IRON, COBALT, AND NICKEL 339 



Ferric oxide, or sesquioxide of iron, Fe 2 O 3 , is found in nature, 

 and is artificially prepared in the form of a red -powder by many 

 methods. Thus after heating green vitriol a red oxide of iron remains, 

 called colcothar, which is used as an oil paint, principally for painting 

 wood. The same substance in the form of a very tine powder (rouge) 

 is used for polishing glass, steel, and other objects. If a mixture of 

 ferrous sulphate with an excess of common salt be strongly heated, 

 crystalline ferric oxide will be formed, having a dark violet colour, and 

 resembling some natural varieties of this substance. When iron pyrites 

 is heated for preparing sulphurous anhydride, ferric oxide also remains 

 behind ; it is used as a pigment. On the addition of alkalis to a 

 solution of ferric salts, a brown precipitate of ferric hydroxide is formed, 

 which when heated (even when boiled in water, that is, at about 100, 

 according to Tomassi) easily parts with the water, and leaves red 

 anhydrous ferric oxide. Pure ferric oxide does not show any mag- 

 netic properties, but when heated to a white heat it loses oxygen and is 

 converted into the magnetic oxide. Anhydrous ferric oxide which has 

 been heated to a high temperature is with difficulty soluble in acids 

 (but it is soluble when heated in strong acids, and also when fused with 

 potassium hydrogen sulphate), whilst ferric hydroxide, at all events 

 that which is precipitated from salts by means of alkalis, is very readily 

 soluble in acids. The precipitated ferric hydroxide has the composition 

 2Fe 2 ;i 3H 2 O, or Fo 4 H 6 9 . If this ordinary hydroxide be rendered 

 anhydrous (at 100), at a certain moment it becomes incandescent 

 that is, loses a certain quantity of heat. This self -incandescence 

 depends on internal displacement produced by the transition of the 

 easily- soluble (in acids) variety into the difficultly-soluble variety, 

 and does not depend on the loss of water, since the anhydrous oxide 

 .undergoes the same change. In addition to this there exists a ferric 

 hydroxide, or hydrated oxide of iron, which, like the strongly-heated 

 anhydrous iron oxide, is difficultly soluble in acids. This hydroxide on 

 losing water, or after the loss of water, does not undergo such self- 

 incandescence, because no such state of internal displacement occurs 

 (loss of energy or heat) with it as that which is peculiar to the ordinary 

 Oxide of iron. The ferric hydroxide which is difficultly soluble in acids 

 has the composition Fe 2 3 ,H 2 O. This hydroxide is obtained by a pro-, 



formed, then part of the ferrous salt still remains ; if there is none, the transformation is 

 complete. The thiocyanate does not give any marked coloration with ferrous salts ; but 

 ,with ferric salts in the most diluted state it forms a bright rod soluble compound, and 

 therefore when transforming a ferric salt into a ferrous salt we must proceed as before, 

 testing a drop of the solution with thiocyanate, when the absence of a red colour will 

 prove the total transformation of the ferric salt into the ferrous state, and if a red colour 

 Is apparent it shows that, the transformation is not yet complete. 



