs who love warmth, are fond of dirt); and as poor people are generally ill-clad, they like the warmth of dirt.
Q. Why are poor people generally averse to ventilation?
A. 1st—Because ventilation increases the oxygen of the air,—the combustion of food,—and the cravings of appetite: and
2ndly—Ventilation cools the air of our rooms: poor people, therefore, (who are generally ill-clad) love the warmth of an ill-ventilated apartment.
Q. Why does flannel, &c. make us warm?
A. Flannel and warm clothing do not make us warm, but merely prevent the body from becoming cold.
Q. How does flannel, &c. prevent the body from becoming cold?
A. Flannel (being a bad conductor) will neither carry off the heat of the body into the cold air, nor suffer the cold of the air to come into contact with our warm bodies; and thus it is that flannel clothing keeps us warm.
Q. Why are frogs and fishes cold-blooded animals?
A. Because they consume so little air; and without a plentiful supply of air, combustion is so slow, that very little animal heat is evolved.
Q. Why is a dead body cold?
A. Air is no longer conveyed to the lungs after respiration has ceased; and, therefore, animal heat is no longer evolved by combustion.
CHAPTER VII.
MECHANICAL ACTION.
1.—PERCUSSION.
Q. How is heat produced by mechanical action?
A. 1.—By Percussion. 2.—By Friction. 3.—By Condensation.
Q. What is meant by percussion?
A. The act of striking; as when a blacksmith strikes a piece of iron on his anvil with his hammer.
Q. Why does beating iron make it red-hot?
A. Beating the iron condenses the particles of the metal; and squeezes out its latent heat, as water from a sponge.
Q. Does cold iron contain heat?
A. Yes; every thing contains heat; but when a thing feels cold, its heat is latent.
Q. What is meant by latent heat?
A. Heat not perceptible to our feeling. When anything contains heat without feeling the hotter for it, that heat is called “latent.” (See p. [31].)
Q. Does cold iron contain latent heat?
A. Yes; and when a blacksmith compresses the particles of the iron by his hammer, he squeezes out this latent heat, and makes the iron red-hot.
Q. How did blacksmiths use to light their matches before the general use of lucifers?
A. They used to place a soft iron nail upon their anvil; strike it two or three times with a hammer; and the point became sufficiently hot to light a brimstone match.
Q. How can a nail (beaten by a hammer) ignite a brimstone match?
A. As the particles of the nail are compressed by the hammer, it cannot contain so much heat as it did before; so some of it flies out (as water flows from a sponge when it is squeezed).
Q. Why does striking a flint against a piece of steel produce a spark?
A. The blow condenses those parts of the flint and steel which strike together, and squeezes out their latent heat.
Q. How does this development of heat produce a spark?
A. A very small fragment (either of the steel or flint) is knocked off red-hot, and sets fire to the tinder on which it falls.
Q. Why is it needful to keep blowing the tinder with the breath?
A. Because blowing the tinder, drives the oxygen of the air towards it.
Q. Where does the oxygen of the air come from, which is blown to the lighted tinder?
A. The air itself is composed of two gases (nitrogen and oxygen) mixed together.
(Every 5 lbs. of common air contain 4 lbs. of nitrogen, and 1 lb. of oxygen.)
Q. What is the good of blowing oxygen gas to lighted tinder?
A. Oxygen gas supports combustion; and lighted tinder is quickened by the breath, in the same way as a dull fire is revived by a pair of bellows.
Q. Why do horses sometimes strike fire with their feet?
A. When iron horse-shoes strike against the flint-stones of the road, very small fragments (either of the shoe or stones) are knocked off red-hot, and look like sparks.
Q. What makes these fragments red-hot?
A. The percussion condenses the part struck, and squeezes out its latent heat.
CHAPTER VIII.
2.—FRICTION.
3.—CONDENSATION.
Q. What is meant by friction?
A. The act of rubbing two things together; as the Indians rub two pieces of wood together to produce fire.
Q. How do the Indians produce fire, by merely rubbing two pieces of dry wood together?
A. They take a piece of dry wood (sharpened to a point), which they rub quickly up and down a flat piece, till a groove is made; and the saw-dust (collected in this groove) soon catches fire.
Q. Why does the saw-dust of the wood catch fire by rubbing?
A. The latent heat of the wood is developed by friction; because the particles of the wood are squeezed closer together, and the heat pours out, as water from a sponge.
(The best woods for this purpose are box-wood against mulberry, or laurel against poplar or ivy.)
Q. Do not carriage wheels sometimes catch fire?
A. Yes; if the wheels be dry,—or fit too tightly,—or revolve very rapidly,—they often catch fire.
Q. Why do wheels catch fire in such cases?
A. The friction of the wheels against the axle-tree is so great, that their latent heat is disturbed, and produces ignition.
Q. What is the use of greasing cart wheels?
A. The grease lessens the friction; and (by diminishing the friction) the latent heat is less disturbed.
Q. Why is the top of a mountain colder than the valley beneath, although it be two or three miles nearer to the sun?
A. 1st—Because the air on a mountain is less compressed, than the air in a valley.
2ndly—It is more rarefied: and
3rdly—It is less heated by reflection.
Q. Why is air colder on a mountain “because it is less compressed?”
A. As the air in a valley is more compressed (by the mass of air above) than that on the top of a mountain, therefore more heat runs out; just as more water runs from a sponge, the closer it is squeezed together.
Q. Why is a mountain-top colder than a valley, “because the air there is more rarefied?”
A. As the air is more rarefied, its heat is diffused over a larger space and is less intense; just as a candle would show less light in a large room, than in a small one.
Q. Why is a mountain-top colder than a valley, “because the air there is less heated by reflection?”
A. Air is not heated by the sun, but by reflection from the surface of the earth; and as there is no earth round a mountain-top to reflect heat, therefore the air there is intensely cold.
Q. Why does rubbing our hands and faces make them feel warm?
A. Chiefly because the friction excites the latent heat of our hands and faces, and makes it sensible to our feeling.
Q. When a man has been almost drowned, why is suspended animation restored by rubbing?
A. The vital heat of the body (which had become latent by the action of the water) is again developed by friction: and, as soon as this animal heat can be excited, the vital powers of the body are restored.
Q. Why do two pieces of ice (rubbed together) melt?
A. Ice contains 140 degrees of latent heat, and (when two pieces are rubbed together) their particles are compressed, and this latent heat rolls out and melts the ice.
Q. Are not forests sometimes set on fire by friction?
A. Yes; when two branches or trunks of trees (blown about by the wind) rub violently against each other, their latent heat is developed, and sets fire to the forest.
Q. What is meant by compression?
A. The act of bringing parts nearer together; as a sponge is compressed by being squeezed in the hand.
Q. Cannot heat be evolved from common air merely by compression?
A. Yes; if a piece of German tinder be placed at the bottom of a glass tube, and the air in the tube compressed by a piston,[11] the tinder will catch fire.
[11] In a common syringe or squirt, the handle part which contains the sucker (and is forced up and down), is called “The Piston.”
Q. Why will the tinder catch fire?
A. Because the air is compressed; and its latent heat being squeezed out, sets fire to the tinder at the bottom of the tube.
CHAPTER IX.
EFFECTS OF HEAT.
1.—EXPANSION.
Q. What are the principal effects of heat?
A. 1.—Expansion. 2.—Liquefaction. 3.—Vaporization. 4.—Evaporation; and 5.—Ignition.
Q. Does heat expand the air?
A. Yes; if a bladder (partially filled with air) be tied up at the neck, and laid before the fire, the air will swell till the bladder bursts.
Q. Why will the air swell, if the bladder be laid before the fire?
A. Because the heat of the fire gets between the particles of air, and drives them further apart from each other; which causes the bladder to expand.
Q. Why do unslit chestnuts crack with a loud noise, when roasted?
A. Chestnuts contain a great deal of air, which is expanded by the heat of the fire; and, as the thick rind prevents the air from escaping, it violently bursts through, slitting the rind, and making a great noise.
Q. What occasions the loud crack or report which we hear?
A. 1st—The sudden bursting of the rind makes a report, in the same way as a piece of wood or glass would do, if snapped in two: and
2ndly—The escape of hot air from the chestnut makes a report also, in the same way as gunpowder, when it escapes from a gun.
Q. Why does the sudden bursting of the rind, or snapping of a piece of wood, make a report?
A. As the attraction of the parts is suddenly overcome, a violent jerk is given to the air; this jerk produces rapid undulations in the air, which (striking upon the ear) give the brain the sensation of sound.
Q. Why does the escape of air from the chestnut, or the explosion of gunpowder, produce a report?
A. Because a quantity of air (suddenly let loose) pushes against the air around, in order to make room for itself; and as the air of the chestnut slaps against the air of the room, a report is made, (as when I slap a book or table).
Q. If a chestnut be slit, it will not crack; why is this?
A. Because the heated air of the chestnut can freely escape through the slit in the rind.
Q. Why does an apple spit and spurt about, when roasted?
A. An apple contains a vast quantity of air, which (being expanded by the heat of the fire) bursts through the peel, carrying the juice of the apple along with it.
Q. Does an apple contain more air, in proportion, than a chestnut?
A. Yes, much more. There is as much condensed air in a common apple, as would fill a space 48 times as big as the apple itself.
Q. Where is all this quantity of air stowed in the apple?
A. The inside of an apple is made up of little cells (like a honey-comb), each of which contains a portion of the air.
Q. When an apple is roasted, why is one part made soft, while all the rest remains hard?
A. When an apple is roasted, the air in the cells next to the fire is expanded and flies out; the cells are broken, and their juices mixed together; so the apple collapses (from loss of air and juice), and feels soft in those parts.
Q. What is meant by the “apple collapsing?”
A. The plumpness gives way, and the apple becomes flabby and shrivelled.
Q. Why do sparks of fire start (with a crackling noise) from pieces of wood laid upon a fire?
A. The air in the wood (expanded by the heat), forces its way through the pores of the log; and carries along with it the covering of the pore, which resisted its passage.
Q. What is meant by the “pores of the wood?”
A. Very small holes in the wood, through which the sap circulates.
Q. What are the sparks of fire, which burst from the wood?
A. Very small pieces of wood red hot, separated from the log by the force of the air, as it bursts from its confinement.
Q. Why does deal make more snapping than any other wood?
A. The pores of deal are very large, and contain much more air than wood of a closer grain.
Q. Why does dry wood make more snapping than green wood?
A. In green wood the pores are filled with sap, and therefore contain very little air; but in dry wood the sap is dried up, and the pores are filled with air instead.
Q. Why does dry wood burn more easily than green or wet wood?
A. Because the pores of dry wood are filled with air, which supports combustion; but the pores of green or wet wood are filled with vapour, which extinguishes flame.
Q. Why does vapour extinguish flame?
1st—Because the coat of water (which wraps the fuel round) prevents the oxygen of the air from getting to the fuel, to form into carbonic acid gas: and
2ndly—Heat is perpetually carried off, by the formation of the sap or water into steam.
(Carbonic acid gas is a compound of carbon and oxygen. The solid part of the fuel is carbon, and one of the gases of the air is oxygen.)
Q. What has carbonic acid gas to do with combustion?
A. Combustion is produced by the chemical action which takes place, while the carbon of fuel unites with the oxygen of air, and forms “carbonic acid gas.” (See p. [36].)
Q. Why do stones snap and fly about, when heated in the fire?
A. The air in the stones (expanded by the heat of the fire), meets with great resistance from the close texture of the stone; and, therefore, bursts forth with great violence, tearing the stone to atoms, and forcing the fragments into the room.
Q. Must not air be very strong, to shatter into atoms a hard stone?
A. Yes. All the dreadful effects of gunpowder are merely the results of the sudden expansion of air.
Q. When bottled ale and porter is set before a fire, why is the cork forced out sometimes?
A. If the bottle be not quite full, there will be air between the liquor and the cork; this air (expanded by the heat of the fire) forces out the cork.
Q. Why does ale or porter froth more, after it has been set before the fire?
A. The froth of ale or porter depends upon the pressure to which it is subjected; and as the air (between the liquor and the cork) is expanded by the heat, it presses against the liquor, and increases the quantity of froth.
Q. Why is the froth of ale and porter increased by pressure?
A. Because the liquor absorbs carbonic acid so long as it is under pressure; and the moment that the pressure is removed, the carbonic acid escapes in foam or froth.
Q. When a boy makes a balloon, and sets fire to the cotton or sponge (which has been steeped in spirits of wine), why is the balloon inflated, or blown out?
A. The air inside the balloon is expanded by the flame, till the whole balloon is blown out without a crumple.
Q. Why does the balloon rise, after it has been inflated by the expanded air?
A. The same quantity of air is expanded to three or four times its original volume; and is made so much lighter than common air, that even when all the paper, wire, and cotton are added, it is still lighter bulk for bulk.
Q. What is meant by being lighter “bulk for bulk?”
A. If the balloon be 3 square feet in size, it is lighter (when inflated) than 3 square feet of common air, and therefore floats through it; as a cork (at the bottom of a tub of water) would rise to the surface.
Q. Why does smoke rush up a chimney?
A. The heat of the fire expands the air in the chimney; and (being thus made lighter than the air around), it rises up the chimney, and carries the smoke in its current.
Q. Why has a long chimney a greater draught than a short one?
A. Because air rises faster and faster the higher it ascends in a chimney flue; the same as a stone falls faster and faster the nearer it approaches to the ground.
Q. Why will a long chimney smoke, unless the fire be pretty fierce?
A. If the fire be not pretty fierce, its heat will not be sufficient to rarefy all the air in the chimney; and then the chimney will smoke.
Q. Why will the chimney smoke, if the fire be not big enough to heat all the air in the chimney flue?
A. Because the cold air (condensed in the upper part of the flue), will sink from its own weight, and sweep the ascending smoke back with it into the room.
Q. What is the use of a cowl upon a chimney-pot?
A. The cowl acts as a screen against the wind, to prevent it from blowing into the chimney.
Q. What harm would the wind do, if it were to blow into a chimney?
A. 1st—It would prevent the smoke from getting out: and
2ndly—The cold air (introduced into the chimney by the wind) would fall down the flue, and drive the smoke with it back into the room.
Q. Why does a smoke-jack turn round in a chimney?
A. The current of hot air up the chimney, striking against the oblique vanes of the smoke-jack, drives them round and round; in the same way as the sails of a wind-mill are driven round by the wind.
Q. Why are some things solid, others liquid, and others gaseous?
A. As heat enters any substance, it drives its particles further asunder; and a solid (like ice) becomes a liquid; and a liquid (like water) becomes a gas.
Q. Why does water simmer before it boils?
A. The particles of water near the bottom of the kettle (being formed into steam sooner than the rest) shoot upwards; but are condensed again (as they rise) by the colder water, and produce what is called “simmering.”
Q. What is meant by simmering?
A. A gentle tremor or undulation on the surface of the water. When water simmers, the bubbles collapse beneath the surface, and the steam is condensed to water again: but when water boils, the bubbles rise to the surface, and steam is thrown off.
Q. Why does a kettle sing when the water simmers?
A. Because the air (entangled in the water) escapes by fits and starts through the spout of the kettle; which makes a noise like a wind instrument, when it is blown into.
Q. Why does not a kettle sing, when the water boils?
A. As all the water is boiling hot, the steam meets with no impediment, but freely escapes in a continuous stream.
Q. When does a kettle sing most?
A. When it is set on a hob to boil.
Q. Why does a kettle sing more when it is set on the side of a fire, than when it is set in the midst of the fire?
A. When the kettle is set on the hob to boil, the heat is applied very partially: one side is hotter than the other, and therefore the steam is more entangled.
Q. Why does a kettle sing, when the boiling water begins to cool again?
A. Because the upper surface cools first; and the steam (still rising from the lower parts of the kettle) is again entangled, and escapes fitfully.
Q. Why does boiling water swell?
A. Water (like air) expands by heat. The heat of the fire drives the particles of water further apart from each other; and (as they are not packed so closely together) they take up more room; or (in other words) the water swells.
Q. What is meant when it is said, “that heat drives the particles of water further apart from each other.”
A. Water is composed of little globules, like very small grains of sand; the heat drives these particles away from each other; and (as they then require more room) the water swells.
Q. Why does boiling water bubble?
A. Water contains air; and (as the water is heated) the air is driven out, and raises a bubble in that part of the water which resists its escape.
Q. Why does a kettle sometimes boil over?
A. Liquids expand very much by heat; if, therefore, a kettle be filled with cold water, some of it must run over as soon as it is expanded by heat.
Q. But I have seen a kettle boil over, although it has not been filled full of water; how do you account for that?
A. If a fire be very fierce, the air is expelled so rapidly, that the bubbles are very numerous; and (towering one above the other) reach the top of the kettle, and fall over.
Q. Why is a pot, which is full to overflowing (while the water is boiling hot), nothing like full, when it has been taken off the fire for a short time?
A. When the water was swelled by boiling heat, it filled the pot even to overflowing; but as soon as the water is condensed by cold, it contracts again, and occupies a much less space.
Q. Why does the water of a kettle run out of the spout when it boils?
A. Because the steam cannot escape so fast as it is formed, and (being confined in the kettle) presses on the water with great power, and forces it out of the spout.
Q. How can the pressure of steam on the surface of the water, force the water through the kettle-spout?
A. In the same manner as the pressure of air on the mercury of a barometer, forces the quicksilver up the glass tube.
Q. What causes the rattling noise so often made by the lid of a saucepan or boiler?
A. The steam (seeking to escape) forces up the lid of the boiler, and the weight of the lid causes it to fall back again: this being done frequently, produces a rattling noise.
Q. If the steam could not lift up the lid of the boiler, how would it escape?
A. If the lid fitted so tightly, that the steam could not raise it up, the boiler would burst into fragments, and the consequences might be fatal.
Q. When steam pours out from the spout of a kettle, the stream begins apparently half an inch off the spout; why does it not begin close to the spout?
A. Steam is really invisible; and the half-inch (between the spout and the “stream of mist”) is the real steam, before it has been condensed by air.
Q. Why is not all the stream invisible, as well as that half-inch?
A. As the steam comes in contact with the colder air, the invisible particles (being condensed), roll one into another, and look like a thick mist.
Q. What becomes of the steam? for it soon vanishes.
A. After it is condensed into mist, it is dissolved by the air, and dispersed abroad as invisible vapour.
Q. And what becomes of the invisible vapour?
A. Being lighter than air, it ascends to the upper regions, where (being again condensed) it contributes to form clouds.
Q. Why does a metal spoon, left in a saucepan, retard the process of boiling?
A. The metal spoon (being an excellent conductor) carries off the heat from the water; and (as heat is carried off by the spoon) the water takes a longer time to boil.
Q. Why will a pot (filled with water) never boil, when immersed in another vessel full of water also?
A. Because water can never be heated above the boiling point: all the heat absorbed by the water after it boils, is employed in converting the water into steam.
Q. How does the conversion of water into steam prevent the inner pot from boiling?
A. The moment the water in the larger pot is boiling hot (or 212°), steam is formed, and carries off some of its heat; therefore, 212 degs. of heat can never pass through it, to raise the inner vessel to the same heat.
Q. Why do sugar, salt, &c. retard the process of boiling?
A. Because they have a tendency to fix water by chemical attraction; and therefore retard its conversion into steam.
Q. If you want water to boil, without coming in contact with the saucepan, what plan must you adopt?
A. Immerse the pot (containing the water you want to boil) in a saucepan containing strong brine, or sugar.
Q. Why would the inner vessel boil, if the outer vessel contained strong brine?
A. Though water boils at 212 degs. of heat, yet brine will not boil till raised to 218 or 220 degs. Therefore, 212 degs. of heat may easily pass through brine to raise the vessel immersed in it to boiling heat, before any of it is carried off by steam.
Q. Why will brine impart to another vessel more than 212°, and water not so much?
A. Because both liquids will impart heat till they boil, and then they can impart heat no longer.
Q. Why can they impart no extra heat after they boil?
A. Because all extra heat is spent in making steam. Hence water will not boil a vessel of water immersed in it, because it cannot impart to it 212 degs. of heat: but brine will, because it can impart more than 212 degs. of heat, without being converted itself into steam.
| Ether boils at | 104 degs. |
| Alcohol boils at | 173-1/2 degs. |
| Water boils at | 212 degs. |
| Water with one-fifth salt at | 219 degs. |
| Syrup boils at | 221 degs. |
| Oil of turpentine at | 304 degs. |
| Sulphuric acid at | 472 degs. |
| Linseed oil at | 640 degs. |
| &c. &c. |
Any liquid which boils at a lower degree can be made to boil if immersed in a liquid which boils at a higher degree. Thus a cup of ether can be made to boil in a saucepan of water. A cup of water in a saucepan of brine or syrup. But a cup of water will not boil if immersed in ether; nor a cup of syrup in water.
Q. Why are clouds higher on a fine day?
A. 1st—Because the air (expanded by heat) drives them higher up: and
2ndly—The clouds themselves are lighter, and therefore more buoyant.
Q. Why are the clouds lighter on a fine day?
A. Because their mists are either absorbed by the dry air, or vapourized by the hot sun.
Q. Why is a cup put topsy-turvy into a fruit-pie?
A. Its principal use is to hold the crust up, and prevent it from sinking, when the cooked fruit gives away under it.
Q. Does not the cup prevent the fruit of the pie from boiling over?
A. No, by no means; it would rather tend to make it boil over, than otherwise.
Q. Why would the cup tend rather to make the fruit boil over?
A. As soon as the pie is put into the oven, the air in the cup will begin to expand, and drive every particle of juice from under it; the pie dish, therefore, will have a cup-full less room to hold its fruit, than if the cup were taken out.
Q. If the juice is driven out of the cup, why is the cup always full of juice, when the pie is cut up?
A. Immediately the pie is drawn, the air in the cup begins to condense again, and occupy a smaller space; in consequence of which, there is no longer enough air to fill the cup, and so juice rushes in to fill up the deficiency.
Q. Why does juice rush into the cup, because the cup is not full of air?
A. As the external air presses upon the surface of the juice, it rushes into the cup unobstructed; as mercury rises through the tube of a barometer through similar pressure.
CHAPTER X.
EXPANSION FROM HEAT.
(Continued.)
Q. Does heat expand every thing else besides air and water?
A. Yes; every thing (that man is acquainted with) is expanded by heat.
Q. Why does a cooper make his hoops red-hot, when he puts them on a tub?
A. 1st—As iron expands by heat, the hoops will be larger when they are red-hot; and will, therefore, fit more easily on the tub: and
2ndly—As iron contracts by cold, the hoops will shrink as they cool down, and girt the tub with a tighter grasp.
Q. Why does a wheelwright make his hoops red-hot, which he fixes on the nave of a wheel?
A. 1st—That they may fit on more easily: and
2ndly—That they may girt the nave more tightly.
Q. Why will the wheelwright’s hoop fit the nave more easily, because they are made red-hot?
A. As iron expands by heat, the hoops will be larger when they are hot; and (being larger) will go on the nave more easily.
Q. Why will the hoops, which have been put on hot, girt the nave more firmly?
A. As iron contracts by cold, the hoops will shrink as they cool down; and, therefore, girt the nave with a tighter grasp.
Q. Why does a farrier put the horse-shoe on hot?
A. That it may stick the closer, when it has contracted by cold.
Q. Why does a stove make a cracking noise, when a fire is very hot?
A. The iron stove expands by heat, and (as it swells) the parts rub both against each other, and against the bricks around, driving them further off; and this produces a cracking noise.
Q. Why does a stove make a similar cracking noise, when a large fire is taken down?
A. The iron stove contracts again, as soon as the fire is removed; and (as it shrinks into a smaller space) the parts rub against each other again, and the bricks are again disturbed; and this produces a cracking noise.
Q. Why does the plaster round a stove crack and fall away?
A. When the fire is lighted, the iron-work (which expands more than the brick-work and plaster) pushes away the bricks and plaster: but when the fire is put out, the metal shrinks again, and leaves the “setting” behind.
Q. Why does the plaster fall away?
A. As a chink is left (between the “setting” and the stove), the plaster will frequently fall away from its own weight.
Q. What other cause contributes to bring the plaster down?
A. As the heat of the fire varies, the size of the iron stove varies also; and this swelling and perpetually contracting, keeps up such a constant disturbance about the plaster, that it cracks and falls off, leaving the fire-place very unsightly.
Q. Why does the mercury of a thermometer rise in hot weather?
A. Heat expands the metal; and as the metal is increased in bulk, it occupies a larger space, (or, in other words, rises higher in the tube.)
Q. Why is a glass broken, when hot water is poured into it?
A. Because the inside of the glass is expanded by the hot water, and not the outside; so the glass snaps for want of flexibility.
Q. Why is not the outside of the glass expanded by the hot water, as well as the inside?
A. Glass is a non-conductor of heat; and, therefore, breaks before the heat of the inner surface is conducted to the outside.
Q. Why does a glass snap, because the inner surface is hotter than the outer?
A. Glass is expanded by heat; and as the inner surface expands, it stretches the outer surface till it snaps.
Q. Why is a china cup broken, if hot water be poured over it, or into it?
A. China is a non-conductor; and, as the inner surface expands by the heat, before the outer one, it forms an arch, and pulls the parts of the cup asunder.
Q. Why does the bottom come off, if a glass beaker be set on a warm hob?
A. Glass is a non-conductor; and, as the bottom of the glass (from the warmth of the hot stove) expands, before the sides are heated, the two parts separate the one from the other.
CHAPTER XI.
2.—LIQUEFACTION.
3.—VAPORIZATION.
Q. What is meant by liquefaction?
A. The state of being melted; as ice is melted by the heat of the sun.
Q. Why is ice melted by the heat of the sun?
A. The heat of the sun (entering the solid ice) forces its particles asunder, till their attraction of cohesion is sufficiently overcome, to convert the solid ice into liquid. (See p. [112].)
Q. Why are metals melted by the heat of fire?
A. The heat of the fire (entering the solid metal) forces its particles asunder, till their attraction of cohesion is sufficiently overcome, to convert the solid metal to a liquid.
Q. Why is water converted to steam by the heat of fire?
A. The heat of the fire (entering the water) divides its globules into very minute bubbles, which (being made lighter than air) fly off from the surface in the form of steam.
Q. Why does not wood melt, like metal?
A. Because the heat of the fire decomposes the wood into gas, smoke, and ashes; and the different parts separate from each other.
Q. What is meant by vaporization?
A. The conversion of liquid into vapour; as water is converted into vapour by the heat of the sun.
Q. What are clouds?
A. Moisture evaporated from the earth, and collected in the upper regions of the air.
Q. What is the difference between a fog and a cloud?
A. Clouds and fogs differ only in one respect. Clouds are elevated above our heads: but fogs come in contact with the surface of the earth.
Q. If clouds are water, why do they float on the air?
A. 1st—The vapour of clouds is composed of very minute bubbles (called ves’cicles), which float like soap bubbles: and
2ndly—Warm air (between the bubbles) keeps them apart, and makes the mass lighter; and the currents of air (which constantly ascend from the warm earth) buoy them up.
Q. Why does vapour sometimes form into clouds, and sometimes rest upon the earth as mist or fog?
A. When the surface of the earth is warmer than the air, the vapour of the earth (being condensed by the chill air) becomes mist or fog. But when the air is warmer than the earth, the vapour rises through the air, and becomes cloud.
Q. Are all clouds alike?
A. No. They vary greatly in density, height, and colour.
Q. What is the chief cause of fog and clouds?
A. The changes of the wind.
Q. How can the changes of the wind affect the clouds?
A. If a cold current of wind blows suddenly over any region, it condenses the invisible vapour of the air into cloud or rain: but if a warm current of wind, blows over any region, it disperses the clouds, by absorbing their vapour.
Q. What countries are the most cloudy?
A. Those where the winds are most variable, as Britain.
Q. What countries are the least cloudy?
A. Those where the winds are not variable, as Egypt.
Q. What distance are the clouds from the earth?
A. Some thin light clouds are elevated above the highest mountain-top; some heavy ones touch the steeples, trees, and even the earth: but the average height is between one and two miles.
(Streaky curling clouds, like hair, are often five or six miles high.)
Q. What clouds are the lowest?
A. Those that are most highly electrified: lightning clouds are rarely more than about 700 yards above the ground; and very often actually touch the earth with one of their edges.
Q. What is the thickness of the clouds?
A. Some clouds are 20 square miles in surface, and above a mile in thickness; while others are only a few yards or inches.
Q. How can persons ascertain the thickness of a cloud?
A. As the tops of high mountains are generally above the clouds; therefore, travellers (who climb the mountains) may pass quite through the clouds, into a clear blue firmament, when they may see the clouds beneath their feet.
Q. Why are the clouds so variable in shape?
A. The shape of clouds depends upon two things:—Their state of electricity, and the wind.
Q. How can electricity affect the shape of clouds?
A. If one cloud be full of electricity, and another not, they will be attracted to each other, and either coalesce,—diminish in size,—or vanish altogether.
Q. Which clouds assume the most fantastic shapes?
A. Those that are the most highly electrified.
Q. What effect have winds on the shape of clouds?
A. They sometimes absorb them entirely: sometimes increase their volume and density; and sometimes change the position of their parts.
Q. How can winds absorb clouds altogether?
A. A warm dry wind will convert the substance of the clouds into invisible vapour, and carry it in its own current.
Q. How can winds increase the bulk and density of clouds?
A. A cold current of wind will condense the invisible vapour of the air, and add it to the clouds as it passes by.
Q. How can winds change the shape of clouds by altering the position of their parts?
A. Because clouds are so voluble and light, that every breath of wind changes the position of those ves’cicles or bubbles.
Q. What are the general colours of the clouds?
A. White and grey, when the sun is above the horizon: but red, orange, and yellow, at sun-rise and sun-set.
The blue sky cannot be considered as clouds at all.
Q. Why are the last clouds of evening generally of a red tinge?
A. Because red rays are the least refrangible of all; and, therefore, are the last to disappear.
Q. What is meant by being “less refrangible”?
A. Being less able to be bent. Blue and green rays being very easily bent (by the resistance of the air) are thrown off from the horizon; but red rays not being bent back in the same way, give a tinge to the evening clouds.
Q. Why are morning clouds generally of a red tinge?
A. Because red rays are the least refrangible of all, and not being bent back by the air (like blue and green), strike upon the horizon, and give a tinge to the morning clouds.
Q. Why is not the reflection of clouds always alike?
A. Because their size, density, and situation in regard to the sun, vary perpetually; so that sometimes one colour is reflected, and sometimes another.
Q. What regulates the motion of the clouds?
A. The motion of the clouds is generally directed by the winds; but sometimes electricity will influence their motion also.
Q. How do you know that clouds move by other influences besides wind?
A. Because we often see in calm weather small clouds meeting each other from opposite directions.
Q. How do you know that electricity affects the motion of the clouds?
A. Because clouds often meet from opposite directions; and (after they have discharged their opposite electricities into each other) vanish altogether.
Q. Into how many classes are the different sorts of clouds generally divided?
A. Into three classes:—viz. Simple, Intermediate, and Compound.
Q. How are simple clouds sub-divided?
A. 1.—Cirrus. 2.—Cum’ulus; and 3.—Stra’tus.
Q. What are cirrus clouds?
A. Clouds like fibres, loose hair, or thin streaks, are called cirrus clouds.
Q. Why are these clouds called cirrus?
A. From the Latin word, cirrus (“a lock of hair, or curl”): they are the most elevated of all clouds.
Q. What do cirrus clouds portend?
A. When the streamers point upwards, the clouds are falling, and rain is at hand: but when the streamers point downwards, expect easterly wind or drought.
Q. What are cum’ulus clouds?
A. Cum’ulus clouds are lumps like great sugar-loaves,—volumes of smoke,—or mountain towering over mountain.
Q. Why are these monster masses called cum’ulus clouds?
A. From the Latin word, cum’ulus (a mass or pile).
Q. What do cum’ulus clouds foreshow?
A. When these piles of cloud are fleecy, and sail against the wind, they indicate rain; but when their outline is very hard, and they come up with the wind, they foretell fine weather.
Cumulus clouds should be smaller towards evening than they are at noon. If they increase in size at sun-set, a thunder-storm may be expected in the night.
Q. What are stra’tus clouds?
A. Creeping mists, especially prevalent in a summer’s evening: these clouds rise at sun-set in low damp places, and are always nearer the earth, than any other sort of cloud.
Q. Why are these mists called stra’tus clouds?
A. From the Latin word, stra’tus (“laid low,” or “that which lies low”).
Q. How are the intermediate clouds sub-divided?
A. Into two sorts. 1.—The Cirro-Cum’ulus; and 2.—The Cirro-Stra’tus.
Q. What are cirro-cum’ulus clouds?
A. When cirrus clouds spring from a massy centre; or when heavy masses of cloud terminate at their edges in long streaks, or what are called “mares’ tails.”
A system of small round clouds may be called cirro-cum’ulus.
Q. What do cirro-cum’ulus clouds generally forebode?
A. Continued drought, or hot dry weather.
Q. What are cirro-stra’tus clouds?
A. They compose what is generally called a “mackarel sky.” This class of clouds always indicate rain and wind; hence the proverb—
“Mackarels’ scales and mares’ tails
Make lofty ships to carry low sails.”
Q. How are compound clouds sub-divided?
A. Compound clouds are also sub-divided into two sorts. 1.—The Cum’ulo-stra’tus; and 2.—The Nimbus.
Q. What is meant by cum’ulo-stra’tus clouds?
A. Those clouds which assume all sorts of gigantic fancy forms; such as vast towers and rocks,—huge whales and dragons,—scenes of battle,—and cloudy giants. This class of clouds is the most romantic and strange of all.
Q. What do the cumulo-stratus clouds foretell?
A. A change of weather; either from fine to rain, or from rain to fine weather.
Q. What are nimbus clouds?
A. Nimbus is the Latin word for “clouds which bring a storm;” and all clouds from which rain falls are so named.
Q. What appearance takes place in the clouds at the approach of rain?
A. The cum’ulus cloud becomes stationary, and cirrus streaks settle upon it, forming cumulo-stratus clouds; which are black at first, but afterwards of a grey colour.
Q. Why do clouds gather round mountain-tops?
A. Because (as they float along) they dash against the mountains; and (being arrested in their motion) collect round the top.
Q. What is the use of clouds?
A. 1st—They act as screens to arrest the radiation of heat from the earth:
2ndly—They temper the heat of the sun’s rays: and
3rdly—They are the great store-houses of rain.
Q. Why is wind said to blow up the clouds?
A. When a dry wind travels over sea, and accumulates more vapour than the air can sustain, it relinquishes a part (as it flies along) in the form of clouds.
Q. Why does wind sometimes drive away the clouds?
A. When wind travels over dry climes or thirsty deserts, it becomes so dry itself, that it absorbs vapour from the clouds, and disperses them.
Q. What is the cause of a red sun-set?
A. Because the vapour of the air is not actually condensed into clouds, but only on the point of being condensed; in which state it bends the red rays of the sun towards the horizon, where they are reflected at sun-set.
Q. Why is a red sun-set an indication of a fine day to-morrow?
A. Because (notwithstanding the cold of sun-set) the vapours of the earth are not condensed into clouds. Our Lord referred to this prognostic in the following words: “When it is evening ye say, it will be fair weather, for the sky is red.” (Matt. xvi. 2.)
Q. What is the cause of a coppery yellow sun-set?
A. Because the vapour of the air is actually condensed into clouds; in which case it “refracts” (or bends) the yellow rays of the sun towards the horizon, where they are reflected at sun-set.
Q. Why is a yellow sunset an indication of wet?
A. Because the vapours of the air are already condensed into clouds; rain, therefore, may be shortly expected.
Q. What is the cause of a red sun-rise?
A. Vapour in the upper region of the air just on the point of being condensed.
Q. Why is a red and lowering sky at sunrise an indication of a wet day?
A. Because the higher regions of the air are laden with vapour, on the very point of condensation, which the rising sun cannot disperse. Hence our Lord’s observation, “In the morning (ye say) it will be foul weather to-day, for the sky is red and lowering.” (Matt. xvi. 3.)
Q. Why is a grey morning an indication of a fine day?
A. Because that air alone contiguous to the earth is damp and full of vapour. There are no vapours in the higher regions of the air to reflect red rays; and hence the morning-light looks grey.
Q. What difference (in the state of the air) is required, to make a grey and red sunrise?
A. In a grey sunrise, only that portion of air contiguous to the earth is filled with vapour; all the rest is clear and dry. But in a red sunrise the air in the upper regions is so full of vapour that the rising sun cannot disperse it.
Q. Why is a grey sunset an indication of wet?
A. If the air on the surface of the earth be very damp at sunset, it is a proof that the air is saturated with vapour, and wet may be expected: hence the proverb—
“Evening red and morning grey
Will set the traveller on his way;
But evening grey and morning red
Will bring down rain upon his head.”
Q. The proverb says, “A rainbow in the morning is the shepherd’s warning:” why is it so?
A. A rainbow can only be formed when the clouds (containing or dropping rain) are opposite the sun: a morning rainbow, therefore, is always in the west, and indicates that bad weather is on the road to us.
Q. Why does a rainbow in the west indicate that bad weather is on the road to us?
A. Because our heavy rains are usually brought by west or south-west winds; and, therefore, clouds which reflect the colour of the rainbow in the west, are coming up with the wind, bringing rain with them.
Q. The proverb says, “A rainbow at night, is the shepherd’s delight;” why is it so?
A. As a rainbow is always opposite to the sun, therefore a rainbow at night is in the east, and indicates that bad weather is leaving us.
Q. Why does a rainbow in the east indicate that bad weather is leaving us?
A. As west and south-west winds bring rain, if the clouds have been driven from the west to the east, they have passed over us, and are going away from us.
Q. What is meant by an aurora borea’lis, or northern light?
A. A luminous white cloud in the north of the sky at night-time. Sometimes streaks of blue, purple, and red,—and sometimes flashes of light, are seen also.
In our island this phenomenon generally rises from a dark cloud (running from the north to the east and west) elevated about 10 or 20 degrees above the horizon: above this dark bed of clouds the luminous white light appears.
Q. What is the cause of the aurora borealis, or northern light?
A. Electricity in the clouds.
Q. Why is the aurora borealis generally a white light?
A. Because the electric fluid passes through air extremely rarefied: and whenever electric fluid passes through air much rarefied, it always produces a white light.
Q. Why are there sometimes different colours in the aurora borealis, such as yellow, red, and purple?
A. Because the electric fluid passes through air of different densities. The most rarefied air produces a white light; the most dry air, red; and the most damp produces yellow streaks.
Q. Does the aurora borealis forbode fine weather or wet?
A. When its corruscations are very bright, it is generally followed by stormy moist unsettled weather.
Q. Why does a haze round the sun indicate rain?
A. Because the haze is caused by very fine rain falling in the upper regions of the air; when this is the case, a rain of 5 or 6 hours continuance, may be expected.
Q. Why is a halo round the moon a sure indication of rain?
A. Because the halo is caused by fine rain falling in the upper regions of the air. The larger the halo the nearer the rain-clouds, and the sooner may rain be expected.
Q. Why does a black mist bring wet weather?
A. The mist is black, because it is overshadowed by dense clouds or masses of vapour; and, therefore, it forebodes wet.
Q. Why does a white mist indicate fine weather?
A. The mist is white, because no clouds blacken it with their shadow; and (as the sky is cloudless) fine weather may be expected.
Q. Why do we feel almost suffocated in a hot cloudy night?
A. Because the heat of the earth (being unable to escape into the upper region of the air, in consequence of the clouds) floats, like a sea of heat, on the surface of the earth.
Q. Why do we feel more sprightly in a clear bright night?
A. Because the heat of the earth can readily escape into the upper regions of the air, and is not confined and pent-in by thick clouds.
Q. Why do we feel depressed in spirits on a wet murky day?
A. 1st—Because when the air is laden with vapour, it has less oxygen.
2ndly—The air being lighter than usual, does not balance the air in our body: and
3rdly—Moist air has a tendency to relax the nervous system.
Q. What is meant by the “air balancing the air” in our body?
A. The human body is filled with air of the same density as that around: if, therefore, we ascend into purer air, or descend into denser air, the balance is destroyed, and we feel oppressed and suffocated.
Q. Why do we feel oppressed and suffocated if the air around is not of the same density as that in our body?
A. If the air around be more dense, it will squeeze our body in by its weight: if it be less dense, the air in our body will blow us out.
Q. Why do persons who ascend in balloons feel pain in their eyes, ears and chest?
A. Because the air in the upper regions is more rare than the air in their bodies; and (till the equilibrium is restored) great pain is felt in all the more sensitive parts of the body.
Q. Why do persons who descend in diving-bells feel pain in their eyes, ears and chest?
A. Because the air in the sea is more dense than the air in their bodies; and (till the equilibrium is restored) great pain is felt in all the more sensitive parts of the body.
Q. Why does the sea heave and sigh just previous to a storm?
A. The density of the air (just previous to a storm) is very suddenly diminished, but the air in the sea is not so quickly affected; therefore the sea heaves and sighs in its effort to restore an equilibrium.
Q. Why is the air so universally still just previous to a tempest?
A. Because the air is suddenly and very greatly rarefied; and (as the density of the air is diminished) its power to transmit sound is diminished also.
Q. How do you know that rarefied air cannot transmit sound so well as dense air?
A. Because the sound of a bell (in the receiver of an air-pump) cannot be heard at all, after the air has been partially exhausted; and a pistol fired on a high mountain would not sound louder than a common cracker.
Q. Why do we feel braced and light-hearted on a fine spring or frosty morning?
A. 1st—Because there is more oxygen in the air on a fine frosty morning, than there is on a wet day: and
2ndly—A brisk and frosty air has a tendency to brace the nervous system.
Q. Why do dogs and cats (confined to a room) feel lazy and drowsy at the approach of rain?
A. 1st—Because the air does not contain its full proportion of oxygen: and
2ndly—Because the damp relaxes their nervous system, and makes them drowsy.
Q. Why do horses neigh, cattle low, sheep bleat, and asses bray, at the approach of rain?
A. 1st—As the air does not contain its full proportion of oxygen, they feel a difficulty in breathing: and
2ndly—As damp relaxes their nerves, they feel languid and uneasy.
Q. Why do candles and fires burn with a bluer flame in wet weather?
A. As the air contains less oxygen in wet weather, the heat of fire is less intense: and the flame is blue, because the fuel is not thoroughly consumed.
Q. Why do hills, &c. appear larger in wet weather?
A. Because (when the air is laden with vapour) the rays of light are more dispersed, and produce a larger reflection; objects, therefore, seen at a distance, appear larger.
Q. Why do trees, &c. in wet weather appear further off than they really are?
A. Because the fog or mist diminishes the light reflected from the object; and as the object becomes more dim, it seems to be further off.
Q. Why does the sun seem larger when he sets and rises, than he does at noon?
A. Because the rays pass through more of the vapoury atmosphere which surrounds the earth; and this vapoury atmosphere acts like a magnifying glass.
It is very manifest that the lines D C are shorter than the lines E C: if, therefore, A be the earth, and D G E the boundary of the atmosphere round the earth, then the rays M E C (at the horizon) will pass through more of the atmosphere, than the rays S D C, which are more elevated.
Q. Why does the moon appear larger at her rising and setting, than when above our heads?
A. Because the rays pass through more of the vapoury atmosphere which surrounds the earth; and this vapoury atmosphere magnifies the moon, just like a magnifying glass.
Q. Why do cats rub their ears when it is likely to rain?
A. Either because the air is full of vapour, and its humidity (piercing between the hair of the cat) produces an itching sensation; or more probably, because the air is overcharged with electricity.
Q. How can the electricity of air produce a sensation of itching?
A. If the air is overcharged with electricity, the hair of the cat is overcharged also; and this makes her feel as if she were covered with cobwebs.
Q. Why does the cat keep rubbing herself?
A. Her hair will not lie smooth, but has a perpetual tendency to become turgid and ruffled; so the cat keeps rubbing her coat and ears, to smooth the hair down, and brush away the feeling of cobwebs.
Q. Why do our heads and skin itch before rain?
A. Probably because the air is overcharged with electricity; and, therefore, a sensation (like that of cobwebs) irritates the skin, and produces an itching.
Q. Why do we hear distant CLOCKS more distinctly when rain is near at hand?
A. Because the air is filled with vapour, and water is a better conductor of sound than dry air.
Q. Why do we hear church-bells further, just previous to rain?
A. Because the air is filled with vapour, and vapour is a better conductor of sound than dry air.
Q. Why do doors swell, when rain is at hand?
A. Because the air is filled with vapour, which (penetrating into the pores of the wood) forces the parts further apart, and swells the door.
Q. Why do doors shrink in dry weather?
A. Because the moisture is absorbed from the wood; and, as the particles are brought closer together, the size of the door is lessened, (or in other words, the wood shrinks).
Q. Why is the air filled with offensive smells previous to a coming rain?
A. Because the volatile parts, (which rise from dunghills, sewers, &c.), being laden with vapour, are unable to rise so readily, as when they are rarefied by a bright sun.
Q. Why do flowers smell sweeter and stronger just previous to rain?
A. Because the volatile parts (which constitute the perfume of flowers) are laden with vapour; and (being unable to rise) are confined to the lower regions of the air.
Q. Why do horses and other animals stretch out their necks, and snuff up the air, just previous to a fall of rain?
A. Because they smell the odour of plants and hay, and delight to snuff in their fragrance.
Q. Why does smoke fall when rain is at hand?
A. The air being less dense in wet weather, cannot buoy up smoke so readily, as when more dry and heavy.
Q. Why do swallows fly low when rain is at hand?
A. Because the insects (of which they are in pursuit) have fled from the cold upper regions of the air, to the warm air near the earth: and as their food is low, the swallows fly low.
Q. Why do these insects seek the lower regions of the air in wet weather, more than in fine weather?
A. Because they are forced downward, by some current of cold air which drives them down.
Q. Why does a downward current of cold air bring rain?
A. Because it condenses the warm vapour; which then descends in rain.
Q. The proverb says, “a single magpie in spring, foul weather will bring:” why is this the case?
A. In cold stormy weather, one magpie alone will leave its warm snug nest in search of food, while the other stays with the eggs or young ones; but in fine mild weather (when their brood will not be injured by cold) both the magpies will fly out together.
Q. Why is it unlucky for anglers to see a single magpie in spring?
A. Because when magpies fly abroad singly, the weather is cold and stormy; but when both birds fly out together, the weather is warm and mild, which is favourable for fishing.
Q. Why do sea gulls fly about the sea in fine weather?
A. Because they live upon fish, which are found near the surface of the sea in fine weather.
Q. Why may we expect stormy rains, when sea gulls assemble on the land?
A. Because the fish (on which they live) leave the surface of the sea in stormy weather, and go down too deep for the gulls to get at them; they are obliged, therefore, to feed on the worms and larvæ which are driven out of the ground at such times.
Q. Why does the petrel always fly to the sea during a storm?
A. Because the petrel lives upon sea insects, which are always to be found in abundance about the spray of swelling waves.
(The Petrel is a bird of the duck-kind, which lives in the open sea. They run on the top of the sea, and are called Petrels, or rather Peter-els, from “St. Peter,” in allusion to his walking on the sea, to go to Jesus.)
Q. Why do candles and lamps spirt when rain is at hand?
A. Because the air is filled with vapour, and the humidity penetrates the wick; where (being formed into steam) it expands suddenly, and produces a little explosion.
Q. Why does a drop of water sometimes roll along a piece of hot iron without leaving the least trace?
A. If the iron be very hot indeed, the bottom of the drop is turned into vapour, before the drop can evaporate; and the vapour thus formed buoys the drop up, without allowing it to touch the iron at all.
Q. Why does it roll?
A. The current of air (which is always passing over the heated surface) drives it along.
Q. Why does a laundress put a little saliva on an ironing-box to know if it be hot enough?
A. If the saliva sticks to the box and is evaporated, the box is not hot enough; but if the saliva runs along the box, it is.
Q. Why is the box hotter if the saliva runs along the box, than if it adheres to it till it is evaporated?
A. If the saliva runs along the box, the iron is hot enough to convert the bottom of the drop of spittle into vapour; but if the saliva will not roll, the box is not hot enough to convert the bottom of the drop of spittle into vapour.
CHAPTER XII.
4.—EVAPORATION.
Q. What is meant by evaporation?
A. The dissipation of liquid by its being converted into vapour.
Q. What effects are produced by evaporation?
A. The liquid vaporized absorbs heat from the body whence it issues; and the body deprived of the liquid by evaporation, loses heat thereby.
Q. If you wet your finger in your mouth, and hold it up in the air, why does it feel cold?
A. The saliva quickly evaporates; and (as it evaporates) absorbs heat from the finger, which makes it feel cold.
Q. If you bathe your temples with ether, why does it allay inflammation and feverish heat?
A. Ether very rapidly evaporates; and (as it evaporates) absorbs heat from the burning head, producing a sensation of cold.
Q. Why is ether better for this purpose than water?
A. Because it requires less heat to convert it into vapour; and therefore it evaporates much more quickly.
(Ether is converted into steam with 104 degs. of heat, but water requires 212 degs. of heat to convert it into steam.)
Q. Why does ether very greatly relieve a scald or burn?
A. Because it evaporates very rapidly; and (while it is converted into vapour) carries off the heat of the burn.
Q. Why do we feel so cold when we have wet feet or clothes?
A. As the wet of our shoes or clothes evaporates, it keeps absorbing heat from the body, which makes it feel cold.
Q. Why do wet feet or clothes give us “cold?”
A. Because the evaporation absorbs heat from the body so abundantly, that it is lowered below its natural standard; and therefore health is injured.
Q. Why is it dangerous to sleep in a damp bed?
A. Because the heat of the body is continually absorbed in converting the damp of the sheets into vapour; and as heat is abstracted from the body, its temperature is reduced below the healthy standard.
Q. Why do we not feel the same sensation of cold, if we throw a macintosh over our wet clothes?
A. The macintosh prevents evaporation, because the steam cannot escape through the air-tight fabric; and (as the wet cannot evaporate from the clothes) no heat is absorbed from our bodies.
Q. Why do not sailors get cold, who are so often wet all day with sea-water?
A. The salt of the sea retards evaporation; and (as the heat of the body is drawn off very gradually) the sensation of cold is prevented.
Q. Why does sprinkling a hot room with water cool it?
A. The heat of the room causes a rapid evaporation of the sprinkled water; and as the water evaporates, it absorbs heat from the room, and cools it.
Q. Why does watering the streets and roads cool them?
A. The hot streets and roads part with their heat to promote the evaporation of the water sprinkled on them.
Q. Why does a shower of rain seem to cool the air in summer-time?
A. The earth (being wet with the rain) parts with its heat to promote evaporation; and as the earth is cooled, it cools the air also.
Q. Why is linen dried by being exposed to the wind?
A. The air (blowing over the linen) promotes evaporation, by removing the vapour from the surface of the wet linen, as soon as it is formed.
Q. Why is linen dried sooner in the open air, than in a confined room?
A. Because the particles of vapour are more rapidly removed from the surface of the linen by evaporation.
Q. Why are wet summers generally succeeded by cold winters?
A. Because the great evaporation (carried on through the wet summer) reduces the temperature of the earth lower than usual, and produces cold.
Q. Why is england warmer than it used to be, when agues were so common?
A. Because it is better drained and better cultivated.
Q. Why does draining land promote warmth?
A. Because it diminishes evaporation; in consequence of which less heat is abstracted from the earth.
Q. Why does cultivation increase the warmth of a country?
A. 1st—Because hedges and belts of trees are multiplied;
2ndly—Because the land is better drained;
3rdly—Because the land is dug and ploughed; and
4thly—Because the vast forests are cut down.
Q. Why do hedges and belts of trees promote warmth?
A. Because they retard evaporation, by keeping off the wind.
Q. If belts of trees promote warmth, why do forests produce cold?
A. 1st—Because they detain and condense the passing clouds:
2ndly—They prevent the access of both wind and sun:
3rdly—The soil of forests is always covered with long damp grass, rotting leaves, and thick brushwood: and
4thly—There are always many hollows in every forest full of stagnant water.
Q. Why do long grass and rotting leaves promote cold?
A. Because they are always damp; and the evaporation which they promote, is constantly absorbing heat from the earth beneath.
Q. Why do digging and ploughing help to make a country warm?
A. Digging and ploughing help to pulverize the soil, by admitting air into it, and this increases its mean temperature.
Q. Why are France and Germany warmer now, than when the vine would not ripen there?
A. Chiefly because their vast forests have been cut down; and the soil is better drained and cultivated.
Q. What becomes of the water of ponds and tubs in summer-time?
A. Ponds and tubs in summer-time are often left dry, because their water is evaporated by the air.
Q. How is this evaporation produced and carried on?
A. The air contains heat, and changes the surface of the water into vapour; this vapour (blending with the air) is soon wafted away; while fresh portions of air blow over the water, and produce a similar evaporation; till the pond or tub is left quite dry.
Q. Why are the wheels of some machines kept constantly wet with water?
A. To carry off the heat (arising from the rapid motion of the wheels) by evaporation, as soon as it is developed.
Q. Why is mould hardened by the sun?
A. Because (when the moisture of the mould has been evaporated by the sun) the earthy particles come into closer contact, and the mass becomes more solid.
Q. Show the wisdom of god in this arrangement.
A. If the soil did not become crusty and hard in dry weather, the heat and drought would penetrate the soil, and kill both seeds and roots.
Q. Why is tea cooled faster in a saucer than in a cup?
A. Because evaporation is increased by increasing the surface; and as tea in a saucer presents a much larger surface to the air, its heat is more rapidly carried off by evaporation.
(The subject of “convection” will be treated of in a future chapter, and would scarcely be understood in this place.)
Q. Why is not the vapour of the sea salt?
A. Because the salt is always left behind, by the process of evaporation.
Q. Why does a white crust appear (in hot weather) upon clothes wetted by sea water?
A. The white crust is the salt of the water left on the clothes by evaporation.
Q. Why does this white crust always disappear in wet weather?
A. In wet weather the moisture of the air dissolves the salt; and, therefore, it no longer remains visible.
Q. Why should not persons, who take violent exercise, wear very thick clothing?
A. When the heat of the body is increased by exercise, perspiration reduces the heat (by evaporation) to a healthy standard: as thick clothing prevents this evaporation, and confines the heat and perspiration to the body, it is injurious to health.
CHAPTER XIII.
COMMUNICATION OF HEAT.
1.—CONDUCTION.
Q. How is heat communicated from one body to another?
A. 1. By Conduction. 2. By Absorption. 3. By Reflection. 4. By Radiation: and 5. By Convection.
Q. What is meant by conduction of heat?
A. Heat communicated from one body to another, by actual contact.
Q. Why does a piece of wood (blazing at one end) not feel hot at the other end?
A. Wood is a bad conductor of heat; and, therefore, heat does not traverse freely through it: hence, though one end of a stick be blazing-hot, the other end may be quite cold.
Q. Why do some things feel so much colder than others?
A. Principally because they are better conductors; and, therefore, draw off the heat from our body (which touches them) so much faster.
Q. What are the best conductors of heat?
A. Dense solid bodies, such as metal and stone.
Q. Which metals are the most rapid conductors of heat?
A. Silver is the best conductor, then copper, then gold or tin, then iron, then zinc, and then lead.
Q. What are the worst conductors of heat?
A. All light and porous bodies, such as hair, fur, wool, charcoal, and so on.
Q. Why are cooking vessels so often furnished with wooden handles?
A. Wood is not a good conductor, like metal; and, therefore, many vessels (which are exposed to the heat of the fire) have wooden handles, lest they should burn our hands when we take hold of them.
Q. Why is the handle of a metal tea-pot made of wood?
A. As wood is a bad conductor, the heat of the boiling water is not so quickly conveyed to the wooden handle, nor so quickly poured into the hand by it, as when the handle is made of metal.
Q. Why would a metal handle burn the hand of the tea-maker?
A. As metal is an excellent conductor, the heat of the boiling water rushes quickly into the metal handle, and into the hand that touches it.
Q. How do you know that a metal handle would be hotter than a wooden one?
A. By touching the metal collar into which the wooden handle is fixed: though the wooden handle is quite cold, this metal collar is intensely hot.
Q. Why do persons use paper or woollen kettle-holders to take hold of a kettle with?
A. Paper and woollen are both very bad conductors of heat; and, therefore, the heat of the kettle does not readily pass through them to the hand.
Q. Does the heat of the boiling kettle never get through the woollen or paper kettle-holder?
A. Yes; but though the kettle-holder became as hot as the kettle itself, it would never feel so hot.
Q. Why would not the kettle-holder feel so hot as the kettle, when it really is of the same temperature?
A. Because (being a very bad conductor) it disposes of its heat so slowly, that it is scarcely perceptible; but metal (being an excellent conductor) disposes of its heat so quickly, that the sudden influx is painful.
Q. Why then does hot metal feel so much more intensely warm than hot wool?
A. Because it gives out a much greater quantity of heat in the same space of time; and the influx of heat is, therefore, more perceptible.
Q. Why does money in our pocket feel so hot, when we stand before a fire?
A. Metal is an excellent conductor; and, therefore, becomes rapidly heated. For the same reason it becomes rapidly cold, when it comes in contact with a body colder than itself.
Q. Why does a pump-handle feel intensely cold in winter?
A. As metal is an excellent conductor, when the hot hand touches the cold pump-handle, the heat passes rapidly from the hand into the iron; and this rapid loss of heat produces a sensation of intense coldness.
Q. Is the iron handle of the pump really colder than the wooden pump itself?
A. No; every inanimate substance (exposed to the same temperature) possesses the same degree of heat.
Q. Why then does the iron handle seem so much colder than the wooden pump?
A. Merely because the iron is a better conductor; and, therefore, draws off the heat from our hand much more rapidly than wood does.
Q. Why does a stone or marble hearth feel to the feet so much colder than a carpet or hearth-rug?
A. Because stone and marble are good conductors, but woollen carpets and hearth-rugs are very bad conductors.
Q. Why does the stone hearth make our feet cold?
A. As soon as the hearth-stone has absorbed a portion of heat from our foot, it instantly disposes of it, and calls for a fresh supply; till the hearth-stone has become of the same temperature as the foot placed upon it.
Q. Do not the woollen carpet and hearth-rug, also, conduct heat from the human body?
A. Yes; (but being very bad conductors) they convey the heat away so slowly, that it is scarcely perceptible.
Q. Is the cold hearth-stone and warm carpet then of the same temperature?
A. Yes; everything in the room is really of the same temperature; but some feel colder than others because they are better conductors.
Q. How long will the hearth-stone feel cold to the feet resting on it?
A. Till the feet and the hearth-stone are both of the same temperature; and then the sensation of cold in the hearth-stone will go off.
Q. Why would not the hearth-stone feel cold, when it is of the same temperature as our feet?
A. Because the heat would no longer rush out of our feet into the hearth-stone, in order to produce an equilibrium.
Q. Why does the hearth-stone (when the fire is lighted) feel so much hotter than the hearth-rug?
A. The hearth-stone is an excellent conductor; and, therefore, parts with its heat more readily than the woollen hearth-rug; which (being a very bad conductor) parts with its heat reluctantly.
Q. Why does parting with heat rapidly make the hearth-stone feel warm?
A. As the heat of the stone rushes quickly into our foot, it raises its temperature so suddenly, that we cannot help perceiving the increase of heat.
Q. Why does the non-conducting power of the hearth-rug prevent its feeling so hot as it really is?
A. Because it parts with its heat so slowly and gradually, that we scarcely perceive its transmission into our feet.
Q. When we plunge our hands into a basin of water, why does it produce a sensation of cold?
A. Though the water (in which we wash) is really warmer than the air of our bed-room; yet because it is a better conductor, it feels colder.
Q. Why does the conducting power of water make it feel colder than the air, though in reality it is warmer?
A. Because it abstracts heat from our hands so rapidly, that we feel its loss; but the air abstracts heat so very slowly, that its gradual loss is hardly perceptible.
Q. Is water a good conductor of heat?
A. No; no liquid is a good conductor of heat; but yet water is a much better conductor than air.
Q. Why is water a better conductor of heat than air?
A. Because it is less subtile; and the conducting power of any substance depends upon its solidity, or the closeness of its particles.
Q. How do you know that water is not a good conductor of heat?
A. Because water may be made to boil at its surface, without imparting sufficient heat to melt ice a quarter of an inch below the boiling surface.
Q. Why are not liquids good conductors of heat?
A. Because the heat (which should be transmitted) produces evaporation, and flies off in the vapour.
Q. Why does a poker (resting on the fender) feel so much colder than the hearth-rug, which is further off the fire?
A. The poker (being an excellent conductor) draws heat from the hand much more quickly than the rug, which is a bad conductor: and, therefore, (though both are equally warm) the poker seems to be much colder.
Q. Why are hot bricks (wrapped in cloth) employed in cold weather to keep the feet warm?
A. Bricks are bad conductors of heat, and cloth or flannel still worse: therefore a hot brick (wrapped in flannel) will retain its heat a very long time.
Q. Why is a tin pan (filled with hot water) employed as a foot warmer?
A. Because polished tin (being a bad radiator of heat) keeps hot a very long time; and warms the feet resting upon it.
Q. What is meant by being a “bad radiator of heat?”
A. To radiate heat is to throw off heat by rays, as the sun; a polished tin pan does not throw off the heat of boiling water from its surface, but keeps it in.
Q. Why is the tin foot-warmer covered with flannel?
A. 1st—To prevent the perspiration of the foot from taking off the polish of the tin:
2ndly—Flannel is a very bad conductor; and, therefore, helps to keep the tin hot longer: and
3rdly—If the feet were not protected, the conducting surface of the tin would feel painfully hot.
Q. What harm would it be if the polish of the tin were injured by the perspiration of our feet?
A. Polished tin throws off its heat very slowly; but dull, scratched, painted, or dirty tin, throws off its heat very quickly: if, therefore, the tin foot-warmer were to lose its polish, it would get cold in a much shorter time.
Q. Why are furnaces and stoves (where much heat is required) built of porous brick?
A. As bricks are bad conductors, they prevent the escape of heat: and are, therefore, employed where great heat is required.
Q. Why are furnace doors, &c. frequently covered with a paste of clay and sand?
A. Because this paste is a very bad conductor of heat; and, therefore, prevents the escape of heat from the furnace.
Q. If a stove be placed in the middle of a room, should it be made of bricks or iron?
A. A stove in the middle of a room should be made of iron; because iron is an excellent conductor, and rapidly communicates its heat to the air around.
Q. Why does the Bible say, that God “giveth snow like wool?”
A. As snow is a very bad conductor of heat, it protects vegetables and seeds from the frost and cold.
Q. How does the non-conducting power of snow protect vegetables from the frost and cold?
A. As snow is a bad conductor, it prevents the heat of the earth from being drawn off by the cold air which rests upon it.
Q. Why are woollens and furs used in cold weather for clothing?
A. Because they are very bad conductors of heat; and, therefore, prevent the warmth of the body from being drawn off by the cold air.
Q. Do not woollens and furs actually impart heat to the body?
A. No; they merely prevent the heat of the body from escaping.
Q. Where would the heat escape to, if the body were not wrapped in wool or fur?
A. The heat of the body would fly off into the air; for the cold air (coming into contact with our body) would gradually draw away its heat, till it was as cold as the air itself.
Q. What then is the principal use of clothing in winter-time?
A. To keep the body air-tight; and prevent the external air (or wind) from coming into contact with it, to absorb its heat.
Q. Why are beasts covered with fur, hair, or wool?
A. Because fur, hair, and wool are very slow conductors of heat; and (as dumb animals cannot be clad like human beings) God has given them a robe of hair or wool, to keep them warm.
Q. Why are birds covered with down or feathers?
A. Because down and feathers are very bad conductors of heat; and (as birds cannot be clad like human beings) God has given them a robe of feathers to keep them warm.
Q. Why are wool, fur, hair, or feathers such slow conductors of heat?
A. Because a great quantity of air lurks entangled between their fibres; and air is a very bad conductor of heat.
Q. If air be a bad conductor of heat, why should we not feel as warm without clothing, as when we are wrapped in wool and fur?
A. Because the air (which is cooler than our body) is never at rest; and, therefore, fresh particles (perpetually passing over our body) keep drawing off the heat little by little.
Q. Why does the ceaseless change of air tend to decrease the warmth of a naked body?
A. Thus:—the air which cases the body absorbs as much heat from it as it can, while it remains in contact; it is then blown away, and makes room for a fresh coat of air, which does the same.
Q. Does the air (which encases a naked body) become by contact as warm as the body itself?
A. It would do so, if it remained motionless; but as it remains only a very short time, it absorbs as much heat as it can in the time, and passes on.
Q. Why do we feel colder in windy weather, than in a calm day?
A. Because (in windy weather) the particles of air pass over us more rapidly; and every fresh particle takes from us some portion of heat.
Q. Show the wisdom of God in making the air a bad conductor.
A. If air were a good conductor (like iron and stone) the heat would be drawn so rapidly from our body, that we must be chilled to death. Similar evils would be felt also by all the animal and vegetable world.
Q. Does not the bad conducting power of air enable persons to judge whether an egg be new or stale?
A. Yes; touch your tongue against the shell at the larger end; if it feels warm to the tongue, the egg is stale; if not, it is new-laid.
Q. Why will the shell of a stale egg feel warm to the tongue?
A. Between the shell and the “white of the egg” there is a small quantity of air, which expands in a stale egg, from the shrinking of the white.
Q. Why does the expansion of air (at the end of an egg) make it feel warm to the tongue?
A. As air is a very bad conductor, the more air an egg contains, the less heat will be drawn from the tongue when it touches the shell.
Q. Why do ladies fan themselves in summer, to make their faces cool?
A. The fan puts the air in motion, and makes it pass more rapidly over their face; and (as the temperature of the air is always lower than that of the human face) each puff of air carries off some portion of heat from the face.
Q. Does fanning the air make the air itself cooler?
A. No; fanning makes the air hotter and hotter.
Q. Why does fanning the air increase its heat?
A. By causing the air continually to absorb heat from the human body which it passes over.
Q. If fanning makes the air hotter, how can it make a person feel cooler?
A. Fanning makes the air hotter, but the face cooler; because it keeps taking the heat out of the face, and giving it to the air.
Q. Why is broth cooled by blowing it?
A. The breath causes a rapid change of air to pass over the broth; and (as the air is not so hot as the broth) it keeps absorbing heat, and thus makes the broth cooler and cooler.
Q. Would not the air absorb heat from the broth just as well without blowing?
A. No; air is a very bad conductor; unless, therefore, the change be rapid, the air nearest the surface of the broth would soon become as hot as the broth itself.
Q. But would not the hot air part with its heat instantly to the circumjacent air?
A. No; not instantly. Air is so bad a conductor, that it parts with its heat very slowly: unless, therefore, the air be kept in continual motion, it would cool the broth very slowly indeed.
Q. Why does wind generally feel cool?
A. Wind is only air in motion; and the more quickly the air passes over our body, the more rapidly it absorbs the heat therefrom.
Q. Why does air absorb heat more quickly by being set in motion?
A. Because every fresh gust of air absorbs a fresh portion of heat; and the more rapid the succession of gusts, the greater will be the quantity of air absorbed.
Q. If the air were hotter than our body, would the wind feel cool?
A. No; if the air were hotter than our body, it would feel insufferably hot.
Q. Why would the air feel intensely hot, if it were warmer than our blood?
A. Because then the wind would add to the heat of our body, instead of diminishing it.
Q. Is the air ever as hot as the human body?
A. Not in this country: in the hottest summer’s day, the air is always 10 or 12 degrees cooler than the human body.
Q. Is the earth a good conductor of heat?
A. No; the power of conducting heat depends upon the continuity of matter; if the particles of which a thing is composed are not continuous, they have very little power to conduct heat.
Q. Why is the earth (below the surface) warmer in winter than the surface itself?
A. Because the earth is a bad conductor of heat; and, therefore (although the ground be frozen) the frost never penetrates above an inch or two below the surface.
Q. Why is the earth (below the surface) cooler in summer than the surface itself?
A. Because the earth is a bad conductor of heat; and, therefore, (although the surface be scorched with the burning sun) the intense heat cannot penetrate to the roots of the plants and trees.
Q. Shew the wisdom of God in making the earth a bad conductor.
A. If the heat and cold could penetrate the earth (as freely as the heat of a fire penetrates iron), the springs would be dried up in summer and frozen in winter, and all vegetation would perish.
Q. Why is water from a spring so cool in summer?
A. As the earth is a bad conductor, the burning rays of the sun can penetrate only a few inches below the surface; in consequence of which, the springs of water are not affected.
Q. Why is it cool under a shady tree in a hot summer’s day?
A. 1st—Because the overhanging foliage screens off the rays of the sun:
2ndly—As the rays of the sun are warded off, the air (beneath the tree) is not heated by the reflection of the earth: and
3rdly—The leaves of trees, being non-conductors, allow no heat to penetrate through them.
Q. Why do the laplanders wear skins, with the fur inwards?
A. The dry skin prevents the wind from penetrating to their body; and as the fur contains a quantity of air between its hairs (which soon becomes heated by the body) the Laplander is clad in a case of hot air, impervious to the cold and wind.
Q. Why does a linen shirt feel colder than a cotton one?
A. Linen is a much better conductor than cotton; and, therefore, (as soon as it touches the body) it more rapidly draws away the heat, and produces a sensation of cold.
Q. Why is the face cooled by wiping the temples with a fine cambric handkerchief?
A. The fine fibres of the cambric have a strong capillary attraction for moisture; and are excellent conductors of heat: thus the moisture and heat are both abstracted from the face, and a sensation of coolness is produced.
“Capillary attraction,” i. e. the attraction of a thread or hair. The wick of a candle is wet with grease, because the melted tallow runs up the cotton from capillary attraction.
Q. Why would not a cotton handkerchief do as well?
A. The coarse fibres of cotton have much less capillary attraction, and are nothing like such good conductors as linen: and, therefore, wiping the face with a cotton handkerchief, increases the sensation of warmth.
CHAPTER XIV.
2.—ABSORPTION OF HEAT.
Q. What is the difference between conducting heat, and absorbing heat?
A. To conduct heat, is to transmit it from one body to another through a conducting medium: to absorb heat, is to suck it up, as a sponge sucks up water.
Q. Give me an example.
A. Black cloth absorbs, but does not conduct heat: thus, if black cloth be laid in the sun, it will absorb the rays very rapidly; but if one end of the black cloth be made hot, it would not conduct the heat to the other end.
Q. Are good conductors of heat, good absorbers also?
A. No; every good conductor of heat is a bad absorber of it; and no good absorber of heat can be a good conductor also.
Q. Is iron a good absorber of heat?
A. No; iron is a good conductor, but a very bad absorber of heat.
Q. Why do the fender and fire-irons (which lie upon it) remain cold, although they are before a good fire?
A. Because the metal fender and fire-irons have very little capacity for absorbing heat; although they are soon made hot (by conduction), when placed in contact with the hot fire or stove.
Q. Why does a kettle boil faster, when the bottom and back are covered with soot?
A. The black soot absorbs heat very quickly from the fire, and the metal conducts it to the water.
Q. Why will not a new kettle boil so fast as an old one?
A. Because the bottom and sides of a new kettle are clean and bright; but in an old kettle are covered with soot.
Q. Why would the kettle be slower boiling, if the bottom and back were clean and bright?
A. Bright metal does not absorb heat, but reflect it (i. e. throw the heat back again); and as the heat is thrown off from the surface of bright metal, therefore, a new kettle is longer boiling.
Q. Why do we wear white linen and a black outer dress, if we want to be warm?
A. The black outer dress quickly absorbs heat from the sun, and conveys it to the body; and the white linen (being a bad absorbent) abstracts no heat from the warm body.
Q. Why do persons wear white dresses in summer time?
A. White throws off the heat of the sun by reflection, and is, therefore, a very bad absorbent of heat; in consequence of which, it never becomes so hot from the scorching sun as dark colours do.
Q. Why do not persons wear white dresses in winter time?
A. White will not absorb heat, like black and other dark colours; and, therefore, white dresses are not so warm as dark ones.
Q. What colours are warmest for dresses?
A. For outside garments black is the warmest, and then such colours as approach nearest to black (as dark blue and green). White is the coldest colour for external clothing.
Q. Why are dark colours (for external wear) so much warmer than light ones?
A. Because dark colours absorb heat from the sun more abundantly than light ones.
Q. How can you prove that dark colours are warmer than light ones?
A. If a piece of black cloth and a piece of white were laid upon snow, in a few hours the black cloth will have melted the snow beneath; whereas the white cloth will have produced little or no effect upon it at all.
N. B. The darker any colour is, the warmer it is, because it is a better absorbent of heat. The order may be thus arranged:—1. Black (warmest of all).—2. Violet.—3. Indigo.—4. Blue.—5. Green.—6. Red.—8. Yellow: and 9. White (coldest of all).
Q. Why are black kid gloves so hot in summer time?
A. 1st—Because the black absorbs the solar heat: and
2ndly—The kid will not allow the heat of the hand to escape through the glove.
Q. Why are lisle thread gloves so cool in summer time?
A. 1st—Because thread absorbs the perspiration of the hands: and
2ndly—It conducts away the heat of our hot hands.
Q. Are Lisle thread gloves absorbents of heat?
A. As Lisle thread gloves are generally of a grey or lilac colour, they do not absorb solar heat.
Q. Why is a plate-warmer made of un-painted bright tin?
A. Bright tin reflects (or throws back) the heat, which issues from the fire in rays; and (by reflecting the heat upon the meat) assists greatly in roasting it.
Q. Why would not the tin reflector do as well if it were painted?
A. If the tin reflector were painted, it would be utterly spoiled, because it would then absorb heat, and not reflect it at all. A plate-warmer should be kept very clean, bright, and free from all scratches.
Q. Why should a reflector be kept so very clean and free from scratches?
A. If a reflector be spotted, dull, or scratched, it will absorb heat, instead of reflecting it; and, therefore, would be of no use whatsoever as a reflector.
Q. Why does hoar-frost remain on tombstones, long after it has melted from the grass and gravel-walks of a church-yard?
A. Tomb-stones being white, will not absorb heat, like the darker grass and gravel; and, therefore, the white tombstones (being so much colder) retain the hoar-frost after it has melted from other things.
Q. If black absorbs heat, why have those who live in hot climates black skins, and not white skins (which would not absorb heat at all)?
A. Though the black skin of the negro absorbs heat more plentifully than the white skin of a European, yet the blackness prevents the sun from blistering or scorching it.
Q. How is it known that the black colour prevents the sun from either blistering or scorching the skin?
A. If you put a white glove on one hand, and a black glove on the other (when the sun is burning hot), the hand with the white glove will be scorched, but not the other.
Q. Which hand will feel the hotter?
A. The hand with the black glove will feel the hotter, but it will not be scorched by the sun; whereas the hand with the white glove (though much cooler) will be severely scorched.
Q. Why does the black skin of a negro never scorch or blister with the hot sun?
A. Because the black colour absorbs the heat,—conveys it below the surface of the skin, and converts it to sensible heat and perspiration.
Q. Why does the white European skin blister and scorch when exposed to the hot sun?
A. Because the white will not absorb the heat; and, therefore, the hot sun rests on the surface of the skin, and scorches it.
Q. Why has a negro black eyes?
A. The black colour of a negro’s eyes defends them from the strong light of the tropical sun. If a negro’s eyes were not black, the sun would scorch them, and every negro would be blind.
Q. Why is water kept cooler (in summer time) in a bright tin pot, than in an earthen one?
A. Because bright metal will not absorb the heat of the summer sun, like an earthen vessel.
Q. Why is boiling water kept hot in a bright tin vessel longer, than in an earthen one?
A. Because bright tin will not suffer the heat of the boiling water to escape in rays, as an earthen vessel does.
CHAPTER XV.
3.—REFLECTION OF HEAT.
Q. What is meant by reflecting heat?
A. To reflect heat, is to throw it back in rays from the surface of the reflecting body, towards the place from whence it came.
Q. What are the best reflectors of heat?
A. All bright surfaces, and light colours.
Q. Are good absorbers of heat good reflectors also?
A. No; those things which absorb heat best, reflect heat worst; and those which reflect heat worst, absorb it best.
Q. Why are those things which absorb heat unable to reflect it?
A. Because if any thing sucks in heat like a sponge, it cannot throw it off from its surface; and if any thing throws off heat from its surface, it cannot drink it in.
Q. Why are reflectors always made of light-coloured and highly polished metal?
A. Because light coloured and highly polished metal makes the best of all reflectors.
Q. Why do not plate-warmers blister and scorch the wood behind?
A. Because the bright tin front throws the heat of the fire back again, and will not allow it to penetrate to the wood behind.
Q. If metal be such an excellent conductor of heat, how can it reflect heat, or throw it off?
A. Polished metal is a conductor of heat, only when that heat is communicated by actual contact; but whenever heat falls upon bright metal in rays, it is reflected back again, and the metal remains quite cool.
Q. What is meant by “heat falling upon metal in rays,” and not “by contact”?
A. If a piece of tin were thrust into a fire, it would be in actual contact with the fire; but if it be held before a fire, the heat of the fire falls upon it in rays.
Q. What is the use of the tin screen or reflector used in roasting?
A. The tin reflector throws the heat of the fire back upon the meat; and, therefore, assists the process of roasting and helps to keep the kitchen cool.
Q. How does a tin reflector tend to keep the kitchen cool?
A. Because it confines the heat to the hearth, and prevents it from being dispersed throughout the kitchen.
Q. Why does a lamp glass diminish the smoke of a lamp?
A. As glass is a reflector, it reflects the heat of the lamp back upon the flame; in consequence of which, less carbon escapes unconsumed (as smoke).
Q. Why are shoes hotter for being dusty?
A. 1st—Because dust absorbs heat: and
2ndly—As it destroys the blackness of our shoes, it prevents them from throwing off the heat of our feet in rays.
Q. Why can we not see into the road or street, when a candle is lighted in a room?
A. Glass is a reflector; and, therefore, throws the rays of the candle back into the room, and thus prevents our seeing into the road or street.
Q. Why can persons in the dark street see into a room (lighted by a candle or lamp)?
A. The pupil of the eye expands greatly, when persons are in the dark; and, therefore, when any one in the dark street looks into a light room, his dilated pupil sees every thing distinctly.
Q. Why does it always freeze on the top of a mountain?
A. Air is heated by the reflection of the earth, and not by the rays of the sun; and, as there is no earth round a mountain-top to reflect heat, therefore, it remains intensely cold.
CHAPTER XVI.
4.—RADIATION.
Q. What is meant by radiation?
A. Radiation means the emission of rays: thus the sun radiates both light and heat; that is, it emits rays of light and heat in all directions.
Q. When is heat radiated from one body to another?
A. When the two bodies are separated by a non-conducting medium: thus the sun radiates heat towards the earth, because the air comes between (which is a very bad conductor).
Q. On what does radiation depend?
A. On the roughness of the radiating surface: thus if metal be scratched, its radiating power is increased, because the heat has more points to escape from.
Q. Does a fire radiate heat?
A. Yes; and because burning fuel emits rays of heat, therefore we feel warm when we stand before a fire.
Q. Why does our face feel uncomfortably hot, when we approach a fire?
A. Because the fire radiates heat upon the face; which (not being covered) feels the effect immediately.
Q. Why does the fire catch the face more than the rest of the body?
A. The rest of the body is covered with clothing, which (being a bad conductor of heat) prevents the same sudden and rapid transmission of heat to the skin.
Q. Do those substances which radiate heat, absorb heat also?
A. Yes. Those substances which radiate most, also absorb most heat: and those which radiate least, also absorb the least heat.
Q. Does any thing else radiate heat, besides the sun and fire?
A. Yes; all things radiate heat in some measure, but not equally well.
Q. What things radiate heat the next best to the sun and fire?
A. All dull and dark substances are good radiators of heat; but all light and polished substances are bad radiators of heat.
Q. Why does a polished metal tea-pot make better tea than a black earthen one?
A. As polished metal is a very bad radiator of heat, it keeps the water hot much longer; and the hotter the water is, the better it “draws” the tea.
Q. Why will not a dull black tea-pot make good tea?
A. Because the heat of the water flies off so quickly through the dull black surface of the tea-pot, that the water is rapidly cooled, and will not “draw” the tea.
Q. Do not pensioners, and most aged cottagers, prefer the little black earthen tea-pot to the bright metal one?
A. Yes; because they set it on the hob “to draw;” in which case, the little black tea-pot will make the best tea.
Q. Why will a black tea-pot make better tea than a bright metal one, if it be set upon the hob to draw?
A. Because the black tea-pot will absorb heat plentifully from the fire, and keep the water boiling hot: whereas, a bright metal tea-pot (set upon the hob) would throw off the heat by reflection.
Q. Then sometimes a black earthen tea-pot is the best, and sometimes a bright metal one?
A. Yes; when the tea-pot is set on the hob “to draw,” the black earth is the best, because it absorbs heat: but when the tea-pot is not set on the hob, the bright metal is the best, because it radiates heat very slowly, and therefore keeps the water hot.
Q. Why does a saucepan which has been used, boil quicker than a new one?
A. Because the bottom and back are covered with soot; and the black soot rapidly absorbs the heat of the glowing coals.
Q. Why should the front and lid of a saucepan be clean and bright?
A. As they do not come in contact with the fire, they cannot absorb heat; and (being bright) they will not suffer the heat to escape by radiation.
Q. In what state should a saucepan be, in order that it may boil quickly?
A. All those parts which come in contact with the fire should be covered with soot, to absorb heat; but all the rest of the saucepan should be as bright as possible, to prevent the escape of heat by radiation.
Q. Why is it said that “Saturday’s kettle boils the fastest?”
A. Because on Saturday the front and top of the kettle are generally cleaned and polished; but the bottom and back of the kettle are never cleaned.
Q. Why should not the bottom and back of a kettle be cleaned and polished?
A. Because they come in contact with the fire, and (while they are covered with black soot) absorb heat freely from the burning coals.
Q. Why should the front and top of a kettle be clean and well polished?
A. Because polished metal will not radiate heat; and, therefore, (while the front and top of the kettle are well polished) the heat is kept in, and not suffered to escape by radiation.
Q. Why is the inside of a kettle and saucepan white?
A. White will not radiate heat: if, therefore, the inside of a boiler be white, the liquor in it is kept hot much longer.
Q. Why is the bottom of a kettle nearly cold, when the water is boiling hot?
A. Black soot is a very bad conductor of heat; and, therefore, the heat of the boiling water is some considerable time, before it gets through the soot which adheres to the bottom of the kettle.
Q. Why is the lid of a kettle so intensely hot, when the water boils?
A. The bright metal lid of the kettle is an admirable conductor of heat; and, therefore, the heat from the boiling water pours into our hand the moment we touch it.
Q. Show the benefit of smoke in cooking.
A. The carbon of the fuel (which flies off in smoke) naturally blackens all culinary vessels set upon the fire to boil, and thus renders them fit for use.
(“Culinary vessels” are vessels used in kitchens for cooking, as saucepans, boilers, kettles, &c.)
Q. How does smoke make culinary vessels fit for use?
A. If it were not for the smoke, (which gathers round a kettle or saucepan) heat would not be absorbed, and the process of boiling would be greatly retarded.
Q. Why is boiling water kept hot best in a bright metal pot?
A. Because bright metal being a bad radiator will not throw off the heat of the boiling water from its surface.
Q. Why is water kept cold in summer-time in a bright metal pot, better than in an earthen vessel?
A. Because bright metal will not absorb heat from the hot air, like an earthen vessel; in consequence of which, the water is kept cooler.
Q. Why are dinner-covers made of bright tin or silver?
A. Light-coloured and highly-polished metal is a very bad radiator of heat; and, therefore, bright tin or silver will not allow the heat of the cooked food to escape through the cover by radiation.
Q. Why should a meat-cover be very brightly polished?
A. If the cover be dull or scratched it will absorb heat from the hot food beneath it; and (instead of keeping it hot) will make it cold.
Q. Why should a silver meat-cover be plain, and not chased?
A. If the cover be chased, it will absorb the heat of the food covered by it; and instead of keeping it hot, will make it cold by absorption.
Q. What is dew?
A. Dew is the vapour of the air condensed, by coming in contact with bodies colder than itself.
Q. Why is the ground sometimes covered with dew?
A. The earth is more heated by solar rays than the air, during the day; but at night, the earth parts with more heat than the air, and becomes (in consequence) 5 or 10 degrees colder.
Q. How does the earth being colder than the air account for the deposition of dew?
A. As soon as the air touches the cold earth, its warm vapour is chilled, and condensed into dew.
Q. Why is the surface of the ground colder in a fine clear night, than in a cloudy one?
A. On a fine clear star-light night, heat radiates from the earth freely, and is lost in open space: but on a cloudy night, the clouds arrest the process of radiation.
Q. Why is dew deposited only on a fine clear night?
A. Because, when the night is clear and fine, the surface of the ground radiates heat most freely; and (being cooled down by this loss of heat) chills the vapour of the air into dew.
Q. Why is there no dew on a dull cloudy night?
A. The clouds arrest the radiation of heat from the earth; and (as the heat cannot freely escape) the surface is not sufficiently cooled down to chill the vapour of the air into dew.
Q. Why is a cloudy night warmer than a fine one?
A. Because the clouds prevent the radiation of heat from the earth; and, therefore, the surface of the earth remains warmer on a dull cloudy night.
Q. Why is dew most abundant in situations most exposed?
A. Because the radiation of heat is not arrested by houses, trees, hedges, or any other thing.
Q. Why is there scarcely any dew under a shady tree?
A. The shady head of the tree both arrests the radiation of heat from the earth, and also radiates some of its own heat towards the earth; and, therefore, the ground (underneath a tree) is not sufficiently cooled down to chill the vapour of the air into dew.
Q. Why is there never much dew at the foot of walls and hedges?
A. 1st—Because the wall or hedge acts as a screen, to arrest the radiation of heat from the earth: and
2ndly—The wall or hedge also radiates some portion of heat towards the earth.
Q. How do these things prevent the deposition of dew?
A. As the ground (beneath a wall, tree, or hedge) is not cooled by the radiation of heat, it remains of the same temperature as the air above it; in consequence of which, the vapours of the air are not chilled by it into dew.
Q. Why is there little or no dew beneath a flower-awning, although that awning be open on all four sides?
A. 1st—Because the awning arrests the radiation of heat from the ground beneath: and
2ndly—It radiates some of its own heat downwards; in consequence of which, the ground beneath an awning is not sufficiently cooled down to chill the vapour of air into dew.
Q. How can a thin covering of bass or even muslin protect trees from frost?
A. Because any covering prevents the radiation of heat from the tree; and if the tree be not cooled down by radiation, the vapour of the air will not be frozen as it comes in contact with it.
Q. Why is the bass or canvass itself (which covers the tree) always drenched with dew?
A. The bass or canvass covering radiates heat both upwards and downwards; and is, therefore, so cooled down, that it readily chills all the vapour of the air (which passes over it) into dew.
Q. Why does snow at the foot of a hedge or wall melt sooner, than in an open field?
A. Because the hedge or wall radiates heat into the snow beneath, which melts it.
Q. Why is there no dew after a windy night?
A. 1st—Because the wind evaporates the moisture, as fast as it is deposited; and
2ndly—It disturbs the radiation of heat, and diminishes the deposition of dew thereby.
Q. Why are VALLEYS & HOLLOWS often thickly covered with dew, although they are sheltered?
A. The surrounding hills prevent the repose of air (in the valleys) from being disturbed; but do not overhang and screen them, so as to arrest their radiation.
Q. Why does dew fall more abundantly on some things than upon others?
A. Because some things radiate heat more freely than others, and therefore become much cooler in the night.
Q. Why are things which radiate heat most freely, always the most thickly covered with dew?
A. Because the vapour of the air is chilled into dew, the moment it comes in contact with them.
Q. What kind of things radiate heat most freely?
A. Grass, wood, and the leaves of plants, radiate heat very freely: but polished metal, smooth stones, and woollen cloth, part with their heat very tardily.
Q. Do the leaves of all plants radiate heat equally well?
A. No. Rough woolly leaves (like those of a holly-hock) radiate heat much more freely, than the hard smooth polished leaves of a common laurel.
Q. Shew the wisdom of God in making grass, the leaves of trees, and all vegetables, excellent radiators of heat.
A. As vegetables require much moisture, and would often perish without a plentiful deposit of dew, God wisely made them to radiate heat freely, so as to chill the vapour (which touches them) into dew.
Q. Will polished metal, smooth stones, and woollen cloth, readily collect dew?
A. No. While grass and the leaves of plants are completely drenched with dew, a piece of polished metal, or of woollen cloth (lying on the same spot) will be almost dry.
Q. Why would polished metal and woollen cloth be dry, while grass and leaves are drenched with dew?
A. Because the polished metal and woollen cloth part with their heat so slowly, that the vapour of the air is not chilled into dew as it passes over them.
Q. Why is a gravel walk almost dry, when a grass plat is covered thick with dew?
A. Grass, (being a good radiator) throws off its heat very freely; but gravel (being a very bad radiator) parts with its heat very reluctantly.
Q. Is that the reason why grass is saturated with dew, and the gravel is not?
A. Yes. When the vapour of warm air comes in contact with the cold grass, it is instantly chilled into dew; but (as the gravel is not so cold as the grass) the vapour of air is not so freely condensed as it passes over the gravel.
Q. Why does dew rarely fall upon hard rocks and barren lands?
A. Rocks and barren lands are so compact and hard, that they can neither absorb nor radiate much heat; and (as their temperature varies but very little) very little dew distils upon them.
Q. Why does dew fall more abundantly on cultivated soils, than on barren lands?
A. Because cultivated soils (being loose and porous) absorb heat freely during the day, and radiate it by night; and (being much cooled by the rapid radiation of heat) as the vapour of the air passes over them, it is plentifully condensed into dew.
Q. Shew the wisdom of God in this arrangement.
A. Every plant and inch of land which needs the moisture of dew, is adapted to collect it; but not a single drop even of dew is wasted, where its refreshing moisture is not required.
Q. Shew the wisdom of God in making polished metal and woollen cloth bad radiators of heat.
A. If polished metal collected dew as easily as grass, it could never be kept dry, and free from rust. Again, if woollen garments collected dew as readily as the leaves of trees, we should be often soaking wet, and subject to constant colds.
Q. Shew how this affords a beautiful illustration of Gideon’s miracle, recorded in the book of Judges, VI. 37, 38.
A. The fleece of wool (which is a very bad radiator of heat) was soaking wet with dew: when the grass (which is a most excellent radiator) was quite dry.
Q. Was not this contrary to the laws of nature?
A. Yes; and was, therefore, a plain demonstration of the power of God, who could change the very nature of things at his will.
Q. Why do our clothes feel damp, after walking in a fine evening in spring or autumn?
A. Because the vapour (condensed by the cold earth) lights upon them, like dew.
Q. Why are windows often covered with thick mist, and the frames wet with standing water?
A. The temperature of the external air always falls at sun-set, and chills the window-glass, with which it comes in contact.
Q. How does this account for the mist and water on a window?
A. As the warm vapour of the room touches the cold glass, it is chilled and condensed into mist; and the mist (collecting into drops) rolls down the window-frame in little streams of water.
Q. Does the glass of a window cool down more rapidly than the air of the room itself?
A. Yes; because the air is kept warm by fires, and the animal heat of the people in the room; in consequence of which, the air of a room suffers very little diminution of heat from the setting of the sun.
Q. Whence arises the vapour of a room?
A. 1st—The very air of the room contains vapour:
2ndly—The breath and insensible perspiration of the inmates increase this vapour: and
3rdly—Hot dinners, the steam of tea, &c. contribute to increase it still more.
Q. What is meant by “the insensible perspiration?”
A. From every part of the human body an insensible and invisible perspiration issues all night and day; not only in the hot weather of summer, but also in the coldest day of winter.
Q. If the perspiration be both insensible and invisible, how is it known that there is any such perspiration?
A. If you put your naked arm into a clean dry glass cylinder, the perspiration of your arm will soon condense on the glass, like mist.
Q. Why are carriage windows very soon covered with thick mist?
A. The warm vapour of the carriage is condensed the moment it touches the cold glass, and covers it over with a thick mist.
Q. Why is the glass window cold enough to condense the vapour of the carriage?
A. Because the inside of the carriage is much warmer than the outside, and the glass window is made cold by contact with the external air.
Q. Where does the warm vapour of the carriage come from?
A. The warm breath and insensible perspiration of the persons riding in the carriage, load the air of it with warm vapour.
Q. What is the cause of the pretty frost-work seen on bed-room windows in winter-time?
A. The breath and insensible perspiration of the sleeper (coming in contact with the ice-cold window) is frozen by the cold glass, and forms those beautiful appearances seen in our bed-rooms in a winter morning.
Q. Why is the glass of a window colder than the walls of a room?
A. Glass is a very excellent radiator; and, therefore, most rapidly parts with its heat.
Q. Why is a tumbler of cold water made quite dull with mist, when brought into a room full of people?
A. Because the hot vapour of the room (coming in contact with the cold tumbler) is condensed upon it; and changes its invisible and gaseous form for that of a thick mist.
Q. Why is a glass made quite dull, by laying a hot hand upon it?
A. The insensible perspiration of the hot hand is condensed upon the cold glass, and thus made perceptible.
Q. Why are wine-glasses made quite dull when they are brought into a room full of company?
A. The hot vapour of the room (coming in contact with the cold wine-glasses) is condensed upon them, and covers them with vapour like dew.
Q. Why does this misty appearance go off after a little time?
A. Because the glass becomes of the same temperature as the air of the room, and will no longer chill the vapour which touches it, and condense it into mist.
Q. Why is a wine-glass (brought out of a cellar into the air) covered with a thick mist in summer-time?
A. The vapour of the hot air is condensed by the cold glass, and covers it as a thick mist.
Q. Why does breathing on a glass make it quite dull?
A. Because the hot breath is condensed by the cold glass; and, therefore, covers it with a thick mist.
Q. Why do walls stand thick with wet in a sudden thaw?
A. The walls (being thick) cannot change their temperature so fast as the thin air can; and, therefore, they retain their cold after the thaw has set in.
Q. How does retaining their cold account for their being so wet?
A. As the vapour of the warm air touches the cold wall, it is chilled and condensed into water, which sticks to the wall, and sometimes trickles down in little streams.
Q. Why does a thick well-built house contract more damp of this kind, than an ordinary one?
A. Because the walls are much thicker; and (if the frost has penetrated far into the bricks) it takes a long time to reduce them to the same temperature as the air.
Q. Why are banisters, &c. damp after a thaw?
A. The wooden banister (being made of some very close-grained, varnished wood) cannot change its temperature so fast as the air; and, therefore, remains cold some time after the thaw has set in.
Q. How does this account for the banisters being damp?
A. The vapour of the warm air (coming in contact with the cold banister) is chilled, and condensed into water upon it.
Q. Why is our breath visible in winter and not in summer?
A. In winter the coldness of the air condenses our breath into visible vapour; but in summer the air is not cold enough to condense it into visible vapour.
Q. Why are our hair and the brim of our hat often covered with little drops of pearly dew in winter-time?
A. The breath (issuing from our mouth and nose) is condensed into drops, as it comes in contact with our cold hair or hat; and (being condensed) hangs there in little dew-drops.
Q. Why does the steam of a railway boiler often pour down, like fine rain, when the steam is “let off?”
A. The steam from the steam-pipe (when the air is cold) is condensed by contact with the chill air, and falls like fine rain.
Q. Why is there less dew when the wind is easterly, than when the wind is westerly?
A. Easterly winds cross the continent of Europe, and, (as they pass over land) are dry and arid; but westerly winds cross the Atlantic Ocean; and (as they pass over water) are moist and full of vapour.
Q. How does the dryness of an eastern wind prevent dew-falls?
A. As the easterly winds are dry, they imbibe the moisture of the air; and, therefore, there is very little left to be condensed into dew.
Q. How does the moistness of a western wind promote dew-falls?
A. As the westerly winds are saturated with vapour, they require a very little reduction of heat to cause a copious deposition of dew.
Q. When is dew most copiously distilled?
A. After a hot day in summer or autumn, with the wind in the west.
Q. Why is dew distilled most copiously after a hot day?
A. Because the surface of the earth radiates heat very freely at sunset; and (becoming thus much colder than the air) chills its vapour, and condenses it into dew.
Q. Does not air radiate heat, as well as the earth and its various plants?
A. No. The air never radiates heat, nor is the air itself made hot by the rays of the sun.
Q. How is the air made hot or cold?
A. By convection of hot or cold currents.
Q. What is meant by “convection of hot and cold currents?”
A. The air (which is heated by the surface of the earth) ascends, warming the air through which it passes. Other air (being warmed in a similar way) also ascends, carrying heat; till all the air is made hot.
Q. Is the air made cold in a similar way?
A. Yes. The air resting on the earth is made cold by contact: this cold air makes the air above it cold; and cold currents or winds shake the whole together, till all becomes of one temperature.
Q. Why is meat very subject to taint on a moon-light night?
A. In a bright moon-light night, meat radiates heat very freely; and is, therefore, soon covered with dew, which produces rapid decomposition.
Q. Why do plants grow rapidly in moon-light nights?
A. In bright moon-light nights rapid radiation is carried on, and dew is plentifully deposited on young plants, which conduces much to their growth and vigour.
Q. Why is evening dew injurious to health?
A. Because the condensed vapours are always laden with noxious exhalations from the earth: this is especially the case in marshy countries.
Q. Is honey-dew a similar thing to dew?
A. No. Honey-dew is a sweet liquid shed by a very small insect (called the aphis), and deposited in autumn on the under surface of favourite leaves.
Q. Does honey-dew injure leaves, or do them good?
A. It injures them very much, because it fills the pores of the leaf with a thick clammy liquid; and, therefore, prevents the leaf from transpiring and absorbing.
Q. What effect has honey-dew upon the appearance of a leaf?
A. After a little time, the leaf (being smothered and starved) begins to turn a dingy yellow.
Q. Are not ants very fond of honey-dew?
A. Yes; and they crawl up the loftiest trees, in order to obtain it.
Q. What is the cause of mist (or earth-fog)?
A. If the night has been very calm, a rapid radiation of heat has taken place in the earth; in consequence of which, the air (resting on the earth) is made so cold, that its vapour is chilled, and condensed into a thick mist.
Q. Why does not the mist become dew?
A. Because the chill of the air is so rapid, that vapour is condensed faster than it can be deposited; and (covering the earth in a mist) prevents any further radiation of heat from the earth.
Q. When the earth can no longer radiate heat upwards, does it continue to condense the vapour of the air?
A. No; the air (in contact with the earth) becomes about equal in temperature with the surface of the earth itself; for which reason, the mist is not condensed into dew, but remains floating above the earth as a thick cloud.
Q. Why does this mist seem to rise higher and higher, and yet remain quite as dense below as before?
A. The air resting on the earth is first chilled, and chills the air resting on it; the air which touches this new layer of mist being also condensed, layer is added to layer; and the mist seems to be rising, when (in fact) it is only deepening.
Q. Why does mist and dew vanish as the sun rises?
A. Because the condensed vapour is again rarefied by the heat of the sun, and separated into invisible particles.
Q. Why is a dew-drop round?
A. Because every part of the drop is equally balanced; and, therefore, there is no cause why one part of the drop should be further from the centre than another.
Q. Why is the dew-drop on a broad leaf sometimes flattened?
A. Whenever two or more drops of dew roll together, they make one large spheroid (or flattened drop).
Q. Why will DEW-DROPS ROLL ABOUT CABBAGE-PLANTS, POPPIES, &c. without wetting the surface?
A. The leaves of cabbages and poppies are covered with a very fine powder; and the dew-drop rolls over this fine powder, as a drop of rain over dust, without wetting the surface.
Q. Why does not the drop of rain wet the dust over which it rolls?
A. Because it is driven from grain to grain by capillary repulsion.
Q. Why does not the dew-drop wet the powder of the cabbage-plant?
A. Because it is driven from grain to grain by capillary repulsion.
Q. Why will dew-drops roll over roses, &c. without wetting their petals?
A. The leaves of a rose contain an essential oil, which prevents them from absorbing the dew immediately.
Q. Why can a swan or duck dive under water without being wetted?
A. Because their feathers are covered with an oily secretion, which repels the water.
Q. What is the cause of mist?
A. When currents of air from land mix with currents of air from water, the currents from the water are condensed into mist by the colder currents blowing from the land.
Q. Why are the currents of air from the land colder than those blowing over water?
A. Because the earth radiates heat very freely, and (being greatly cooled down) cools the air also which comes in contact with it.
Q. Why is not the air, which passes over water, so cool as that which passes over land?
A. Because water does not cool down at sun-set, so fast as the land does; and, therefore, the air in contact with it is warmer.
Q. Why does not water cool down so fast as land?
A. 1st—Because the surface of water is perpetually changing, and as fast as one surface is made cold, another is presented: and
2ndly—The moment water is made cold it sinks, and warmer portions of water rise to occupy its place: therefore, before the surface of water is cooled, the whole volume must be made cold; which is not the case with land.
Q. What is the cause of a “pea-soup” london fog?
A. These fogs (which occur generally in the winter time) are occasioned thus:—Some current of air (being suddenly cooled) descends into the warm streets, preventing the rise of the smoke, and forcing it back in a mass towards the earth.
Q. Why are there not always fogs every night?
A. Because the air will always hold in solution a certain quantity of vapour, (which varies according to its temperature): and when the air is not saturated with vapour, it may be condensed without parting with it.
Q. Why are there ever fogs at night?
A. If the air be pretty well saturated with vapour during the day, as soon as its capacity for holding vapour is lessened by the cold night, it deposits some of the superabundant vapour in the form of dew or fog.
Q. Why is there very often a fog over marshes and rivers at night-time?
A. The air of marshes is almost always near saturation; and, therefore, the least depression of temperature, will compel it to relinquish some part of its moisture in dew or fog.
Q. What is the difference between dew and rain?
A. In dew, the condensation is made near the earth’s surface:
In rain, the drops fall from a considerable height; but the cause of both is the same, viz.—cold condensing the vapour of the air, when it is near the point of saturation.
Q. Why does mist and fog vanish at sunrise?
A. Because the condensed particles are again changed into invisible vapour, by the heat of the sun.
Q. What is the difference between a mist and fog?
A. Mist is generally applied to vapours condensed on marshes, rivers, and lakes.
Fog is generally applied to vapours condensed on land, especially if those vapours are laden with smoke.
Q. What is the reason why condensed vapour sometimes forms into clouds, and sometimes into fog?
A. If the surface of the earth be hotter than the air, then the vapour of the earth (being chilled by the cold air) becomes fog: but if the air be hotter than the earth, the vapour rises through the air, and becomes cloud.
Q. If cold air produces fog, why is it not foggy on a frosty morning?
A. 1st—Because less vapour is formed on a frosty day; and
2ndly—The vapour is frozen upon the ground before it can rise from the earth, and becomes hoar-frost.
Q. Why are fogs more general in autumn than in spring?
A. In spring the earth is not so hot as it is in autumn. In autumn the earth is generally warmer than the air; and, therefore, the vapour (issuing from the earth) is condensed into fog by the chill air.
Q. Why are fogs more common in valleys than on hills?
A. 1st—Because valleys contain more moisture than hills: and
2ndly—They are not exposed to so much wind, (which dissipates the vapour).
Q. How does wind dissipate fogs?
A. Either by blowing them away; or else by dissolving them into vapour again.
Q. What is hoar-frost?
A. There are two sorts of hoar-frost: 1.—Frozen dew: and 2.—Frozen fog.
Q. What is the cause of the ground hoar-frost, or frozen dew?
A. Very rapid radiation of heat from the earth; in consequence of which, the surface is so cooled down, that it freezes the dew condensed upon it.
Q. Why is hoar-frost seen only after a very clear night?
A. Unless the night has been very clear indeed, the earth will not have thrown off heat enough by radiation, to freeze the vapour condensed upon its surface.
Q. Why does hoar-frost very often cover the ground and trees, when the water of rivers is not frozen?
A. Hoar-frost is not the effect of cold in the air, but the cold of the earth (produced by excessive radiation); in consequence of which, the dew (condensed upon it) is frozen.
Q. Why is the hoar-frost upon grass and vegetables much thicker than that upon lofty trees?
A. Because the air (resting on the surface of the ground) is much colder after sun-set, than the air higher up; in consequence of which, more vapour is condensed and frozen there.
Q. Why is the air (resting on the surface of the earth) colder than that in the higher regions?
A. Because the earth radiates more heat than the leaves of lofty trees; and, therefore, condenses and freezes the vapour of the air more rapidly.
Q. Why are evergreens often frost-bitten, when lofty trees are not?
A. Evergreens do not rise far above the surface of the earth; and (as the air contiguous to the earth is much colder than that in the higher regions) therefore, the low evergreen is often frost-bitten, when the lofty tree is uninjured.
Q. Why are tomb-stones covered with hoar-frost, long after it has melted from every object around?
A. White is a very bad absorbent of solar heat; and, therefore, the white tomb-stone remains too cold to thaw the frost congealed upon its surface.
Q. Why is there little or no hoar-frost under shrubs and shadowy trees?
A. 1st—Because the leafy shrubs and trees arrest the process of radiation from the earth: and
2ndly—Shrubs and trees radiate a little heat towards the earth; and, therefore, the ground beneath is never cold enough to congeal the little dew which rests upon it.
Q. What is the cause of that hoar-frost which arises from frozen fog?
A. The thick fog (which invested the earth during the night) is condensed by the cold frost of early morning, and congealed upon every object with which it comes in contact.
CHAPTER XVII.
5.—CONVECTION.
Q. What is meant by the convection of heat?
A. Heat communicated by being carried to another thing or place; as the hot water resting on the bottom of a kettle, carries heat to the water through which it passes. (see p. [246]).
Q. Are liquids good conductors of heat?
A. No; liquids are bad conductors; and are, therefore, made hot by convection.
Q. Why are liquids bad conductors of heat?
A. Because heat converts a liquid into steam, and flies off with the vapour, instead of being conducted through the liquid.
Q. Explain how water is made hot?
A. The water nearest the fire is first heated, and (being heated) rises to the top; other cold water succeeds, is also heated, and rises in turn; and this interchange keeps going on, till all the water boils.
Q. Why is water in such continual ferment, when it is boiling?
A. This commotion is mainly produced by the ascending and descending currents of hot and cold water.
The escape of air from the water contributes also to increase this agitation.
Q. How do these two currents pass each other?
A. The hot ascending current passes close by the metal sides of the kettle; while the cold descending current passes down the centre.
Q. Why does boiling water bubble?
A. The bubbles are portions of steam (formed at the bottom of the vessel) which rise to the surface, and escape into the air.
Q. Why does a kettle run over, when the water boils?
A. As the heat insinuates itself between the particles of water, it drives them asunder; and (as the particles of water are driven apart from each other) the same vessel will no longer hold the expanded water, and some runs over.
Q. Why does a kettle sing, when it is about to boil?
A. Water contains a great deal of air, which (being expanded by the heat of the fire) escapes by fits through the spout of the kettle; which sings in the same way as a trumpet does, when a person blows in it.
Q. Why does water boil?
A. Boiling is the effect of a more violent escape of air from the heated water; when, therefore, the air is not permitted to escape, water will never boil.
Q. Why is heat applied to the bottom, and not to the top of a kettle?
A. Because the heated water always ascends to the surface, heating the water through which it passes: if, therefore, heat were applied to the top of a vessel, the water below the surface would never be heated.
Q. As the lower part of a grate is made red-hot by the fire above, why would not the water boil, if fire were applied to the top?
A. The iron of a grate is an excellent conductor; and, therefore, if one part be heated, the heat is conducted to every other part: but water is a very bad conductor, and will not diffuse heat in a similar way.
Q. How do you know that water is a bad conductor of heat?
A. When a blacksmith immerses his red-hot iron in a tank of water, the water which surrounds the red-hot iron is made boiling hot, but the water below the surface remains quite cold.
Q. If you wish to cool liquids, where should the cold be applied?
A. To the top of the liquid; because the cold portion will always descend, and allow the warmer parts to come in contact with the cooling substance.
Q. Does boiling water get hotter by being kept on the fire?
A. No; not if the steam be suffered to escape.
Q. Why does not boiling water get hotter, if the steam be suffered to escape?
A. Because as fast as the water boils, it is converted into steam; and the steam carries away the additional heat, as fast as it is communicated.
Q. Is steam visible or invisible?
A. Steam is invisible; but when it comes in contact with the air (being condensed into small drops) it instantly becomes visible.
Q. How do you know that steam is invisible?
A. If you look at the spout of a boiling kettle, you will find that the steam (which issues from the spout) is always invisible for about half an inch; after which, it becomes visible.
Q. Why is the steam invisible for only half an inch, and not either all invisible or all visible?
A. The air is not able to condense the steam as it first issues from the spout, but when it spreads and comes in contact with a larger volume of air, the invisible steam is readily condensed into visible drops.
Q. Why is our breath visible in winter-time?
A. Because it is condensed by the cold air into small drops, which are visible to the eye.
Q. Why do steam-engines sometimes burst?
A. Steam is very elastic; and this elasticity increases in a greater proportion than the heat which produces it; unless, therefore, some vent be freely allowed, the steam heaves and swells, till it bursts the vessel which confined it.
Q. What becomes of the steam, after it has been condensed?
A. It is dissolved by the air, and forms a part of its invisible vapour.
Q. Is air a good conductor?
A. No; air is a very bad conductor, and is heated (like water) by convection.
Q. How is a room warmed by a stove?
A. The air nearest the fire is made hot first; the cold air descends, is heated also, and rises in turn; and this goes on, till all the air of the room is warmed.
Q. Why are fires placed on the floor of a room, and not towards the ceiling?
A. As heated air always ascends, if the fire were not near the floor, the lower part of the air (which we want to be the warmest) would never be benefited by the fire at all.
Q. If you take a poker out of the fire, and hold the hot end downwards, why is the handle so intensely hot?
A. Because the hot end of the poker heats the air around it, and this hot air (in its ascent) scorches the poker, and the hand which holds it.
Q. How should a red-hot poker be carried so as not to burn our fingers?
A. With the hot end upwards; because then the air (heated by the poker) would not pass over our hand to scorch it.
Q. Why is a poker (resting on the fender) cold; but if it leans against the stove, intensely warm?
A. The poker is an excellent conductor; while, therefore, it rests against the hot stove, the heat of the stove is conducted into the poker; but when it rests on the fender, it does not come in contact with the hot stove.
Q. Why does it feel so cold, when it rests on the fender?
A. Not being so warm as our hand, it imbibes the heat from it with such rapidity, that our loss of heat is palpable, and produces the sensation of coldness.
Q. Why are flues (which are carried through a church or room) always blackened with black lead?
A. In order that the heat of the flue may be more readily diffused throughout the room. Black lead radiates heat more freely than any other known substance.
Q. Why do country people touch the thick end of an egg with their tongue, to know if it be stale or not?
A. The thick end of an egg always contains a little air (between the shell and the white); but, when the egg is stale, the white shrinks, and the air expands.
Q. How can the tongue tell from this, whether the egg be stale or fresh laid?
A. As air is a very bad conductor, if the egg be stale, it will feel much warmer to the tongue, than if it be new-laid.
Q. Why will the big end of an egg feel warmer to the tongue, because it contains more air?
A. As air is a bad conductor, it will draw off the heat of the tongue very slowly, and, therefore, appear warm; but when there is only a very little air in the egg (as the white is a pretty good conductor), the heat of the tongue will be more rapidly drawn off, and the egg appear colder.
Q. Why is the large end of an egg cracked, when put into a saucepan to boil?
A. To let the air out; if the large end were not cracked, the air (expanded by the heat) would enter the white of the egg, and give it an offensive taste.