The Elementary Principles of Chemistry

solids generally increases with increase of temperature, but in some instances it decreases. The solubility of gases diminishes with increase of temperature, and it increases with increase of pressure. A solvent, when it has dissolved the maximum of a given substance, is said to be saturated. Unsaturated solutions of solid or non-volatile substances may be concentrated by evaporating the solvent (see Exp. 21/4), and the solute may be recovered, often unchanged, by making the evaporation complete (see Exps. 17/ib and 18/A). Crystallization.?If saturated or nearly saturated solu- 21/4 tions remain undisturbed, so that slow cooling and evaporation may take place, the solid in many instances separates in definite geometric forms, as you saw in your own experiments with alum and copper sulphate. This is crystallization. It may accompany solidification, not only from the state of solution, but also of fusion (seen in the sulphur experiments, Nos. 11 and 12, and 12/j), and even from the gaseous condition (seen in the experiment with iodine, No. 20/j, the sublimate of which is beautifully crystalline). In the crystallization of mixtures each individual substance crystallizes by itself, in its own peculiar form, and therefore crystallization is a most important means of separation and purification. Furthermore, the peculiarities of form?that is, the shape and disposition of the faces, the dimension of the angles, etc.?are definite and constant characteristics of the substanc...