CRYSTALLIZATION, the art of obtaining a substance in the form of crystals. It is an important process in chemistry, since it permits the purification of a substance or the separation of the constituents of a mixture. Generally a substance is more soluble in a solvent at a high temperature than at a low, and consequently, if a boiling strong solution be allowed to cool, the substance will separate in virtue of the diminished solubility. The slower the cooling the larger and more perfect will be the crystals formed. If, as sometimes occurs, such a solution refuses to crystallize, the expedient of "inoculating" the solution with a minute crystal of the same substance, or with a similar substance, may be adopted; shaking the solution, or the addition of a drop of another solvent, may also occasion the desired result. "Frac tional crystallization" consists in repeatedly partially crystallizing the salt content of a solution so as to separate the substances of different solubilities. Examples are especially presented in the study of the rare-earths. Other conditions under which crystals are formed are given in the article CRYSTALLOGRAPHY.