ISOMORPHISM, i's,'cinOr'fiz'm (from iso morphous, from Gk. //roc. isos, equal -I- Aung, morphd7, form). The relationship existing be tween solid substances which arc similar in their chemical composition and constitution, have simi lar crystalline forms, and are callable of forming homogeneous mixed crystals (`solid solutions'), and each of which is capable of growing in a saturated solution of another, the latter then forming a mantle around the first as a nucleus. Ostwald proposes to define the relationship be tween two isomorphous substances as the capac ity of one to cause immediate crystallization in a supersaturated solution of another, just as a crystal of any substance is capable of causing its own supersaturated solution to crystallize immediately. it must, however. be observed that while the relationship of isomorphism certainly exists, our knowledge of its intimate nature is very vague, and neither of the above definitions gives adequate expression to all of the known facts. The ordinary carbonates of calcium (cal cite), magnesium (magnesite). iron (siderite), inammnese (rhodochrosite), and zinc (smith sonite) all form crystals of the same crystal system. and for the most part of the same crystal class (see CRYSTA1d.0GItAPlIy) ; and, further. their corresponding interfacial angles approach to the same values. Not all substances closely related in their chemical composition are iso morphous. and, exceptionally, substances which have no chemical relationship have similar sym metry of crystals and angles in close corre spondence. (Such an aecidental resemblance of the crystal forms of chemically unrelated sub stances is described as `isogonism2) In the case of salts, the chemical component which seems mainly to condition the symmetry of the mole cule is the acid radical, the metal having less influence upon the crystal's symmetry. though
affecting the size of crystal angles. (See NOR PHOTROPISM. ) The metals of a series of iso morphous salts are said to be isomorphous ele ments, and it is found that for the most part they are of the same or related groups of ele ments. See PERIODIC LAW.
When a substance has been found to form crystals of more than one kind it is said to be dimorphous, trimorphous, or polymorphous, the term dimorphous being used in a general sense to describe substances which exhibit three or more as well as two kinds of crystals. Sulphur crystallizes from fusion in long needle-like crys tals of monoclinic symmetry, but from a solution in carbon bisulphide in orthorhombic crystals. Under other special conditions it assumes other kinds of symmetry. Carbonate of lime, which is generally found crystallized in nature as hex agonal (trigonal) crystals (calcite), is less fre quently found in crystals belonging to the ortho rhombic system, and with different physical properties from those of calcite. As explained above, calcite belongs in an isomorphous series with carbonate of magnesia. carbonate of iron, carbonate of zinc, etc. Aragonite, on the other hand. is a member of a different isomorphous group in which are carbonate of strontium, car bonate of barium, and carbonate of lead. A double group of this kind connected by a dimor phous substance (carbonate of lime as calcite and aragonite) is known as an isodimorphous group or series. See CRYSTALLOGRAPHY; CHEM ISTRY; ATOMIC WEIGHTS.