Conversations on Chemistry Part 79

Yes it is. This power of corroding gla.s.s has been used for engraving, or rather etching, upon it. The gla.s.s is first covered with a coat of wax, through which the figures to be engraved are to be scratched with a pin; then pouring the fluoric acid over the wax, it corrodes the gla.s.s where the scratches have been made.

CAROLINE.

I should like to have a bottle of this acid, to make engravings.

MRS. B.

But you could not have it in a _gla.s.s_ bottle, for in that case the acid would be saturated with silex, and incapable of executing an engraving; the same thing would happen were the acid kept in vessels of porcelain or earthen-ware; this acid must therefore be both prepared and preserved in vessels of silver.

If it be distilled from fluor spar and vitriolic acid, in silver or leaden vessels, the receiver being kept very cold during the distillation, it a.s.sumes the form of a dense fluid, and in that state is the most intensely corrosive substance known. This seems to be the acid combined with a little water. It may be called _hydro-fluoric acid_; and Sir H. Davy has been led, from some late experiments on the subject, to consider _pure_ fluoric acid as a compound of a certain unknown principle, which he calls _fluorine_, with hydrogen.

Sir H. Davy has also attempted to decompose the fluoric acid by burning pota.s.sium in contact with it; but he has not yet been able by this or any other method, to obtain its basis in a distinct separate state.

We shall conclude our account of the acids with that of the MURIATIC ACID, which is perhaps the most curious and interesting of all of them.

It is found in nature combined with soda, lime, and magnesia. _Muriat of soda_ is the common sea-salt, and from this substance the acid is usually disengaged by means of the sulphuric acid. The natural state of the muriatic acid is that of an invisible permanent gas, at the common temperature of the atmosphere; but it has a remarkably strong attraction for water, and a.s.sumes the form of a whitish cloud whenever it meets any moisture to combine with. This acid is remarkable for its peculiar and very pungent smell, and possesses, in a powerful degree, most of the acid properties. Here is a bottle containing muriatic acid in a liquid state.

CAROLINE.

And how is it liquefied?

MRS. B.

By impregnating water with it; its strong attraction for water makes it very easy to obtain it in a liquid form. Now, if I open the phial, you may observe a kind of vapour rising from it, which is muriatic acid gas, of itself invisible, but made apparent by combining with the moisture of the atmosphere.

EMILY.

Have you not any of the pure muriatic acid gas?

MRS. B.

This jar is full of that acid in its gaseous state--it is inverted over mercury instead of water, because, being absorbable by water, this gas cannot be confined by it. --I shall now raise the jar a little on one side, and suffer some of the gas to escape. --You see that it immediately becomes visible in the form of a cloud.

EMILY.

It must be, no doubt, from its uniting with the moisture of the atmosphere, that it is converted into this dewy vapour.

MRS. B.

Certainly; and for the same reason, that is to say, its extreme eagerness to unite with water, this gas will cause snow to melt as rapidly as an intense fire.

This acid proved much more refractory when Sir H. Davy attempted to decompose it than the other two undecompounded acids. It is singular that pota.s.sium will burn in muriatic acid, and be converted into potash, without decomposing the acid, and the result of this combustion is a _muriat of potash_; for the potash, as soon as it is regenerated, combines with the muriatic acid.

CAROLINE.

But how can the potash be regenerated if the muriatic acid does not oxydate the pota.s.sium?

MRS. B.

The pota.s.sium, in this process, obtains oxygen from the moisture with which the muriatic acid is always combined, and accordingly hydrogen, resulting from the decomposition of the moisture, is invariably evolved.

EMILY.

But why not make these experiments with dry muriatic acid?

MRS. B.

Dry acids cannot be acted on by the Voltaic battery, because acids are non-conductors of electricity, unless moistened. In the course of a number of experiments which Sir H. Davy made upon acids in a state of dryness, he observed that the presence of water appeared always necessary to develop the acid properties, so that acids are not even capable of reddening vegetable blues if they have been carefully deprived of moisture. This remarkable circ.u.mstance led him to suspect, that water, instead of oxygen, may be the acidifying principle; but this he threw out rather as a conjecture than as an established point.

Sir H. Davy obtained very curious results from burning pota.s.sium in a mixture of phosphorus and muriatic acid, and also of sulphur and muriatic acid; the latter detonates with great violence. All his experiments, however, failed in presenting to his view the basis of the muriatic acid, of which he was in search; and he was at last induced to form an opinion respecting the nature of this acid, which I shall presently explain.

EMILY.

Is this acid susceptible of different degrees of oxygenation?

MRS. B.

Yes, for though we cannot deoxygenate this acid, yet we may add oxygen to it.

CAROLINE.

Why, then, is not the least degree of oxygenation of the acid called the _muriatous_, and the higher degree the _muriatic_ acid?

MRS. B.

Because, instead of becoming, like other acids, more dense, and more acid by an addition of oxygen, it is rendered on the contrary more volatile, more pungent, but less acid, and less absorbable by water.

These circ.u.mstances, therefore, seem to indicate the propriety of making an exception to the nomenclature. The highest degree of oxygenation of this acid has been distinguished by the additional epithet of _oxygenated_, or, for the sake of brevity, _oxy_, so that it is called the _oxygenated_, or _oxy-muriatic acid_. This likewise exists in a gaseous form, at the temperature of the atmosphere; it is also susceptible of being absorbed by water, and can be congealed, or solidified, by a certain degree of cold.

EMILY.

And how do you obtain the oxy-muriatic acid?

MRS. B.

In various ways; but it may be most conveniently obtained by distilling liquid muriatic acid over oxyd of manganese, which supplies the acid with the additional oxygen. One part of the acid being put into a retort, with two parts of the oxyd of manganese, and the heat of a lamp applied, the gas is soon disengaged, and may be received over water, as it is but sparingly absorbed by it. --I have collected some in this jar--

CAROLINE.

It is not invisible, like the generality of gases; for it is of a yellowish colour.

MRS. B.

The muriatic acid extinguishes flame, whilst, on the contrary, the oxy-muriatic makes the flame larger, and gives it a dark red colour. Can you account for this difference in the two acids?

EMILY.

Yes, I think so; the muriatic acid will not supply the flame with the oxygen necessary for its support; but when this acid is further oxygenated, it will part with its additional quant.i.ty of oxygen, and in this way support combustion.

MRS. B.