CRYSTALLOGRAPHY, the science which describes or delineates the form of crystals. In A. D. 1672, Rome de Lisle published his "Essay on Crystallog raphy," but the honor of being regarded as the founder of the science is given to the Abbe Rene-Just Haiiy. He was born at St. Just, in what is now called the department of Oise, and, among other works, published his "Essay on the Structure of Crystals," in 1784, as also his "Treatise on Mineralogy" and his "Treatise on Crystallography," both in 1822—the year of his death. His view was that all the varieties of crystals which a particular mineral may assume are derivable from one simple form, which is the type of the mineral. That form he attempted to ascertain in each individual case. Essentially the same view is still held. Imaginary lines may be supposed to be drawn through a sim ple crystal longitudinally from end to end, transversely from side to side, or in either of those ways, or obliquely from angle to angle, around which imaginary lines all the particles of matter compos ing the crystal may be supposed to ar range themselves. Such imaginary lines
are called the axes of the crystal. If skillfully chosen they become somewhat more than imaginary lines, for they may coincide with the optical axes of the crystal if it possess double refraction. According to the number, relative length, position, and inclination to each other of these lines depends the outward form of the crystal.
Dana enumerates the following "sys tems of crystallization": (1) Having the axes equal—the Iso metric system.
(2) Having only the lateral axes equal — the Tetragonal and Hexagonal systems.
(3) Having the axes unequal —the orthorhombic, monoclinic, and triclinic systems. See CRYSTAL.