CAROLINE.

True; I should have recollected that. --The product of the decomposition of water by red-hot charcoal, therefore, is carbonated hydrogen gas, and carbonic acid gas.

MRS. B.

You are perfectly right now.

Carbon is frequently found combined with hydrogen in a state of solidity, especially in coals, which owe their combustible nature to these two principles.

EMILY.

Is it the hydrogen, then, that produces the flame of coals?

MRS. B.

It is so; and when all the hydrogen is consumed, the carbon continues to burn without flame. But again, as I mentioned when speaking of the gas-lights, the hydrogen gas produced by the burning of coals is not pure; for, during the combustion, particles of carbon are successively volatilized with the hydrogen, with which they form what is called a _hydro-carbonat_, which is the princ.i.p.al product of this combustion.

Carbon is a very bad conductor of heat; for this reason, it is employed (in conjunction with other ingredients) for coating furnaces and other chemical apparatus.

EMILY.

Pray what is the use of coating furnaces?

MRS. B.

In most cases, in which a furnace is used, it is necessary to produce and preserve a great degree of heat, for which purpose every possible means are used to prevent the heat from escaping by communicating with other bodies, and this object is attained by coating over the inside of the furnace with a kind of plaster, composed of materials that are bad conductors of heat.

Carbon, combined with a small quant.i.ty of iron, forms a compound called plumbago, or black-lead, of which pencils are made. This substance, agreeably to the nomenclature, is _a carburet of iron_.

EMILY.

Why, then, is it called black-lead?

MRS. B.

It is an ancient name given to it by ignorant people, from its shining metallic appearance; but it is certainly a most improper name for it, as there is not a particle of lead in the composition. There is only one mine of this mineral, which is in c.u.mberland. It is supposed to approach as nearly to pure carbon as the best prepared charcoal does, as it contains only five parts of iron, unadulterated by any other foreign ingredients. There is another carburet of iron, in which the iron, though united only to an extremely small proportion of carbon, acquires very remarkable properties; this is steel.

CAROLINE.

Really; and yet steel is much harder than iron?

MRS. B.

But carbon is not ductile like iron, and therefore may render the steel more brittle, and prevent its bending so easily. Whether it is that the carbon, by introducing itself into the pores of the iron, and, by filling them, makes the metal both harder and heavier; or whether this change depends upon some chemical cause, I cannot pretend to decide. But there is a subsequent operation, by which the hardness of steel is very much increased, which simply consists in heating the steel till it is red-hot, and then plunging it into cold water.

Carbon, besides the combination just mentioned, enters into the composition of a vast number of natural productions, such, for instance, as all the various kinds of oils, which result from the combination of carbon, hydrogen, and caloric, in various proportions.

EMILY.

I thought that carbon, hydrogen, and caloric, formed carbonated hydrogen gas?

MRS. B.

That is the case when a small portion of carbonic acid gas is held in solution by hydrogen gas. Different proportions of the same principles, together with the circ.u.mstances of their union, produce very different combinations; of this you will see innumerable examples. Besides, we are not now talking of gases, but of carbon and hydrogen, combined only with a quant.i.ty of caloric sufficient to bring them to the consistency of oil or fat.

CAROLINE.

But oil and fat are not of the same consistence?

MRS. B.

Fat is only congealed oil; or oil, melted fat. The one requires a little more heat to maintain it in a fluid state than the other. Have you never observed the fat of meat turned to oil by the caloric it has imbibed from the fire?

EMILY.

Yet oils in general, as salad-oil, and lamp-oil, do not turn to fat when cold?

MRS. B.

Not at the common temperature of the atmosphere, because they retain too much caloric to congeal at that temperature; but if exposed to a sufficient degree of cold, their latent heat is extricated, and they become solid fat substances. Have you never seen salad oil frozen in winter?

EMILY.

Yes; but it appears to me in that state very different from animal fat.

MRS. B.

The essential const.i.tuent parts of either vegetable or animal oils are the same, carbon and hydrogen; their variety arises from the different proportions of these substances, and from other accessory ingredients that may be mixed with them. The oil of a whale, and the oil of roses, are, in their essential const.i.tuent parts, the same; but the one is impregnated with the offensive particles of animal matter, the other with the delicate perfume of a flower.

The difference of _fixed oils_, and _volatile_ or _essential oils_, consists also in the various proportions of carbon and hydrogen. Fixed oils are those which will not evaporate without being decomposed; this is the case with all common oils, which contain a greater proportion of carbon than the essential oils. The essential oils (which comprehend the whole cla.s.s of essences and perfumes) are lighter; they contain more equal proportions of carbon and hydrogen, and are volatilized or evaporated without being decomposed.

EMILY.

When you say that one kind of oil will evaporate, and the other be decomposed, you mean, I suppose, by the application of heat?

MRS. B.

Not necessarily; for there are oils that will evaporate slowly at the common temperature of the atmosphere; but for a more rapid volatilization, or for their decomposition, the a.s.sistance of heat is required.

CAROLINE.

I shall now remember, I think, that fat and oil are really the same substances, both consisting of carbon and hydrogen; that in fixed oils the carbon preponderates, and heat produces a decomposition; while, in essential oils, the proportion of hydrogen is greater, and heat produces a volatilization only.

EMILY.

I suppose the reason why oil burns so well in lamps is because its two const.i.tuents are so combustible?

MRS. B.