OXIDES OF IRON. 125 



a chemical operation, and depends mainly, upon the 

 fact th«it, at a high temperature, carbon has a 

 stronger affinity for oxygen than iron has ; and hence, 

 when the native oxide of iron is heated with coal or 

 charcoal, it is decomposed, and carbonic acid gas and 

 metallic iron are the results of the process. 



276. When those ores are smelted which consist 

 principally of oxide of iron, they are at once heated 

 with carbon ; but when the clay iron-stones are used 

 — and they are the ores most commonly employed — 

 they are first submitted to a preparatory process, 

 something like the burning of lime, in order to ex- 

 pel the carbonic acid gas which they contain ; and 

 when thus converted into oxide of iron, they are 

 mixed with carbon and lime, the use of the latter be- 

 ing to combine with the silica or silicic acid and alu- 

 mina, and form with them fusible silicates called the 

 slag, which greatly assists in the melting and running 

 together of the newly-reduced iron ; and, besides, by 

 covering the metallic iron with a glassy coat, they 

 protect it from further oxidation from the oxygen of 

 the air. 



277. Iron is able to form two distinct compounds 

 with oxygen, according to the quantity of that ele- 

 ment with which it combines : when it is combined 

 with two-sevenths of its weight of oxygen, it consti- 

 tutes a black substance, which is called the protoxide, 

 and when combined with three-sevenths, forms a 

 brownish-red substance, called the peroxide. These 

 oxides are both bases, and each forms a distinct series 



11* 



