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Allotropy, or Physical Isomerism, the term applied to the property possessed by many substances of differing in physical attributes, while remaining identical in chemical structure. Thus the Allotropy of elements is illustrated by the differences of crystalline form, colour, etc., which are assumed by the same element under different conditions, viz. the different varieties of carbon (as charcoal, graphite, and diamond), phosphorus, and sulphur. The Allotropy of compounds is illustrated by the varieties of silica (quartz, agate, and amorphous silica), mercuric sulphide (red and black), and so on. As a rule, the passage of one allotropic form into another is closely connected with change of temperature.