ALLOTROPY, a-lotiro-pi, or ALLOT ROPISM (from the Greek altos, °another,' and tropos, °manner"), the property exhibited by certain substances of existing in two or more different states distinguished from each other by different properties. The most familiar case of allotropy is afforded by carbon, which exists in a number of allotropic modifications, of which charcoal, graphite and the diamond are familiar examples. Allotropy is not ex hibited by the metals to any marked degree (see, however, SILVER). Sulphur exhibits many allotropic forms, of which the following are the best known: (1) It occurs in rhombic crys tals, i having a specific gravity of 2.07, melting at 235° F., and soluble in carbon disulphide; (2) in monoclinic crystals, having a specific *gravity of 1.96, melting at 243° F., and soluble in car bon disulphide; (3) in an amorphous plastic state, insoluble in carbon disulphide; (4) im mediately above its melting point it is thin, clear and amber-colored; (5) at about F. it be comes thick and dark; and (6) at about F. it is again thin, but remains dark.
Ozone is a familiar allotropic form of oxygen, produced when the silent electric dis charge is allowed to act upon oxygen. It is known that the molecule of oxygen contains two atoms, and that the molecule of ozone con tains three atoms. This suggests that allotropy, in all cases, may be due to a similar change in the number of atoms present in a molecule; but so little is known of the ultimate structure of solids and liquids that speculation of this sort is of no great value.
Most of the non-metallic elements hare allotropic modifications, and remarkable cases of allotropy are observed among chemical com pounds. In the case of a compound, two states of a substance having the same chemical com position are said to be isomeric when their con stituents are combined by different modes of atomic linkage; and they are said to be allo tropic when the kind of atomic linkage is the same in both cases. See 'SOS/CRISS/.