Conversations on Chemistry Part 81

MRS. B.

True; I avoided noticing it then, because you could not have understood it without some previous knowledge of the oxy-muriatic acid, which I have but just introduced to your acquaintance.

Sir H. Davy's idea is that muriatic acid, instead of being a compound, consisting of an unknown basis and oxygen, is formed by the union of oxy-muriatic gas with hydrogen.

EMILY.

Have you not told us just now that oxy-muriatic gas was itself a compound of muriatic acid and oxygen?

MRS. B.

Yes; but according to Sir H. Davy's hypothesis, oxy-muriatic gas is considered as a simple body, which contains no oxygen--as a substance of its own kind, which has a great a.n.a.logy to oxygen in most of its properties, though in others it differs entirely from it. --According to this view of the subject, the name of _oxy-muriatic acid_ can no longer be proper, and therefore Sir H. Davy has adopted that of _chlorine_, or _chlorine gas_, a name which is simply expressive of its greenish colour; and in compliance with that philosopher's theory, we have placed chlorine in our table among the simple bodies.

CAROLINE.

But what was Sir H. Davy's reason for adopting an opinion so contrary to that which had hitherto prevailed?

MRS. B.

There are many circ.u.mstances which are favourable to the new doctrine; but the clearest and simplest fact in its support is, that if hydrogen gas and oxy-muriatic gas be mixed together, both these gases disappear, and muriatic acid gas is formed.

EMILY.

That seems to be a complete proof; is it not considered as perfectly conclusive?

MRS. B.

Not so decisive as it appears at first sight; because it is argued by those who still incline to the old doctrine, that muriatic acid gas, however dry it may be, always contains a certain quant.i.ty of water, which is supposed essential to its formation. So that, in the experiment just mentioned, this water is supplied by the union of the hydrogen gas with the oxygen of the oxy-muriatic acid; and therefore the mixture resolves itself into the base of muriatic acid and water, that is, muriatic acid gas.

CAROLINE.

I think the old theory must be the true one; for otherwise how could you explain the formation of oxy-muriatic gas, from a mixture of muriatic acid and oxyd of manganese?

MRS. B.

Very easily; you need only suppose that in this process the muriatic acid is decomposed; its hydrogen unites with the oxygen of the manganese to form water, and the chlorine appears in its separate state.

EMILY.

But how can you explain the various combustions which take place in oxy-muriatic gas, if you consider it as containing no oxygen?

MRS. B.

We need only suppose that combustion is the result of intense chemical action; so that chlorine, like oxygen, in combining with bodies, forms compounds which have less capacity for caloric than their const.i.tuent principles, and, therefore, caloric is evolved at the moment of their combination.

EMILY.

If, then, we may explain every thing by either theory, to which of the two shall we give the preference?

MRS. B.

It will, perhaps, be better to wait for more positive proofs, if such can be obtained, before we decide positively upon the subject. The new doctrine has certainly gained ground very rapidly, and may be considered as nearly established; but several competent judges still refuse their a.s.sent to it, and until that theory is very generally adopted, it may be as well for us still occasionally to use the language to which chemists have long been accustomed. --But let us proceed to the examination of salts formed by muriatic acid.

Among the compound salts formed by muriatic acid, the _muriat of soda_, or common salt, is the most interesting.* The uses and properties of this salt are too well known to require much comment. Besides the pleasant flavour it imparts to the food, it is very wholesome, when not used to excess, as it a.s.sists the process of digestion.

Sea-water is the great source from which muriat of soda is extracted by evaporation. But it is also found in large solid ma.s.ses in the bowels of the earth, in England, and in many other parts of the world.

[Footnote *: According to Sir H. Davy's views of the nature of the muriatic and oxy-muriatic acids, dry muriat of soda is a compound of sodium and chlorine, for it may be formed by the direct combination of oxy-muriatic gas and sodium. In his opinion, therefore, what we commonly call muriat of soda contains neither soda nor muriatic acid.]

EMILY.

I thought that salts, when solid, were always in the state of crystals; but the common table-salt is in the form of a coa.r.s.e white powder.

MRS. B.

Crystallisation depends, as you may recollect, on the slow and regular reunion of particles dissolved in a fluid; common sea-salt is only in a state of imperfect crystallisation, because the process by which it is prepared is not favourable to the formation of regular crystals. But if you dissolve it, and afterwards evaporate the water slowly, you will obtain a regular crystallisation.

_Muriat of ammonia_ is another combination of this acid, which we have already mentioned as the princ.i.p.al source from which ammonia is derived.

I can at once show you the formation of this salt by the immediate combination of muriatic acid with ammonia. --These two gla.s.s jars contain, the one muriatic acid gas, the other ammoniacal gas, both of which are perfectly invisible--now, if I mix them together, you see they immediately form an opake white cloud, like smoke. --If a thermometer was placed in the jar in which these gases are mixed, you would perceive that some heat is at the same time produced.

EMILY.

The effects of chemical combinations are, indeed, wonderful! --How extraordinary it is that two invisible bodies should become visible by their union!

MRS. B.

This strikes you with astonishment, because it is a phenomenon which nature seldom exhibits to our view; but the most common of her operations are as wonderful, and it is their frequency only that prevents our regarding them with equal admiration. What would be more surprising, for instance, than combustion, were it not rendered so familiar by custom?

EMILY.

That is true. --But pray, Mrs. B., is this white cloud the salt that produces ammonia? How different it is from the solid muriat of ammonia which you once showed us!

MRS. B.

It is the same substance which first appears in the state of vapour, but will soon be condensed by cooling against the sides of the jar, in the form of very minute crystals.

We may now proceed to the _oxy-muriats_. In this cla.s.s of salts the _oxy-muriat of potash_ is the most worthy of our attention, for its striking properties. The acid, in this state of combination, contains a still greater proportion of oxygen than when alone.

CAROLINE.

But how can the oxy-muriatic acid acquire an increase of oxygen by combining with potash?

MRS. B.

It does not really acquire an additional quant.i.ty of oxygen, but it loses some of the muriatic acid, which produces the same effect, as the acid which remains is proportionably super-oxygenated.*

If this salt be mixed, and merely rubbed together with sulphur, phosphorus, charcoal, or indeed any other combustible, it explodes strongly.