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SILICATES. 



By modifying the composition of a silicate, it may 

 always be rendered decomposable by an acid. For 

 example, a silicate containing the following elements : 



Silica, Lime, 



Alumina, Magnesia, 



Iron, Potassa, 



Manganese, Soda. 



This combination occurs in porphyry, gneiss, and 

 granite. 



In the first place it is necessary to observe the action 

 of heat upon the silicate, and if there is a loss in weight, 

 to determine its nature, whether it consists of water or 

 fluorine. 



Fig. 45. 



In most cases, the water contained in a silicate 

 evaporates at a red heat, and it is only necessary to 



