in natural history, are defined to be compound inflammable metallic bodies, of a hard and solid substance, of an obscurely and irregularly foliaceous structure, of a bright glittering appearance, naturally constituting whole strata, though sometimes found in detached masses; very freely giving fire with steel; not fermenting with acid menstruums; and when put into the fire, yielding a blue sulphureous flame, and afterwards calcining into a purple powder. There are only three known species of this genus: 1. The silver-coloured marcasite, found in vast abundance in lead and tin mines. 2. The gold-coloured marcasite. 3. The heavy pale-white marcasite.

Marcasites were at first supposed to be almost all pure gold or silver, according to their colour; but experience has shewn, that if they contain any metal at all, no method has hitherto been found of working them to advantage. In Germany, indeed, they extract sulphur and vitriol from the silver marcasite, which two substances are always contained in it; and besides these, it has usually a quantity of arsenic. It has been recommended as a styptic, after being calcined: but as the arsenic may not be all carried off by that operation, its use as a medicine seems extremely dangerous.