324 ALBERT D. I^ROKAIV 



The experiments were conducted as before. The results show 

 clearly that the free acid, rather than the ferric chloride, in the pres- 

 ence of manganese dioxide exercises the great solvent action, as the 

 same amount of chlorine was present in both cases. Essentially, 

 the most favorable conditions for the solution of gold are those in which 

 free chlorine may be liberated. 



^^^ J- IMcCaughey.' in studying the solubility of gold in ferric 

 salt solutions, found that ferrous sulphate, even in very small amounts, 

 had a marked etTect in depressing the solubility of gold. Conceivably 

 this mav be a factor to be considered, and with tliis in view experiment 

 8 was performed, to determine whether ferrous sulphate, in the pres- 

 ence of sulphuric acid and manganese dioxide, would be quickly 

 oxidized to the ferric salt, according to the following equation: 

 S. 2FeSO-^2H,S04+MnO, = Fe,(S04)3 + MnS04+2H,0 



After acidifying loo c.c. of 1.6 N. FeSO^ solution with sulphuric 

 acid, it was shaken vigorously with 5 g. of powdered J\InO_,. After 

 five minutes the solution was filtered off. No ferrous iron was 

 detected by the ferricyanide test, showing that the iron had been 

 completely oxidized to the ferric state. The experiments were not 

 done in such a way that the A-elocity of the oxidation could be 

 measured, but the result shows that ferrous sulphate in acid solution 

 is quickly oxidized by manganese dioxide, hence the suppression 

 of the solution of gold by ferrous salts may be disregarded if man- 

 ganese dioxide is present. It is interesting to note in this connection 

 that the analysis of a sample of water from the Comstock Lode,^ 

 showing the greatest amount of ferric iron, showed a considerable 

 amount of manganese, but no ferrous iron; a condition exactly in 

 accord with the right-hand side of the abo\-e equation. 



McCaughey's experiments on the solubility of gold in ferric salt 

 solutions were made with stronger solutions than are known to occur 

 in mine waters, but his results show that gold is attacked by ferric 

 salts in the presence of hydrochloric acid.-^ AMiile no loss of gold 



I McCaughey, Jour. Am. Cheni. Soc, XXXI, 1269. 

 ' See analysis 3, p. 326. Nos. 5 and 6 illustrate the same fact. 

 3 If gold is dissolved by ferric chloride the reaction might be expected to be as follows: 

 Au + 3FeCl3 = AUCI3 + 3FeCla 

 This would seem to be a reversible action, as ferrous salts are commonly used as a pre- 



