26 Dr THOMAS THOMSON on some Experiments on Gold. 



the gold. For I had tried the iron solution before mixing it 

 with the muriate of gold, by means of prussiate of potash, 

 which had struck with it a pretty strong blue, shewing, that the 

 iron was not at all in the state of protoxide, but had been at least 

 partially peroxidized ; for protoxide of iron is precipitated 

 white, and not blue, by prussiate of potash. This partial oxy- 

 dizement had been induced by the air existing in the distilled 

 water, and partly also by the air adhering to the crystals, when 

 they were put into the water. For when I let fall a small crystal 

 of protosulphate of iron into prussiate of potash, the precipitate 

 was not quite white ; but had a very sensible blue tinge. 



2. The preceding experiment was repeated with additional 

 precautions, to prevent the peroxydizement of the iron in the 

 protosulphate. 25 grains of gold were employed in the experi- 

 ment, and 104.25 grains of protosulphate of iron ; the distilled 

 water was boiled for half an hour before it was used, and the 

 protosulphate of iron crystals were thrown into the boiling-hot 

 liquid, which was added to the solution of gold as quickly as 

 possible. The gold solution in this second experiment was not 

 neutral, but had an excess of acid, from a notion that this excess 

 might have a tendency to prevent so much air from being con- 

 tained in the liquid as seemed to have been the case in the pre- 

 ceding experiment. The gold obtained weighed 24'9 grains ; so 

 that the loss was only O'l grain, which is little more than one- 

 tenth of the loss sustained in the first experiment. 



Even in this experiment, the iron was not absolutely in the 

 state of protoxide ; for the solution gave a whitish blue preci- 

 pitate with prussiate of potash. 



This last experiment coming within l^th of the theoretic 

 quantity, I was satisfied with it. We see that the 25 grains of 

 gold, dissolved in the muriatic acid, had been combined with 3 

 grains of oxygen. For six times 4'5 grains of protoxide of iron 



