194 ACIDIFIABLE COMBUSTIBLES. 



the preceding experiment, after being sufficiently 

 washed was left on the filter till it was dry enough 

 to be moved. It was then placed (still on the 

 filter) in a linen bag, which was suspended at 

 some height above my sand bath, in a tempera- 

 ture of 85, where it was left for several days, 

 till it ceased to lose weight. By this treatment, 

 the silica became a very white, dry powder, 

 which weighed 38*4 grains. Being exposed to 

 a red heat, the weight was reduced to 29*97 

 grains ; so that it had been a compound of 29*97 

 silica, and 8-43 water. Now, 29'97 : 8-43 : : 4 

 : 1*125 = the water united to 4 of silica. Here 

 we see that the silica and water were united in 

 the true atomic proportions. And this is an ex- 

 periment which I have often made successfully. 

 Thirteenth g. If the silicated potash, or soda, be digested 



hydrate of . . f 



silica. m a sufficient quantity or water, we may obtain 

 a complete solution, which is neither precipitat- 

 ed by acids nor alkalies. When this solution is 

 concentrated by evaporation, it assumes the form 

 of a transparent, colourless, tremulous jelly, con- 

 sisting of silica and water, united together in the 

 form of a hydrate. If we throw this jelly upon 

 the filter, and wash it with hot water till the al- 

 kali be completely removed, the silica remains 

 in the state of a matter, so transparent and co- 

 lourless, as scarce to be perceptible at first. If 

 we allow this jelly to dry on the filter till it ceases 

 to feel wet, or to moisten bibulous paper, it still 



