THE EXTRACTION OF GOLD, ETC. 309 



chromate, chlorate, perchlorate, nitrate nor nitrite enabled 

 the solution to dissolve gold leaf, and ferric hydrate and 

 bleaching powder were also without effect. The addition 

 of pyrolusite gave a doubtful result, and lead dioxide caused 

 very slow dissolution of the gold. Doubtless the presence 

 of hydrogen tended to hinder these substances from assist- 

 ing the cyanide, but under similar conditions if ferric 

 chloride, chlorine, or iodine dissolved in potassium iodide 

 were added, gold was dissolved slowly. The action was 

 more decided if an addition was made of potassium ferri- 

 cyanide or permanganate, sodium dioxide, hydrogen per- 

 oxide or barium dioxide. Finally, gold dissolved rapidly 

 when bromine was added. 



It is probable that ferric chloride and the halogens act 

 without any intervention of oxygen, the superfluous potas- 

 sium being converted directly into a haloid salt according 

 to some such equation as the following : — 



2 Au + 4 KCy + Cl 2 = 2KAuCy 2 + 2KCI, 

 some cyanide of chlorine being formed in the portions of 

 the solution where gold is not present. 



The essential point is that some substance, having a 

 strong affinity for potassium, must be present to assist in 

 the displacement of that element by gold in the cyanide 

 molecule. Whether the removal of the potassium is effected 

 by oxygen or a halogen is immaterial if the mass or con- 

 centration of one of these agents is sufficient, and although 

 their minimum effective concentration is unknown it is 

 clear that, as cyanide is not a solvent without the aid of 

 one of them, it is useless to increase the strength of cyanide 

 without a corresponding increase in the amount of the 

 oxidiser. This is the reason why strong solutions of 

 cyanide are not better solvents of gold than weak ones, 

 unless means are taken to increase the quantity of available 

 oxygen. Moreover, since cyanide acts directly on the sul- 

 phides of the base metals (which are usually present in 

 gold ores), without requiring the assistance of oxygen, a 

 greater waste of the solvent results in proportion as the 

 solution is stronger. The reason for the necessity of circulat- 

 ing the solution in treating ores is also clear, as the very 



