Isomorphism (Gr. equal, and form), in chemistry, the property possessed by certain bodies of replacing each other in compounds without causing in these an essential change of crystalline form. The bodies that thus replace each other possess themselves similar forms, and are said to be isomorphous. Familiar examples of this mutual replacement in minerals are of the protoxides of iron and manganese, and of lime and magnesia. Chlorine, bromine, and iodine possess this relation toward each other; also arsenic and phosphorus; and the acids of these elements. The term, as proposed by Mitscherlich, strictly signifies similarity of form; it is now applied to substances which are not only similar in their crystalline form, but are analogous in their chemical composition. The study of isomorphism has greatly facilitated the classification of compounds and the determination of atomic weights.