146 METALS AND THEIR COMBINATIONS. 



iron is always present as a double atom, and consequently ferric 

 chloride for instance has not the composition FeCl 3 , but Fe 2 Cl 6 . 

 As the reactions of iron in ferrous and ferric compounds differ 

 considerably, they must be studied separately. 



Reduced iron, Ferrum reductum. This is metallic iron, ob- 

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

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

 over ferric oxide, heated in a glass tube : 



Fe 2 O 3 + 6H = 3H 2 O 4- 2Fe. 



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 hydrate Fe 2 HO may be obtained by the addi- 

 tion of any alkaline hydrate 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 

 the absorption of oxygen : 



FeSO 4 + 2NH 4 HO = (NH 4 ) 2 SO 4 -f Fe2HO; 

 2(Fe2HO) + O + H 2 O = Fe 2 6HO. 



The precipitation of ferrous hydrate 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 pale green color. 



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

 obtained by heating ferric hydrate to expel water : 



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



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



Ferric hydrate, Ferri oxidum hydratum, Fe 2 6HO = 213.8 (Hy- 

 drated oxide of iron, Per- or sesqui-oxide, Red oxide of iron), is 

 obtained by the precipitation of ferric sulphate or ferric chloride 

 by ammonium or sodium hydrate : 



Fe 2 3SO 4 + 6JSTH 4 HO = 3[(NH 4 ) 2 S0 4 ] + Fe 2 6HO. 



The precipitation is complete, no iron remaining in solution 

 as in the case of ferrous salts. 



