Conversations on Chemistry Part 72

_Sulphat of soda_, commonly called Glauber's salt, is another medicinal salt, which is still more bitter than the preceding. We must prepare some of these compounds, that you may observe the phenomena which take place during their formation. We need only pour some sulphuric acid over the soda which I have put into this gla.s.s.

CAROLINE.

What an amazing heat is disengaged! --I thought you said that cold was produced by the melting of salts?

MRS. B.

But you must observe that we are now _making_, not _melting_ a salt.

Heat is disengaged during the formation of compound salts, and a faint light is also emitted, which may sometimes be perceived in the dark.

EMILY.

And is this heat and light produced by the union of the opposite electricities of the alkali and the acid?

MRS. B.

No doubt it is, if that theory be true.

CAROLINE.

The union of an acid and an alkali is then an actual combustion?

MRS. B.

Not precisely, though there is certainly much a.n.a.logy in these processes.

CAROLINE.

Will this sulphat of soda become solid?

MRS. B.

We have not, I suppose, mixed the acid and the alkali in the exact proportions that are required for the formation of the salt, otherwise the mixture would have been almost immediately changed to a solid ma.s.s; but, in order to obtain it in crystals, as you see it in this bottle, it would be necessary first to dilute it with water, and afterwards to evaporate the water, during which operation the salt would gradually crystallise.

CAROLINE.

But of what use is the addition of water, if it is afterwards to be evaporated?

MRS. B.

When suspended in water, the acid and the alkali are more at liberty to act on each other, their union is more complete, and the salt a.s.sumes the regular form of crystals during the slow evaporation of its solvent.

Sulphat of soda liquefies by heat, and effloresces in the air.

EMILY.

Pray what is the meaning of the word _effloresces_? I do not recollect your having mentioned it before.

MRS. B.

A salt is said to effloresce when it loses its water of crystallisation on being exposed to the atmosphere, and is thus gradually converted into a dry powder: you may observe that these crystals of sulphat of soda are far from possessing the transparency which belongs to their crystalline state; they are covered with a white powder, occasioned by their having been exposed to the atmosphere, which has deprived their surface of its l.u.s.tre, by absorbing its water of crystallisation. Salts are, in general, either _efflorescent_ or _deliquescent_: this latter property is precisely the reverse of the former; that is to say, deliquescent salts absorb water from the atmosphere, and are moistened and gradually melted by it. Muriat of lime is an instance of great deliquescence.

EMILY.

But are there no salts that have the same degree of attraction for water as the atmosphere, and that will consequently not be affected by it?

MRS. B.

Yes; there are many such salts, as, for instance, common salt, sulphat of magnesia, and a variety of others.

_Sulphat of lime_ is very frequently met with in nature, and const.i.tutes the well-known substance called _gypsum_, or _plaster of Paris_.

_Sulphat of magnesia_, commonly called _Epsom salt_, is another very bitter medicine, which is obtained from sea-water and from several springs, or may be prepared by the direct combination of its ingredients.

We have formerly mentioned _sulphat of alumine_ as const.i.tuting the common _alum_; it is found in nature chiefly in the neighbourhood of volcanos, and is particularly useful in the arts, from its strong astringent qualities. It is chiefly employed by dyers and calico-printers, to fix colours; and is used also in the manufacture of some kinds of leather.

Sulphuric acid combines also with the metals.

CAROLINE.

One of these combinations, _sulphat of iron_, we are already well acquainted with.

MRS. B.

That is the most important metallic salt formed by sulphuric acid, and the only one that we shall here notice. It is of great use in the arts; and, in medicine, it affords a very valuable tonic: it is of this salt that most of those preparations called _steel medicines_ are composed.

CAROLINE.

But does any carbon enter into these compositions to form steel?

MRS. B.

Not an atom: they are, therefore, very improperly called steel: but it is the vulgar appellation, and medical men themselves often comply with the general custom.

Sulphat of iron may be prepared, as you have seen, by dissolving iron in sulphuric acid; but it is generally obtained from the natural production called _Pyrites_, which being a sulphuret of iron, requires only exposure to the atmosphere to be oxydated, in order to form the salt; this, therefore, is much the most easy way of procuring it on a large scale.

EMILY.

I am surprised to find that both acids and compound salts are generally obtained from their various combinations, rather than from the immediate union of their ingredients.

MRS. B.

Were the simple bodies always at hand, their combinations would naturally be the most convenient method of forming compounds; but you must consider that, in most instances, there is great difficulty and expense in obtaining the simple ingredients from their combinations; it is, therefore, often more expedient to procure compounds from the decomposition of other compounds. But, to return to the sulphat of iron.

--There is a certain vegetable acid called _Gallic acid_, which has the remarkable property of precipitating this salt black--I shall pour a few drops of the gallic acid into this solution of sulphat of iron--

CAROLINE.

It is become as black as ink!