ISOMORPHISM, from isos, equal, and morphē, shape or form. A vast number of substances are distinguished by crystalline forms, which have been arranged into six classes or systems founded upon the relation of their axes of symmetry to each other, the study of which belongs to the science of Crystallography. (See MINERALOGICAL SCIENCE.) Certain elementary substances, which are analogous to each other in chemical properties, have the power of assuming a similar crystalline form, and may be substituted for each other in a compound without producing any change in its crystalline form. Such bodies are said to be isomorphous. Thus, magnesia, oxide of copper, protoxide of iron, and oxide of nickel, are isomorphous: when they enter into combination with the same acid, and with similar proportions of water of crystallization, they furnish crystals which are identical in form, and if of the same colour cannot be distinguished by the eye. Common alum is a sulphate of alumina and potash; but the alumina may be displaced by peroxide of iron, the potash by ammonia, or by soda, and yet the shape of the crystal remain the same. Mixtures of isomorphous salts cannot be separated by the method of crystallization, unless of very different degrees of solubility.