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198. The most widely diffused and abundant ofall the metallic oxides, as well as that which is themost important and valuable in the arts, is the oxideof iron, which exists in different quantities in a greatvariety of stones, is very common in soils, and isconstantly present in very small quantity in the bloodof animals.
199. Iron is very rarely indeed found native in itspure metallic state, but is usually met with in the form ofan oxide, either pure or combined with carbonic acid,and mixed with alumine and silica. Thus the richblack and red iron ores of Cumberland and other placesare nearly pure oxide of iron, whilst the common clayiron-stones, as they are called, of Staffordshire andWales , are principally carbonate of iron, mixed withvarious proportions of alumine and silica,
200. The important art of smelting iron is entirely achemical operation, and depends mainly upon the fact,that at a high temperature, carbon has a stronger affinityfor oxygen than iron has; and hence when the nativeoxide of iron is heated with coal or charcoal, it is decom-posed, and carbonic acid gas and metallic iron are theresults of the process.
201. When those ores are smelted which consist prin-cipally of oxide of iron, they are at once heated withcarbon; but when the clay iron-stones are used—andthey are the ores most commonly employed—they arefirst submitted to a preparatory process something likethe burning of lime, in order to expel the carbonic acid