SECONDARY ENRICHMENT OF SILVER ORES 23 
never rise so high as to bar the precipitation of silver as sulphide 
from solutions in which it is carried as chloride. 
MECHANISM OF THE SOLUTION OF SILVER MINERALS IN 
METEORIC WATERS 
The work of Gottschalk and Buehler, coupled with that of the 
writer, renders possible a statement of the method by which the 
silver sulphides are taken into solution in the ground-water. The 
potentials of the minerals studied by Gottschalk and Buehler, 
measured against that of copper, include marcasite, --o.37 volt; 
argentite, -+o.27 volt; pyrite, --o.18 volt. If we assume then an 
ore-body of argentite and pyrite, the pyrite, whose potential is 
lower than that of the argentite, will be oxidized, while the argen- 
tite is protected from direct oxidation. The silver can then be 
taken into solution only by the secondary action of the oxidation 
products of the pyrite, ferric sulphate, aid sulphuric acid. But 
if the iron sulphide of the deposit be marcasite—a rare case—then 
we might expect a direct oxidation of the silver sulphide to sulphate 
by the electrolytic action set up since marcasite possesses a higher 
potential than argentite. 
Moreover, the iron sulphides are the only sulphides which can 
effect the solution of the argentite. The potential of argentite is 
higher than that of the sulphide of any other metal except iron, 
hence it can undergo direct oxidation only in the presence of iron 
sulphide; and since no sulphides but those of iron produce by 
oxidation the solvents ferric sulphate and sulphuric acid, argentite 
can undergo indirect oxidation likewise only in the presence of 
iron sulphide. Therefore Weed’s deduction, that the secondary 
enrichment of silver is dependent on the presence of pyrite in the 
lodes, appears to be confirmed. 
FORMATION OF SILVER SULPHIDE 
The formation of the complex silver minerals by simultaneous 
precipitation of the component sulphides was not attempted, on 
account of lack *of facilities for handling large volumes of dilute 
solutions. It was thought, however, that these might be formed 
by substitution of silver for arsenic or antimony in the pre-existing 
