ALLOTROPY (Feel. Gk. a2.rrporia, alto fropia, variety, from t27,2.oc, altos, other ±rporroc, tropos, turn, way, guise), or ALLOTROPISM. A term used in chemistry to denote the exist 1•11CC of an element in several forms differ ing from each other in their physical proper ties. By the silent discharge of electricity in an atmosphere containing ordinary oxygen, the latter is transformed into ozone. Ozone can be readily shown to be made up of nothing but the element oxygen ; yet oxygen gas and ozone exhibit important differences in their properties; thus ozone (Gk. smelling) has a peculiar and characteristic odor, while oxygen gas is odorless; ozone reacts much more readily with various substances; it has bleaching and disinfectant properties not pos sessed by oxygen gas, and it is much denser than oxygen. Phosphorus affords another example of allotropism. In ordinary circumstances, and when freshly prepared, phosphorus is a pale yellow solid of the consistence and aspect of wax, and to some extent flexible and translucent. It requires to be placed in a vessel with water to keep it from taking fire spontaneously, and it is very poisonous. The same element, when dried and kept for sonic time at a moderately high temperature, passes, weight for weight— without addition or subtraction of matter—into a substance known to chemists as amorphous phosphorus. The color of this new variety is brownish red; and it exists as a powder, which has no odor. does not take fire, and is not known
to be poisonous at all. Three allotropic modifica tions of the element carbon are known: diamond, graphite, and amorphous carbon (pure lamp black). The different varieties of sulphur, boron. silicon, etc., furnish other examples of allotropism. Though comparatively few ele ments have been obtained in more than one form, there seems to be no reason why, in general, any other chemical element should be incapable of existing in two or more allotropic modifications. The existence of allotropic varieties brings to the mind the polymo•phkm of crystalline substances and the isomerism of organic compounds. From the point of view of the atomic theory, the differ ent allotropic modifications of an element are, probably, made up of molecules containing differ ent numbers of atoms, or else of atoms differently combined: thus a molecule of ordinary oxygen contains two oxygen atoms, and its c"eunical symbol is 0„; a molecule of ozone three oxygen atoms, and its chemical symbol is O. A similar explanation of the nature of allotropy in solid elements is, however, purely hypothetical; for nothing at all is known of the ultimate structure of solids. Consult: D. Berthe lie l'allotropir des corps simples (Paris, 1894) ; and Onvrard. Etats allotropiques des corps simples (Paris, 1894).