SECONDARY PRECIPITATION OF GOLD 255 
of sulphuric acid was added to suppress the precipitation of ferrous 
sulphide. The solution and precipitate were allowed to stand 
over night, then filtered, and the precipitate was taken into solution 
with fuming nitric acid and a small amount of concentrated hydro- 
chloric acid. If any sulphide were present it would be oxidized 
to sulphate under these conditions. The solution was diluted 
and filtered; barium chloride solution gave no precipitate, showing 
the absence of sulphates, and thus proving that no gold sulphide 
was present in the precipitate. It is conceivable under these 
conditions that the hydrogen sulphide may have reduced some 
of the ferric salt before the gold began to precipitate and that the 
ferrous salt precipitated the gold. If this had been the case the 
precipitate should have shown the presence of sulphur, according 
to the reaction: 
Fe,(SO,);+H.S = 2FeSO,+H.SO,+S 
which takes place before the precipitation of ferrous sulphide 
begins. The action of the nitric acid in the above experiment 
would have oxidized such precipitated sulphur to sulphuric acid, 
and as the test showed the absence of sulphates it is evident that 
the above reaction did not take place, but that the hydrogen 
sulphide acted on the gold chloride. The gold sulphide, Au.S,, 
was probably formed first, then quickly oxidized to metallic gold 
and sulphuric acid by the ferric sulphate present. 
A further experiment along the same line was made as follows: 
Gold sulphide was precipitated and placed in contact with ferric 
sulphate, free from ferrous salt. After two weeks the solution 
contained a noticeable amount of ferrous salt, showing that part 
of the ferric salt had been reduced by the gold sulphide, but the 
appearance of the precipitate had not changed appreciably. Fresh 
ferric salt solution was added with similar results, and after stand- 
ing three months small particles of what appeared to be gold could 
be seen. They could not be separated from the gold sulphide 
however, and the evidence that they were particles of gold is far 
from conclusive. 
From these experiments it seems likely that the presence of 
ferric salts tends to cause the precipitation of free gold rather than 
