CRYSTAL (ice), in chemistry and mineralogy, a clear transparent body, which, by the mutual attracti'n of its particles, has assumed the form of some one of the regular geometric solids, being bounded by a certain number of plane surfaces. A crystal consists of three parts. First, plane surfaces, called faces, which are said to be similar when they are equal to each other and are similarly situated; dissimilar, when they are unequal or have a different position. Second, edges, formed by the meeting of two faces. They are said to be similar in hen formed by similar faces; dissimi lar, by dissimilar faces. Equal edges are formed when the faces are inclined at the same angle to one another; un equal, when they are inclined at different angles. Third, solid angles, formed by the meeting of three or more faces; and in this case also they are similar and dis similar, equal and unequal angled edges. The chemist procures crystals either by fusing the bodies by heat and then allow ing them gradually to cool, or by dis solving them in a fluid and then ab stracting the fluid by slow evaporation. The method of describing and classifying crystals (now universally adopted) is based upon certain imaginary lines drawn through the crystal, and called its axes. There are seven of these systems, six of which refer to three axes and one to four, and they are subdivided accord ing as the axes are at right angles (orchometric) or not (chinometric). When the axes are equal and at right angles the system is called isometric. When the angles are right angles, but only two are equal, the system is called tetragonal. When the three axes are at right angles but all unequal, the system is called orthorhombic. The classes are as follows: First, the monometric, regu lar, or cubic system, in which the axes are equal and at right angles to one another; second, the square prismatic or dimetric system, in which the axes are at right angles to each other, and while two are equal, the third is longer or shorter; third, the right prismatic, rhombic, or trimetric system, in which the axes are at right angles to each other, but all are of different lengths; fourth, the hexagonal or rhombohedral system, which has four axes, three in one plane inclined to each other at 60 degrees, the fourth perpendicular to this plane; fifth, the monoclinic or oblique system, in which two axes are at right angles and the third is inclined to their plane; sixth, the diclinic or doubly oblique system, in which two axes are at right angles, the third oblique to both; seventh, the triclinic system, in which the three axes are inclined to each other at any angle other than a right angle.
The power of forming crystals is pos sessed by a great majority of inorganic combinations whether natural or artifi cial, and also by a large number of organic chemical bodies. The degree of this capacity varies greatly in different substances, so that certain chemical com binations are found only in crystals, others rarely. Bodies which entirely lack the power of forming crystals or crystalline aggregates are called amor phic. The size of crystals also varies greatly. Some are very large, others are only aggregations of microscopic crys tals. The infinitesimally small crystals are called microliths. Crystals grow by the deposit of new horizontal layers on their surfaces, always keeping the characteristic angles exactly the same. Even when the growth of crystals in dif ferent directions takes place, with un equal rapidity, and distorted forms arise, as is often the case, the law still holds good, the inclination of the adjacent planes and the angles which they enclose are the same. Hence the importance and the value of crystallometry and the science of crystallography or crys talology. Crystals occur with an almost infinite variety of forms—calcareous spar having alone more than 200 forms in more than a thousand different com binations, and some crystals have as many as 300 different sides. But all crystals may be grouped in accordance with certain systems.