PYRITES, OR SULPHURET OP IRON.

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balls of pyrites get exposed to the air, and fall downon the beach, whence they are collected for the use ofmanufacturers. Many springs of water contain a smallquantity of iron, in consequence of which they have apeculiar inky taste; this is usually derived from thegradual oxidation of sulphuret of iron,—and from thesame reason soils also sometimes contain traces of thissalt.

211. Sulphuret of iron is likewise very commonlyfound in coal, being sometimes dispersed throughout itin the form of little yellow shining particles, andsometimes as layers or lumps of the solid sulphuret ;its presence in coal is for some purposes highly objec-tionable ; because, whether the coal is burnt in its crudestate, or after being converted into coke, a quantity ofsulphurous acid gas is always formed by the imperfectcombustion of the sulphur; and that nauseous-smellinggas causes serious mischief in several operations in thearts, and consequently in such cases coal free fromsulphuret of iron can only be employed. It is the pre-sence of this substance in coal that causes the strongsuffocating smell of sulphurous acid sometimes given outby coal and coke fires.

212. The oxides of iron are quite insoluble in water,but many of the salts of iron, like the sulphate, arereadily soluble in it; the solutions of these salts are alldecomposed when a little alkali is added to them,—thiscombines with the acid, and the oxide of iron is separatedas an insoluble powder.