SECONDARY ENRICHMENT OF SILVER ORES 25 
salt would precipitate silver from solution as metal, so that a mix- 
ture of sulphur and metallic silver might result. Sulphur in the 
ordinary form will not react with silver, except under very high 
pressures,* which cannot be assumed under the conditions existing. 
However, as has been recently shown,? precipitated sulphur is 
not crystalline, but amorphous, and might therefore be expected to 
possess a greater chemical activity than crystalline sulphur. To 
test this, an acid solution of ferric sulphate was partially reduced 
to ferrous salt by passing in hydrogen sulphide, after which it was 
allowed to stand for a few minutes in order that all the hydrogen 
sulphide in solution might be used up. Some silver previously 
precipitated by reaction of ferrous sulphate with silver sulphate 
was then added. Reaction did not occur at once, but at the end of 
twenty-four hours all of the silver had been altered from silvery 
white flakes to black silver sulphide. To make certain that the 
black mass did not consist of marcasite, as might be suspected from 
the work of Allen,3 some sulphur was precipitated by treating 
sodium thiosulphate with sulphuric acid, filtered, washed, and 
added to some washed silver prepared as before. The results were 
the same, hence the end product of the reaction of hydrogen sul- 
phide with silver solutions is silver sulphide, whether it is the first 
product of precipitation or not. 
The conclusion may also be drawn from this reaction, that if 
in any deposit native silver were the first product precipitated, 
and at some subsequent time sulphur were also formed there, then 
the sulphur and silver would combine to form argentite, the com- 
pleteness of the alteration depending on the amount of sulphur. 
Such a hypothesis might possibly account for the mixtures of 
secondary argentite and native silver found at depths in some 
deposits, notably at Creede, Colo. 
SUMMARY 
Secondary sulphide enrichment of a primary silver deposit is 
brought about by reactions of silver or its sulphides with the 
sulphides of iron and their products of oxidation. 
1 Spring, loc. cit. 
2 Brownlee, Jour. Am. Chem. Soc., XXIX (1907), 1032. 
3 Allen, loc. cit. 
