Isomorphism

Ī'so-mōrph'ism, n. [Fr. isomorphisme.] (Crystallog.) A similarity of crystalline form; as, (a.) Between substances of like composition or atomic proportions, as between arsenic acid and phosphorous acid, each containing five equivalents of oxygen. (b.) Between compounds of unlike composition or atomic proportions, as between the metal arsenic and oxide of iron, the rhombohedral angle of the former being 85° 41', of the latter 86° 4'.

☞ The first of these is sometimes distinguished as isomerous or isonomic isomorphism; the second as heteromerous or heteronomic isomorphism. Dana.