Opaque, stones, pyrites, and minereis, when regularly formed, are said to be crystallized, as well as transparent salts and stones. Ice is a true crystallization, consisting of long masses flattened on one side, and joined together in such a manner, that the smaller are inserted into the sides of the greater, making uniformly the same angle. Melted metals, and other bodies, such as wax and starch, which become solid when congealed, assume a regular arrangement, if gradually cooled.
In order to perform this process in perfection, the evaporation should be gentle, and not continued longer than till some drops of the liquor, poured on a glass plate, discover filaments of crystal. As soon as this appears, the vessel is to be immediately removed from the fire into a cooler place, and covered with a cloth, to prevent the access of cold air, which would form pellicles. From a variety of experiments, we have observed that crys-tallization may be remarkably promoted, by throwing into the vessel a few small crystals of the same nature.
Another method of crystallizing salts, is, by adding to a solution or salt a substance which does not act upon the latter, but which has a greater affinity with the water, and will serve to deprive the salt of a portion of that liquid which holds it in a state of solution. Spirit of wine will effect this purpose in many salts; and, if judiciously added, Mill cause them to separate freely from the menstruum, or fluid, and form large and beautiful crys tals.
Salts have this peculiar property, that, however minutely they may be divided, when formed into crystals, they will re-assume their proper figures ; so that they may, with equal facility, be divested of their saltness and their figure.— Crystallization, therefore, is one of the most important agents in chemistry, as it enables us to discover compound solutions of salts ; to ascertain their purity or impurity ; and, lastly, to separate different salts from each other.