26 H. C. COOKE 
When the iron sulphide present is pyrite, the silver sulphide is 
converted into sulphate wholly by the sulphuric acid and the 
ferric sulphate produced by the oxidation of the pyrite, according 
to the equation on p. 5. 
When marcasite is present, as it rarely is in quantity, silver sul- 
phide may be oxidized in part directly to silver sulphate by electro- 
lytic action,. and in part may be converted into sulphate by the 
action of the oxidation products of the marcasite. 
Sulphuric acid and ferric sulphate exert a powerful solvent 
action both on silver sulphide and on its companion sulphides, 
such as galena, chalcocite, orpiment, and stibnite. Of these, silver 
sulphide is the least affected. In all cases the action is much more 
powerful when ferric sulphate is present than when sulphuric acid 
acts alone. 
Except in the case of galena, the solvent action does not seem to 
be proportional to the concentration of the ferric sulphate present. 
This suggests that the sulphuric acid is really the active agent, and 
that the ferric sulphate acts principally as an agent for the removal 
from solution of hydrogen sulphide formed during the reaction. 
A mixture of sulphuric acid and ferric sulphate has a powerful 
solvent action on metallic silver, hence, in an ore-body containing 
much pyrite, little native silver may be expected in the gossan; 
and conversely, if much native silver be found in the gossan, the 
ore-body cannot have contained much pyrite, and little secondary 
enrichment should be expected. 
The presence of ferric sulphate in the ground-waters increases 
the solubility of silver sulphate in them. This is probably due to 
the formation of a complex ion AgSO, by the silver, when ferric 
sulphate is present in solution. 
Equilibrium in silver-bearing solutions between ferric, ferrous, 
and silver sulphates is such that the reduction of ferric solutions to 
the ferrous condition by any means will rapidly precipitate the 
silver in the metallic form. Precipitation of silver will not cease 
till all the ferric salt is reduced to the ferrous state. Hence the 
vertical extent of the zone of precipitation and its proximity to the 
surface will depend on the rapidity with which this reduction goes 
on. If reduction be slow, native silver may thus be formed even 
