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 mass; 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 assumes 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 lustre, 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 constitutes 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 constituting 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.