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in crystallography, the condition in which a solid chemical compound exists in more than one crystalline form; the forms differ somewhat in physical and, sometimes, chemical properties, although their solutions and vapours are identical. The existence of different crystalline or molecular forms of elements is called allotropy, although it has been suggested that the meaning of allotropy should be restricted to different molecular forms of an element, such as oxygen (O2) and ozone (O3), and that polymorphism be applied to different crystalline forms of the same species, whether a compound or an element. Differences in the crystalline forms of many elements and compounds were discovered during the 1820s by Eilhardt Mitscherlich, a German chemist.