IRON. 295 



and the conversion of the crystalline structure to a fibrous condition, in which 

 state steel is more elastic, tougher, and of greater tensile strength. 



Iron forms two series of compounds, distinguished as ferrous and 

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

 apparently trivalent. 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 diifers con- 

 siderably, they must be studied separately. Ferrous oxide and 

 hydroxide are more strongly basic than ferric oxide and hy- 

 droxide. 



Reduced iron, Ferrum 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 : 



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



The official article should have at least 90 per cent, of metallic 

 iron. 



Ferrous oxide, FeO (Monoxide or suboxide of iron). This com- 

 pound is little known in the separate state, as it has (like most ferrous 

 compounds) a great tendency to absorb oxygen from the air. The 

 ferrous hydroxide, Fe(OH) 2 , may be obtained by the addition of any 

 alkaline hydroxide to the solution of any ferrous salt, when a white 

 precipitate is produced which rapidly turns bluish -green, dark-gray, 

 black, and finally brown, in consequence of absorption of oxygen 

 (see Plate I., 2) : 



FeSO 4 + 2NH.OH = (NH 4 ) 2 SO 4 + Fe(OH) 2 ; 

 2Fe(OH) 2 + O + H 2 = Fe 2 (OH) 6 . 



The precipitation of ferrous hydroxide is not complete, some iron 

 always remaining in solution. 



Ferrous oxide is a strong base, uniting with acids to form salts, 

 which have usually a palo-groen color. 



Ferric oxide, Fe 2 O 3 . A reddish-brown powder, which may be 

 obtained by heating ferric hydroxide to expel water : 



2Fe(OH) 3 = Fe 2 O 3 + 3K.O. 

 It is a feeble base ; its salts show usually a brown color. 



