IRON, COBALT, AND NICKEL 835 



that is, corresponding to the suboxide FeO and answering to magnesium 

 compounds and hence two atoms of hydrogen are replaced by one 

 atom of iron. Strongly oxidising acids like nitric acid may transform 

 the ferrous salt which is forming into the higher degree of oxidation or 

 ferric salt (corresponding with the sesquioxide, Fe 2 O 3 ), but this is a 

 secondary reaction. Iron, although easily soluble in dilute nitric acid, 

 loses this property when plunged into strong fuming nitric acid ; after 

 this operation it even loses the property of solubility in other acids 

 until the external coating formed by the action of the strong nitric 

 acid is mechanically removed. This condition of iron is termed the 

 passive state. The passive condition of iron depends on the formation, 

 on its surface, of a coating of oxide due to the iron being acted on by 

 the lower oxides of nitrogen contained in the fuming nitric acid. 16 

 Strong nitric acid which does not contain these lower oxides, does not 

 render iron passive, but it is only necessary to add some alcohol or 

 other reducing agent which forms these lower oxides* in the nitric acid, 

 and the iron will assume the passive state. 



Iron readily combines with non-metals for instance, with chlorine, 

 iodine, bromine, sulphur, and even with phosphorus and carbon ; but 

 on the other hand the property of combining with metals is but little 

 developed in it that is to say, it does not easily form alloys. Mercury, 

 which acts on most metals, does not act directly on iron, and the iron 

 amalgam, or solution of iron in mercury, which is used for electrical 

 machines, is only obtained in a particular way namely, with the 

 co-operation of a sodium amalgam, in which the iron dissolves and by 

 means of which it is reduced from solutions of its salts. 



When iron acts on acids it forms ferrous salts of the type FeX 2 , 

 and in the presence of air and oxidising agents they change by degrees 

 into ferric salts of the type FeX 3 . This faculty of passing from the 

 ferrous to the ferric state is still further developed in ferrous hydroxide. 

 If sodium hydroxide be added to a solution of ferrous sulphate or 

 green vitriol, FeSO 4 , 17 a white precipitate of ferrous hydroxide, FeH 2 O 2 , 



16 Powdery reduced iron is passive with regaxd to nitric acid of a specific gravity of 

 1*87, but when heated the acid acts en it. This passiveness disappears in the magnetic 

 field. Saint-Edme attributes the passivenesa of iron (and nickel) to the formation of 

 nitride of iron on the surface of the metal, because he observed that when heated in dry 

 hydrogen ammonia is evolved by passive iron. 



Eemsen observed that if a strip of iron be. immersed in acid and placed in the mag- 

 netic field, it is principally dissolved at its middle part that is, the acid acts more feebly 

 at the poles. According to Etard (1891) strong nitric acid dissolves iron in making it 

 passive, although the action is a very slow one. 



17 Iron vitriol or green vitriol, sulphate of iron or ferrous sulphate, generally crys- 

 tallises from solutions, like magnesium sulphate, with seven molecules of water, 

 FeS0 4 ,7H 2 O. This salt is not only formed by the action of iron on sulphuric acid, but 



