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?