IRON. 167 



forming a fusible glass, called cinder or slag. The iron and slag 

 collect at the bottom of the furnace, where they separate by gravity, 

 and are run off every few hours. 



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

 pure, but always contains, besides silicon (also sulphur, phosphorus, 

 and various metals), a quantity of carbon varying from 2 to 5 per 

 cent. It is the quantity of this carbon and its condition which im- 

 parts to the different kinds of iron different properties. Steel contains 

 from 0.16 to 2 per cent., wrought- or bar-iron but very small quanti- 

 ties, of carbon. Wrought-iron is made from cast-iron by the process 

 known as puddling, which is a burning-out of the carbon by oxida- 

 tion, accomplished by agitating the molten mass in the presence of 

 an oxidizing flame. Steel is made either from cast-iron by partially 

 removing the carbon, or from wrought-iron by recombining it with 

 carbon i. e., by agitating together molten wrought- and cast-iron in 

 proper proportions. 



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 yield- 

 ing 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 ex- 

 ception 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 hydroxide (rust) are formed. 



Iron forms two series of compounds, distinguished as ferrous and 

 ferric compounds ; in the former, iron is bivalent, in the latter, appa- 

 rently trivalent, because, as shown above, the double atom exerts a 

 valence of six. Almost all ferrous compounds show a tendency to 

 pass into ferric compounds when exposed to the air, or more readily 

 when treated with oxidizing agents, such as nitric acid, chlorine, etc. 

 As the reaction of iron in ferrous and ferric compounds differs con- 

 siderably, they must be studied separately. 



Reduced iron, Perrum reductum. This is metallic iron, obtained 

 as a very fine, grayish-black, lustreless powder by passing hydrogen 

 gas (purified and dried by passing it through sulphuric acid) over 

 ferric oxide, heated in a glass tube : 



Fe 2 3 + 6H = 3H 2 O + 2Fe. 



