190 ACIDIFTABLE COMBUSTIBLES. 



experiments, is below %. But the weight just 

 deduced is from a much greater number of ex- 

 periments, and ought, therefore, to be much 

 nearer the truth. 

 Ratio be- jjj j t occurpe d to me, that, as silica, when 



tween the 



atom of si- heated with carbonate of potash, has the proper- 



lica and . r . A . 



carbonic ty of disengaging the carbonic acid gas and unit- 

 ing with the potash in its place, I might be able 

 to form some estimate of the atomic weight of 

 silica by mixing determinate quantities of it with 

 pure dry carbonate of potash, exposing the mix- 

 ture to a strong heat in a platinum crucible, and 

 noting the loss of weight. 



I found, in the first place, that carbonate of 

 potash, obtained by exposing crystals of the bi- 

 carbonate to a red heat, lost no sensible weight 

 though kept for a considerable time in a state of 

 fusion. I found, in the second place, that, to make 

 a successful experiment, it was necessary to em- 

 ploy, at least, eight times as much carbonate of 

 potash as of silica. When less was employed, 

 the loss of weight was always much too small. 

 Twelve grains of pure silica in fine powder, and 

 anhydrous carbonate of potash, were mixed to- 

 gether, and kept in a state of fusion for 13 hours 

 in a platinum crucible. The loss of weight 

 amounted to 16*67 grains. 



It appears from this experiment, that 16'G7 

 grains of carbonic acid are displaced by 12 

 grains of silica : but upon examining the cruci- 



