64 



are sometimes soluble substances capable of crystallising from 

 solution in water, and sometimes insoluble earthy powders, e.g., 

 Carbonate of Lime. 



In the case of Iron and some other metals, there are two 

 Oxides which are both capable of forming salts when acted on by 

 acids. These Oxides of Iron are supposed to be composed of 

 one atom of Iron combined with one atom of Oxygen, having 

 the formula, Fe 0, and of two atoms of Iron combined with 

 three of Oxygen, having the formula Fe20s ; they may be dis- 

 tinguished as Protoxide and Peroxide respectively, and the salts 

 formed by their action on acids as Protosalts and Persalts. 



The protoxide and protosalts have the power of uniting with 

 the oxygen of the air and passing into the state of peroxide and 

 persalts ; and, conversely, the peroxide and persalts may be 

 reduced, i.e., deprived of oxygen, by the action of substances 

 having a strong attraction for oxygen, and are thus converted 

 to protoxide and protosalt. 



The protoxide and protosalts are as a rule but slightly coloured, 

 generally being of a greyish, greenish, or pale sandy colour ; 

 while the proxide and persalts are much more strongly coloured, 

 being generally of various shades between yellow, brown, and 

 deep red. 



Now the compounds of iron are almost invariably present in 

 all rocks in varying proportions, the compounds being generally 

 protoxide, protocarbonate, protosilicate, peroxide, andpersilicate; 

 and since the essential materials of the rock frequently have but 

 little colour of themselves, the colour of the rock will frequently 

 depend to a very considerable extent upon the kind of iron com- 

 pound present. 



The hydrated (i.e., combined with water) protoxide, when 

 freshly prepared by artificial means, is a greyish coloured 

 precipitate, but on exposure to the air immediately begins to 

 absorb oxygen, changing from a greyish to a greenish, and 

 finally to a rusty-brown colour, eventually becoming converted 

 into the hydrated peroxide. 



The Protocarbonate is of very considerable interest with 



