Dr THOMAS THOMSON on some Experiments on Gold. 25 



To understand the reason for taking 208'5 grains of proto- 

 sulphate of iron, the reader has only to call to mind, that this 

 salt is composed of 



1 atom sulphuric acid, 5 

 1 atom protoxide of iron, 4'5 

 7 atoms water, 7'875 



17-^75 



so that 17*375 grains of this salt contain the equivalent of 1 

 atom of protoxide of iron. As 2 atoms of peroxide of gold were 

 to be reduced, it was necessary to employ 12 atoms of protoxide 

 of iron. Now, 17'375 X 12 = 208'5. So that 208'5 grains of 

 protosulphate of iron, contain the equivalent of 12 atoms of pro- 

 toxide of iron. 



The gold, precipitated by 208*5 grains of protosulphate of 

 iron, was collected on a filter, washed and dried, and exposed to 

 a red heat. It weighed 48'04 grains, or 1-96 grain less than the 

 quantity originally dissolved. An additional dose of protosul- 

 phate of iron being poured into the original gold solution, a far- 

 ther precipitate of gold was obtained, which weighed 1*67 grains. 

 Thus all the gold was recovered, with the exception of O29 

 grain, which I believe was lost, in consequence of the improper 

 method taken to wash the gold. This was done by decantation. 

 Now the films of gold were so extremely fine, that they were 

 very apt to swim on the surface of the liquid. And though I 

 was at great pains to avoid throwing any of the gold away, a few 

 of these flocks might have escaped my observation. And as the 

 decantation was repeated a good many times, I think a loss of 

 0*29 grain might have been sustained. 



The gold precipitated by 208'5 grains of protosulphate of 

 iron was almost 2 grains less than it ought to have been. I was 

 prepared to expect this diminution of weight before I weighed 



VOL. XI. PART I. D 



