236. _PYRITES, or MARCASITE, is a mineral substance, formed by a combination of iron with sulphur._ _It is usually of a bronze, yellow, or brownish colour, very various in form, being massive, globular, club─shaped, oval, or crystallized; and so hard as to strike fire with flint._ Few minerals are more common than this, as it occurs, in some state or other, in almost every rock and vein. It is often found among coals; and, when heated, decrepitates with a loud unpleasant noise and sulphureous smell. To the decomposition of this mineral it is that the hot temperature of almost all the mineral waters may be ascribed. The name of pyrites, which in the Greek language signifies _firestone_, has been obtained by this mineral from its property of striking sparks from steel. It was formerly used for fire─arms, as we now use flints. In commerce it is known by the name of marcasite. Some years ago it was much used, particularly in France, for the making of buttons and buckles; and was cut and polished, by lapidaries, for trinkets, particularly for the rims and hands of watches, and various kinds of female ornaments. If skilfully cut in the form of small rose diamonds, although an opaque substance, it has somewhat the appearance of a diamond. In the tombs of the Peruvian princes, with whom a considerable portion of their valuables was always interred, there have been found polished plates of marcasite, which appear to have served them as mirrors. This mineral is never worked as an ore of iron; and it is principally valued on account of the sulphur which can be obtained from it by means of heat; and the green vitriol, or copperas (208), which it affords by exposure to the air. Ignorant persons frequently mistake iron pyrites for gold; but it is easily distinguished from that precious metal by its brittleness. It breaks when hammered, whereas gold is malleable, or may be extended by hammering: it also strikes fire with steel, which gold will not. 237. _RED OCHRE, REDDLE, or RED CHALK, is an iron ore of blood─red colour, which is sometimes found in powder, and sometimes in a hardened state. It has an earthy texture, and stains the fingers when handled._ The principal use of red chalk is for drawing: the coarser kinds are employed by carpenters and other mechanics, and the finer kinds by painters. For the latter purpose it should be free from grit, and not too hard. In order to free it from imperfections, and render it better for use, it is sometimes pounded, washed, mixed with gum, and cast into moulds of convenient shape and size. Under the name of reddle, this substance is much used for the marking of sheep; and (when mixed with oil) for the painting of pales, gates, and the wood─work of out─buildings. Another kind of iron ore, or rather a compound of the ores of iron and manganese, is called _umber_. This mineral, which is of a brown colour, is found in beds in the island of Cyprus, and is used as a kind of paint both in a raw state and burnt.