DIMORPHOUS (Gr. dis, twice; morphe, shape or form) is the term applied to a sub stance when it exhibits the property of crystallizing in two distinct forms or systems. See CRYSTALLOGRAPHY. Thus, sulphur, as found crystallized naturally, and as obtained by the spontaneous evaporation of its solution in bisulphuret of carbon, or in chloride of sulphur, presents itself in crystals of the form of octahedra, with a rhombic base, and thus belongs to the prismatic system; but when sulphur is heated to fusion, and then slowly cooled. prismatic crystals of an amber color are obtained, which belong to the oblique system. The latter form of sulphur is not permanent, and the crystals gradually become, opaque, and pass into the form of numberless octahedra. In their turn, the octohedral crystals, when kept at a temperature of 230° F. for some time, pass into the prismatic form. Carbon is another illustration of dimorphism. Thus, carbon
crystallizes in the diamond in the regular system as the octohedron and allied forms; while in the condition cA graphite or black lead (q.v.), as obtained by the cooling of its solution in fused cast-iron, it is in the form of hexagonal crystals belonging to the rhombohedral system. Carbonate of lime and iodide of mercury are also good examples of dimorphism; and some substances, such as the sulphate of nickel, sulphate of zinc, seleniate of nickel, and the seleniate of zinc, crystallize in three different systems, and are thus trimcnphous.
a fortified t. and important military station in Russia on the Dwina, reached by the St. Petersburg and Warsaw railroad, 120 m. s.c. of Riga; pop. '69, 29,613. It has many manufactories, and a flourishing trade.