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OXIDES OP IKON.

gas which they contain; and when thus converted intooxide of iron, they are mixed with carbon and lime, theuse of the latter being to combine with the silicaor silicic acid and alumine, and form with them afusible compound called a slag; which greatly assistsin the melting and running together of the newly-reduced iron.

202. Iron is able to form two distinct compoundswith oxygen, according to the quantity of that gas withwhich it combines : when it is combined with two-sevenths of its weight of oxygen, it constitutes a blacksubstance, which is called the protoxide, and whencombined with three-sevenths, forms a brownish-redsubstance, called the peroxide. These oxides are bothbases, and each forms a distinct series of salts bycombining with acids; but the salts formed by theprotoxide have always a tendency to absorb oxygen fromthe air, and thus become converted into the salts of theperoxide.

203. The colour of a great many stones and soils isprincipally caused by the presence of a small quantityof either the peroxide of iron, or of a mixture of bothits oxides.

204. The rusting of iron, which proceeds so rapidlywhen iron is exposed to damp air, is caused by theattraction which the metal has for oxygen. It is veryremarkable that iron is unable to combine with the freeoxygen always in the air, but is able to take it from itscompound with hydrogen ; for we find that in dry air,