SECONDARY ENRICHMENT OF SILVER ORES ay | 
gradually changed to the ferrous state as the solutions descend, 
by reaction with hydrogen sulphide sulphides and other reducing 
agents. A descending solution containing ferric, ferrous, and silver 
sulphates may thus react with the sulphides with which it comes in 
contact, perhaps taking more silver into solution, until the concen- 
tration of the ferrous salt present reaches a definite relation to that 
of the ferric and silver salts, expressible by the equation 
Cre Cag _K 
Cre oC Ae. = 
e g 
where 
Cre” . . . . =concentration of ferric ion in solution 
Cr’ ... . =concentration of ferrous ion in solution 
Cag’ ... . =concentration of silver ion in solution 
Cag ... . =concentration of metallic silver in solution 
K =a constant 
Since C4g is extremely small and a constant, we may divide 
both members of the equation by it, and obtain 
Cre!” _ K 
Cre” x Cag’ Cag 
= K,,a constant. 
It is evident that, given this constant, and an analysis of the 
earth waters containing the salts mentioned, at various depths, the 
horizon at which solution would cease and precipitation would begin 
can be determined. It becomes, therefore, a matter of importance 
to determine the value of the constant. To accomplish this, a 
ferrous sulphate solution was made of about F/20 in strength, by 
dissolving chemically pure ferrous sulphate in N/50 sulphuric acid 
solution. The ferrous content of this solution was then determined 
by standard potassium permanganate, the total] iron by reduction 
with test lead and titration with permanganate, and the total 
sulphate by precipitation as barium sulphate. To too-c.c. portions 
of this solution were then added quantities of a saturated silver 
sulphate solution varying from toc.c. to too c.c.; the flasks were 
immediately corked, the whole securely sealed with melted paraffin 
to prevent access of air, and set away to come to equilibrium. 
German standard pipettes were used throughout in measuring the 
solutions. Precipitation of silver, which came down as a cloud of 
silvery flakes, took place almost at once, and equilibrium was 
