SECONDARY ENRICHMENT OF SILVER ORES 15 
nuggets.‘ The conclusion might also be drawn that the formation 
of native silver would take place to but a limited extent in the 
gossans of deposits containing much pyrite. 
The solvent action of the acids in the above table is ascribed to 
the presence of air, which sets up an oxidation potential between the 
silver and the solution, and thus aids the solution of the silver. 
Acids cannot attack silver directly in the absence of air, since the 
solution tension of the silver is less than that of the hydrogen which 
would thereby be given off. This fact may account in part for the 
stability of native silver precipitated below the zone of oxidation. 
ACTION OF FERRIC SULPHATE ON SILVER SULPHATE 
During the course of the above experiments, in some cases un- 
usually high concentrations of silver were noted in the solutions. 
The solubility of silver sulphate in pure water at room temperature, 
22° C., determined by measuring the strength of a solution that had 
been standing with frequent shaking for some months in contact 
with solid silver sulphate in the dark, was 6.90 gm. per liter or 
0.0211 F. Euler? gives a figure somewhat higher than this, 7.70 gm. 
per liter at 17°C. Wright and Thompson give 7.28 per liter at 
18°C. The addition of an ionized sulphate, such as ferric sul- 
phate, would be expected to decrease the solubility of the silver 
sulphate; since in a saturated solution the ion product [Ag]? <[SO,] 
must be a constant. This is not the case here, however. Experi- 
ments carried out by leaving solid silver sulphate in contact with 
ferric sulphate solutions of varying strengths for a month or more 
in the dark show that the concentration of the silver is actually 
very considerably higher in the ferric solutions than in the aqueous. 
The following table (p. 16) shows the results obtained. 
It will be noted that even in the most dilute of these solutions the 
solubility of the silver sulphate is increased by about ro per cent. 
The results in the higher concentrations, while interesting from a 
scientific point of view, are of no significance as regards secondary 
tJ. M. McLaren, Gold (1908), p. 22. 
2 Euler, Zeit. physik. Chem., XLIX (1904), 314. 
3 Phil. Mag. (5), XVII (1884), 288. 
