Marcassita Pharm. Paris. Pyrites or Marcasite: a hard fossil; striking fire with steel, copiously, and in large sparks; becoming vitriolic, either by simple exposure to the air, or by calcination and subsequent exposure.

This mineral varies extremely in its appearances. It is found of a bright brass yellow, of a greenish, of a grey or whitish colour, and of different intermediate or mixt shades: in masses, rarely of any great size, globular, oblong and flattish, cubical, octoedral, dodecaedral; some-times covered with a coat Or crust, but oftener bare; internally sometimes striated, and some-times of an even and simple structure (a). It is met with in different places of this kingdom, and in mod parts of the world; on the surface of the earth, on the sea shores, in clay pits, embedded in earthy and stony bodies of various kinds.

The pyritae consist, in general, of sulphur, iron, and unmetallic earth: in some, a little copper is joined to the iron; and in some, copper is the prevailing metal. In some, particularly the yellow kind, the quantity of sulphur is large: in others, particularly the white, both the sulphur and metal are in small proportion.

If artificial mixtures of sulphur with iron or copper be gently calcined, the inflammable principle of the sulphur exhales, and its acid remains united with the metal, forming therewith a saline vitriolic compound: a mixture of iron filings and sulphur, moistened with water, suffers a like change without external heat, and if the quantity is large, bursts spontaneoufly into fire. A resolution of the same kind happens in the natural pyritae on exposing them to the air and rain; provided, where they are very fulphureous, a part of the sulphur be previously dissipated by calcination. On this exposure, they all become powdery and acquire a vitriolic taste, the serrugineous much more easily than those which have any admixture of copper: some shoot out efflorescences of vitriol upon the surface: from others, the saline matter, washed off by rain, is found to consist chiefly of the sulphureous or vitriolic acid. If the pyritae, even such as have the least sulphureous and metallic impregnation, as those from which the English vitriol is made, be laid in large heaps, they grow hot, and take fire, and emit, during the burning, strong diffusive sulphureous vapours, (a).

(a) See Henckel's Pyritologia oder kiefs-historie.

The pyritae, in substance, are never used medicinally; but in their products they are very important. It is from these, that common sul-phur is extracted, in Sweden and Saxony; that the native vitriols are produced in caverns of the earth or on its surface; that the greatest quantities of artificial vitriol are prepared; and that the chalybeate mineral waters are supposed to receive their impregnation: see the respective articles.