DECOMPOSITION OF FELSPAR. 85 



Most of the insoluble silicates containing alkaline bases are 

 decomposed by the action of hot water, particularly when that 

 water contains an acid. In the middle of the last century, the 

 ignorance of this fact led chemists to believe that water might 

 be converted into an earth. 



When water is distilled in glass vessels, it is found to contain 

 always a certain quantity of earthy substances, which may be 

 detected by evaporation, even if the water has been subjected tc 

 many repeated distillations. Lavoisier proved that part of the 

 glass was dissolved in this operation by the boiling water ; and 

 further, that the diminution in the weight of the glass vessel cor- 

 responded exactly to the quantity of earthy residue left by the 

 evaporation of the water. When the distillation of water is 

 effected in metallic vessels no such residue can be obtained. 



The action of water upon the silicates contained in glass may 

 be observed in the opacity which gradually comes over the win- 

 dows of hot-beds, these being exposed in a great degree to the 

 influence of the air. This action is more marked in the win 

 dows of stables, where the carbonic acid formed by the processes 

 of respiration of the animals, and by the decay of animal matter, 

 accelerates the decomposition. 



Silica being an acid of a very feeble character, the decompo- 

 sition of the soluble silicates is effected even by carbonic acid. 



A solution of soluble glass may be converted into a gelatinous 

 mass by saturating it with carbonic acid gas. The same decom- 

 position must take place in very dilute sotutions, although we 

 cannot detect in them any separation of silica, which remains 

 dissolved in the water. 



The decomposition of silicates by the combined action of water 

 and of acids proceeds with a rapidity proportional to the quantity 

 of alkalies contained in them. 



We find numerous examples in the inorganic kingdom of a con- 

 tinued and progressing process of decomposition of the silicates 

 contained in the various kinds of rocks ; this decomposition is ef- 

 fected by the action of carbonic acid, and of water. 



A consideration of the preceding observations shows clearly 

 that porcelain clay or kaolin has been formed by the decompos- 

 ing action of water on the silicates of potash and soda contained 



