a genus of inflammable substances composed of sulphur, which has dissolved or saturated itself with metals. Thus there are many kinds of pyrites; as of gold, arsenic, iron, &c. It is also the principal ore of sulphur; particularly that called martial pyrites, copperas-flone, or marcasite. This is very common, containing a quantity of sulphur in proportion to the iron; and, when thoroughly inflamed, burns by itself. It is either of a compact texture, steel-grained, coarse-grained, or crystallized. In this last form, it floats mostly into cube and octohedral figures, though it is met with also in innumerable other forms.

The liver-coloured marcasite has an appearance between that of the preceding and the blue copper-ore. The iron predominates in this kind, so that it is less fit than the other for extracting sulphur from it, or for the smelting of copper-ores. It is formed of a compact texture, coarse-grained, and steel-grained. See Chemistry, no 619 and 654; Mineralogy, p. 109; and Metallurgy, p. 479.