SECONDARY ENRICHMENT OF SILVER ORES 27 
at considerable depths. Native silver so precipitated will be com- 
paratively stable in the presence of earth waters. Acids will not 
attack it in the absence of oxygen, since the solution pressure of 
hydrogen is greater than that of silver. Ferric sulphate cannot 
attack it so long as the ground-waters contain enough silver sul- 
phate to preserve equilibrium between the ferric and ferrous salts. 
Precipitated sulphur may convert it into sulphide. 
The presence of sodium chloride in ground-water does not 
increase the solubility of silver chloride therein, unless the amount 
of sodium chloride rises above 34.3 gm. per liter; on the contrary, 
it decreases the solubility. The solubility-curve of silver chloride 
in sodium chloride solutions is shown in Fig. 1. Terming the 
sodium chloride solution which contains the least amount of silver 
chloride the ‘‘minimum solution,” then solutions which contain 
less sodium chloride than this minimum solution will precipitate 
silver chloride on taking up more sodium chloride; solutions with 
more sodium chloride than the minimum solution will precipitate 
silver chloride on dilution. 
Silver chloride solutions containing sodium chloride in any 
concentration may form secondary silver sulphide wherever they 
encounter hydrogen sulphide or any other substance that yields 
sulphur ion even in minute quantity. 
Cerargyrite appears to be stable in presence of sulphuric acid 
and ferric sulphate. 
The action of dilute silver solutions on realgar, orpiment, and 
stibnite results in the substitution into these minerals of some silver; 
but the amounts so substituted were found to be so small that it 
appears doubtful whether the complex sulpho-salts of silver can 
be formed in this way. 
Precipitated sulphur combines with precipitated silver at ordi- 
nary temperature and pressure to form silver sulphide. Silver 
sulphide is also formed by the direct reaction of hydrogen sulphide 
with silver-bearing solutions. 
CONCLUSION 
‘The accuracy of the application of laboratory investigations to 
geologic problems must be verified by field observations; other- 
wise, however correct the experimental work, it cannot be claimed 
