PERCOMPOUNDS OF IRON. 5(>l 



iron, Fe O, Fe 2 O 3 .* The hydrated peroxide produced by the 

 oxidation of iron pyrites, of which it retains the form, contains 



1 eq. of water, or 10.31 per cent, and that from the oxidation 

 of the carbonate of iron, 3 eq. of water, or 14.71 per cent, to 



2 eq. of peroxide, (Mitscherlich, Lehrbuch, II. 23, 1840.) The 

 hydrate is the yellow colouring matter of clay, and with silica 

 and clay it forms the varieties of ochre. 



When metallic iron is oxidated gradually in a large quantity 

 of water, there forms around it a light precipitate of a bright 

 orange yellow, which is a ferric hydrate, according to Berzelius, 

 and of which the empirical formula is 2Fe 2 O 3 4-3HO, the 

 usual composition of brown hematite. When iron is oxidated 

 in deep water, it is converted, according to Mr. E. Davy, into 

 the magnetic oxide, which is possibly formed by cementation 

 from the hydrated peroxide. The hydrated peroxide is also 

 obtained, by precipitation from the persalts of the metal, by 

 ammonia and by a hydrated or carbonated alkali ; but never 

 pure, as when an insufficient quantity of alkali is added, a sub- 

 salt containing acid falls, and when the alkali is added in excess, 

 a portion of it goes down in combination with the peroxide, and 

 cannot be entirely removed by washing. When ammonia is 

 used, the water and excess of the precipitant can be expelled by 

 ignition, and the pure peroxide obtained. The latter is not 

 magnetic, and after ignition dissolves with difficulty in acids. 

 When ignited strongly, it loses oxygen and becomes magnetic. 



The peroxide of iron and its compounds are strictly isomor- 

 phous with alumina and the compounds of that earth, and 

 remarkably analogous to them in properties. It is a weak base, 

 of which the salts have a strong acid reaction, and are decom- 

 posed by all the magnesian carbonates, as well as by the mag- 

 nesian oxides themselves. The solutions of its salts, which are 

 neutral in composition, have generally a yellow tint, but they 

 are all capable, when rather concentrated, of dissolving a great 

 excess of peroxide and then become red. Very dilute solutions 

 of the neutral salts of peroxide of iron are decomposed by 

 ebullition, and the peroxide entirely precipitated, the acid of 

 the salt then uniting with water as a base, (Scheerer.) 



*One of the hydrates, probably this one, occurs vei'y rarely crystallized in very 

 small crystals, derived from the cube or octohedron, (lierthier, Traite", II. 225), 

 that is, in the form of the magnetic oxide of iron a circumstance of great inte- 

 rest, if it is an instance of the isomorphism of hydrogen with iron or a 

 metal. 



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