Allotropism (Gr. in another manner), a word first employed by Berzelius to denote the property in virtue of which the same element can have different chemical characters. There exists a vast series of phenomena, of which polymorphism constitutes the first term, allotropism the intermediate, and isomerism the extreme term. Sulphur, which crystallizes from its solution in octahedra of the fourth system, when crystallized by means of fusion forms prisms having a rhombic base, of the fifth system; this is a polymorphous body. Phosphorus being heated changes its properties; if we heat it still further, it regains its original condition. It can therefore exist in two different states, but it is always phosphorus. This is a phenomenon of allotropism. The formiate of ethyl, C3H6O2, and the acetate of methyl, C3H6O2, are two perfectly distinct bodies, although they have the same quantitative composition; they are isomeric. To take an illustration from natural history, allotropism only makes races, isomerism creates distinct species. Isomeric bodies receive distinct names; but sulphur, phosphorus, oxygen, carbon, etc, in their modified forms, that is, in their allotropic conditions, are still sulphur, phosphorus, and oxygen.

Originally the word allotropism was only applied to elements; later it was also applied to compounds. Tartaric acid which turns the plane of polarization of light to the right, tartaric acid which turns it to the left, and that which does not deviate it at all, are considered by some chemists to be the same compound in several allotropic states. (See Isomerism.)