168 METALS AND THEIR COMBINATIONS. 



The official article should have at least 80 per cent, of metallic 

 iron. 



Ferrous oxide, PeO (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 4 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 pale-green color. 



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

 obtained by heating ferric hydroxide to expel water : 

 Fe 2 (OH) 6 =a Fe 2 O 3 -f 3H 2 O. 



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



Ferric hydroxide, Ferric hydrate, Ferri oxidum hydratum, 

 Fe 2 (OH) 6 =213.8 (Hydrated oxide of iron. Per- or sesqui-oxide, Red 

 oxide of iron), is obtained by precipitation of ferric sulphate or ferric 

 chloride by ammonium or sodium hydroxide (see Plate I., 3) : 

 Fe 2 (S0 4 ) 3 + 6NH 4 OH = 3[(NH 4 ) 2 SOJ + Fe 2 (OH) 6 . 



Precipitation is complete, no iron remaining in solution as in the 

 case of ferrous salts. 



Ferric hydroxide is a reddish-brown powder, sometimes used as 

 an antidote in arsenic poisoning ; for this purpose it is not used in 

 the dry state, but after having been freshly precipitated and washed, 

 it is mixed with water, and this mixture used. Kecently precipitated 

 and consequently highly divided ferric hydroxide combines more 

 readily with arsenous acid than the hydroxide which has been kept 

 some time, or which has been dried, and thereby assumed a denser 

 condition. 



Hydrated oxide of iron with magnesia, U. S. P., is a mixture made by adding 

 magnesia to a solution of ferric sulphate, when magnesium sulphate and ferric 



