CRYSTALS; CRYSTALLOGRAPHY. Al though the study of crystals as a science, apart from their practical applications in the arts, does not belong to the present work, it may be useful to explain certain terms and properties, which are constantly in use when crystals are spoken of. Mineralogists have adopted certain groups of primary and se condary forms, as a means of classifying crys tals. The primary forms, according to one system of crystallography (for there are more systems than one), consist of the cube, the square prism, the right rhombic prism, the oblique rhombic prism, the doubly oblique prism, and the rhomboid or rhombohedron. The na ture of these forms may be best explained by showing the relations between them. The cube has the well-known form of a die, with six equal surfaces. If the lateral edges of the cube be supposed to be longer or shorter than the terminal edges, a square prism would be pro duced ; if two opposite lateral edges of a square prism could be pressed towards each other, the parallelism being kept, a right rhombic prism would he formed ; if this prism rould be pressed in the direction of either of the diagonals of its terminal plane, so as to make the figure overhang the base in that Erection, an oblique rhombic prism would be represented; and if again pressed in the di rection of the other diagonal, so that it should overhang the base in both directions, a doubly oblique prism would be formed. If a cube be made to stand on one of its angles by placing the fingers on an opposite one, and if, while held in this position, the two angles could be pressed nearer together or drawn further apart, the altered cube would become a rhomboid.
The secondary forms of crystals are sup posed to be occasioned by some natural in fluence operating upon the first germ of the crystal, and continuing during the period of its increase in size. The number of known
secondary forms belonging to each system is already very great; in one mineral, carbonate of lime, they amount to many hundreds. Crystals occur more frequently in the se condary than the primary forms. Twin crys tals are produced by the union of two or more crystals according to some regular plan, so that, if any number of twin crystals of the same kind of mineral should be found they would be fashioned in the same manner.
The crystallisation of salts from solution in fluids generally takes place when the solutions are considerably evaporated, but the degree of evaporation is very different for different sub stances. Some salts begin to crystallise at the surface very soon after evaporation com mences ; and others (for example sugar) must be evaporated to the consistence of a thick syrup before any crystals will be formed. Hot fluids will generally dissolve more matter than cold ones, and crystals are frequently produced during the cooling of the hot solu tion. Some soluble substances however can not be brought to crystallise under any cir cumstances hitherto tried; but, on the solvent evaporating, a thick pasty matter is left, which by further evaporation becomes a hard solid mass. Camphor affords an instance of the formation of crystals by volatilization. The slags of furnaces will frequently be found to contain crystallised matter ; and the common rolls of sulphur when broken will frequently present small cavities lined with thin needle. like crystals. Becquerel, Crosse, Fox, and others, by the electrical induction of chemical action, have effected the crystallisation of mineral bodies which are wholly insoluble in any fluid which does not subject them to im mediate chemical change.