IROX. 145 



The heat necessary for this decomposition and fusion of the 

 ore is produced by the combustion of the coal, maintained by 

 the oxygen of the air blown into the furnace. The same air, 

 however, would also burn (oxidize) the iron in the lower and 

 hotter part of the furnace, were it not protected from coming 

 in contact with it by a thin film of a fusible silicate (slag), formed 

 by the combination of the sand and calcium of the limestone, 

 which is added for that purpose. The iron and slag collect at 

 the bottom of the furnace, and are allowed to run off every few 

 hours. 



Iron thus obtained is known as cast-iron, and is not pure, 

 but always contains, besides traces of silica (occasionally also 

 sulphur, phosphorus, and various metals), a quantity of carbon 

 varying from 2 to 5 per cent. It is the quantity of this carbon 

 which imparts to the different kinds of iron different properties. 

 Steel contains from 0.5 to 2 per cent., wrought- or bar-iron from 

 0.03 to 0.3 per cent, of carbon. Wrought-iron ie made from 

 cast-iron by the process known as puddling, which is a burning 

 out of the carbon by oxidation. Steel is made either from 

 cast-iron by partially removing the carbon, or from wrought- 

 iron by recombining it with carbon. 



Properties. The high position which iron occupies among 

 the useful metals is due to a combination of valuable properties 

 not found in any other metal. Although possessing nearly 

 twice as great a tenacity or strength as any of the other 

 metals commonly used in the metallic state, it is yet one of 

 the lightest, its specific gravity being about 7.7. Though being 

 when cold the least yielding or malleable of the metals in 

 common use, its ductility when heated is such that it admits 

 of being rolled into the thinnest sheets and drawn into the 

 finest wire, the strength of which is so great that a wire of one- 

 tenth of an inch in diameter is capable of sustaining 700 pounds. 

 Finally, iron is, with the exception of platinum, the least fusible 

 of all the useful metals. 



Iron is little affected by dry air, but is readily acted upon by 

 moist air, when ferric oxide and ferric hydrate (rust) are formed. 



Iron forms two series of compounds, distinguished as ferrous 

 and ferric compounds ; in the former, iron is bivalent, in the 

 latter trivalent, but it is most likely that the trivalent atom of 



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