Conversations on Chemistry Part 93

CAROLINE.

I have often seen both brandy and spirit of wine burnt; they produce a great deal of flame, but not a proportional quant.i.ty of heat, and no smoke whatever.

MRS. B.

The last circ.u.mstance arises from their combustion being complete; and the disproportion between the flame and heat shows you that these are by no means synonymous.

The great quant.i.ty of flame proceeds from the combustion of the hydrogen to which, you know, that manner of burning is peculiar. --Have you not remarked also that brandy and alcohol will burn without a wick? --They take fire at so low a temperature, that this a.s.sistance is not required to concentrate the heat and volatilise the fluid.

CAROLINE.

I have sometimes seen brandy burnt by merely heating it in a spoon.

MRS. B.

The rapidity of the combustion of alcohol may, however, be prodigiously increased by first volatilising it. An ingenious instrument has been constructed on this principle to answer the purpose of a blow-pipe, which may be used for melting gla.s.s, or other chemical purposes. It consists of a small metallic vessel (PLATE XIV. Fig. 2.), of a spherical shape, which contains the alcohol, and is heated by the lamp beneath it; as soon as the alcohol is volatilised, it pa.s.ses through the spout of the vessel, and issues just above the wick of the lamp, which immediately sets fire to the stream of vapour, as I shall show you--

EMILY.

With what amazing violence it burns! The flame of alcohol, in the state of vapour, is, I fancy, much hotter than when the spirit is merely burnt in a spoon?

MRS. B.

Yes; because in this way the combustion goes on much quicker, and, of course, the heat is proportionally increased. --Observe its effect on this small gla.s.s tube, the middle of which I present to the extremity of the flame, where the heat is greatest.

CAROLINE.

The gla.s.s, in that spot, is become red hot, and bends from its own weight.

MRS. B.

I have now drawn it asunder, and am going to blow a ball at one of the heated ends; but I must previously close it up, and flatten it with this little metallic instrument, otherwise the breath would pa.s.s through the tube without dilating any part of it. --Now, Caroline, will you blow strongly into the tube whilst the closed end is red hot.

EMILY.

You blowed too hard; for the ball suddenly dilated to a great size, and then burst in pieces.

MRS. B.

You will be more expert another time; but I must caution you, should you ever use this blow-pipe, to be very careful that the combustion of the alcohol does not go on with too great violence, for I have seen the flame sometimes dart out with such force as to reach the opposite wall of the room, and set the paint on fire. There is, however, no danger of the vessel bursting, as it is provided with a safety tube, which affords an additional vent for the vapour of alcohol when required.

The products of the combustion of alcohol consist in a great proportion of water, and a small quant.i.ty of carbonic acid. There is no smoke or fixed remains whatever. --How do you account for that, Emily?

EMILY.

I suppose that the oxygen which the alcohol absorbs in burning, converts its hydrogen into water and its carbon into carbonic acid gas, and thus it is completely consumed.

MRS. B.

Very well. --_Ether_, the lightest of all fluids, and with which you are well acquainted, is obtained from alcohol, of which it forms the lightest and most volatile part.

EMILY.

Ether, then, is to alcohol, what alcohol is to brandy?

MRS. B.

No: there is an essential difference. In order to obtain alcohol from brandy, you need only deprive the latter of its water; but for the formation of ether, the alcohol must be decomposed, and one of its const.i.tuents partly subtracted. I leave you to guess which of them it is--

EMILY.

It cannot be hydrogen, as ether is more volatile than alcohol, and hydrogen is the lightest of all its ingredients: nor do I suppose that it can be oxygen, as alcohol contains so small a proportion of that principle; it is, therefore, most probably, carbon, a diminution of which would not fail to render the new compound more volatile.

MRS. B.

You are perfectly right. The formation of ether consists simply in subtracting from the alcohol a certain proportion of carbon; this is effected by the action of the sulphuric, nitric, or muriatic acids, on alcohol. The acid and carbon remain at the bottom of the vessel, whilst the decarbonised alcohol flies off in the form of a condensable vapour, which is ether.

Ether is the most inflammable of all fluids, and burns at so slow a temperature that the heat evolved during its combustion is more than is required for its support, so that a quant.i.ty of ether is volatilised, which takes fire, and gradually increases the violence of the combustion.

Sir Humphry Davy has lately discovered a very singular fact respecting the vapour of ether. If a few drops of ether be poured into a wine-gla.s.s, and a fine platina wire, heated almost to redness, be held suspended in the gla.s.s, close to the surface of the ether, the wire soon becomes intensely red-hot, and remains so for any length of time. We may easily try the experiment. . . . .

CAROLINE.

How very curious! The wire is almost white hot, and a pungent smell rises from the gla.s.s. Pray how is this accounted for?

MRS. B.

This is owing to a very peculiar property of the vapour of ether, and indeed of many other combustible gaseous bodies. At a certain temperature lower than that of ignition, these vapours undergo a slow and imperfect combustion, which does not give rise, in any sensible degree, to the phenomena of light and flame, and yet extricates a quant.i.ty of caloric sufficient to react upon the wire and make it red-hot, and the wire in its turn keeps up the effect as long as the emission of vapour continues.

CAROLINE.

But why should not an iron or silver wire produce the same effect?

MRS. B.

Because either iron or silver, being much better conductors of heat than platina, the heat is carried off too fast by those metals to allow the acc.u.mulation of caloric necessary to produce the effect in question.

Ether is so light that it evaporates at the common temperature of the atmosphere; it is therefore necessary to keep it confined by a well ground gla.s.s stopper. No degree of cold known has ever frozen it.

CAROLINE.

Is it not often taken medicinally?

MRS. B.

Yes; it is one of the most effectual antispasmodic medicines, and the quickness of its effects, as such, probably depends on its being instantly converted into vapour by the heat of the stomach, through the intervention of which it acts on the nervous system. But the frequent use of ether, like that of spirituous liquors, becomes prejudicial, and, if taken to excess, it produces effects similar to those of intoxication.