Читать книгу A Guide to the Scientific Knowledge of Things Familiar - Ebenezer Cobham Brewer - Страница 10

Table of Contents

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.