Читать книгу A Guide to the Scientific Knowledge of Things Familiar - Ebenezer Cobham Brewer - Страница 11
ОглавлениеQ. 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.)
