You recollect that when a solid body is dissolved either by water or caloric it is not decomposed; but that its integrant parts are only suspended in the solvent. When the solution is made in water, the integrant particles of the body will, on the water being evaporated, again unite into a solid mass by the force of their mutual attraction. But when the body is dissolved by caloric alone, nothing more is necessary, in order to make its particles reunite, than to reduce its temperature. And, in general, if the solvent, whether water or caloric, be slowly separated by evaporation or by cooling, and care taken that the particles be not agitated during their reunion, they will arrange themselves in regular masses, each individual substance assuming a peculiar form or arrangement; and this is what is called crystallisation.
EMILY.
Crystallisation, therefore, is simply the reunion of the particles of a solid body that has been dissolved in a fluid.
MRS. B.
That is a very good definition of it. But I must not forget to observe, that heat and water may unite their solvent powers; and, in this case, crystallisation may be hastened by cooling, as well as by evaporating the liquid.
CAROLINE.
But if the body dissolved is of a volatile nature, will it not evaporate with the fluid?
MRS. B.
A crystallised body held in solution only by water is scarcely ever so volatile as the fluid itself, and care must be taken to manage the heat so that it may be sufficient to evaporate the water only.
I should not omit also to mention that bodies, in crystallising from their watery solution, always retain a small portion of water, which remains confined in the crystal in a solid form, and does not reappear unless the body loses its crystalline state. This is called the water of crystallisation. But you must observe, that whilst a body may be separated from its solution in water or caloric simply by cooling or by evaporation, an acid can be taken from a metal with which it is combined only by stronger affinities, which produce a decomposition.