70 PYRITES, OR SULPHURET OF IRON. i

removed from the fire in which it was heated. In fact,iron, when thus strongly heated, would catch fire and j

continue to burn like charcoal, if it were not that the J

crust of oxide formed protects the metal from further j

oxidation, and soon stops the combustion. <

208. One of the most common and abundant of the ;

ores of iron is called pyrites, which is a compound of <

iron and sulphur ; it is not used in the manufacture of !

iron, because it is very difficult to separate the sulphur j

completely from the iron, and the native oxides and 1

carbonate are far more convenient sources of the metal.

209. Pyrites, or sulphuret of iron, is however asubstance of considerable importance in the arts, beingone of the sources of sulphur, which is obtained by lieat-ing pyrites in an oven, so constructed that the sulphurwhich is expelled in the state of vapour from the pyrites

is cooled and condensed into the solid form in a different ■part of the oven. J

210. Pyrites, when exposed to the air, soon crumbles >

down, and undergoes a very curious change, in conse-quence of absorbing and combining with oxygen. Both >the iron and the sulphur combine with oxygen, and :form oxide of iron and sulphuric acid; and hence theresult of this action is sulphate of iron, or common j

green vitriol, a salt much used in the arts for a variety 1

of purposes. Pyrites is most abundantly found in the jform of variously-shaped balls imbedded in chalk ;and as the chalk-hills on the sea-side gradually wearaway from the action of the sea and weather, these