192 ACIDIFIABLE COMBUSTIBLES. 



bonate of soda, and the fused mass digested in 



of silica. 



hot water, we may either obtain a solution, or a 

 white insoluble flocky residue may be left, ac- 

 cording to the quantity of water which we em- 

 ploy. This floky residue is a hydrate of silica, 

 which may be separated by the filter, washed by 

 repeated effusions of water, and dried in a tem- 

 perature which must not exceed 85 or 90. In 

 summer, when the weather happened to be very 

 warm, I have repeatedly obtained an accurate 

 atomic hydrate of silica by drying it in the open 

 air ; but in general, unless the temperature be 

 artificially raised, the silica retains a small excess 

 of water. This hydrate, when properly prepar- 

 ed, is composed of 



Silica . . 4 



Water . . 1-125 



5-1 25 



Now, 1-1&5 is 1 atom of water, and 4 must be 

 just 2 atoms of silica ; so that the hydrate is 

 a compound of 1 atom water, and 2 atoms si- 

 lica ; and an atom of silica weighs exactly 2. 



I shall relate two experiments which I made 

 to obtain this hydrate, the first an unsuccessful, 

 and the second a successful one, that the read- 

 er may be enabled to judge how great a surplus 

 of water the silica may retain. 



About the middle of May, 1823, I fused a 



