THE EXTRACTION OF GOLD, ETC. 307 



Faraday 1 in 1857 made long series of experiments on the 

 subject, the discovery remained for nearly half a century 

 one of the unused and apparently useless chemical data 

 which help to fill text-books for a time and are then omitted 

 as of no interest and finally almost forgotten. When it 

 was found, as the result of repeated trials in 1886, that 

 alkaline cyanides in dilute solution are fairly stable sub- 

 stances, their solvent action on gold became of value to 

 mankind, and since then an ever-increasing army of workers 

 has been carefully experimenting on the action of cyanide 

 not only on gold but on other metals, on sulphides, oxides, 

 and silicates, on wood and a hundred other substances, with 

 the result that the data accumulated would fill a volume by 

 themselves. 



Much light has been thrown, for example, on the exact 

 mechanism of the chemical change which ensues when 

 cyanides act on gold. Eisner found that the air at the top 

 of an inverted test-tube containing gold dipping into a 

 solution of cyanide of potassium had lost its oxygen after 

 twenty-four hours, and considered that this had been con- 

 sumed in dissolving the gold, although the oxidation of 

 cyanide to cyanate might have accounted for its disappear- 

 ance. Faraday discovered that if gold leaf is floated on the 

 surface of a solution of cyanide it is dissolved many times 

 more quickly than if it is completely immersed and so pro- 

 tected from the air. It was subsequently proved by Mac- 

 laurin 2 that pure gold is not soluble in a solution of pure 

 cyanide if oxygen is completely excluded, and that dissolu- 

 tion is greatly increased if the liquid is thoroughly aerated 

 and especially if oxygen is continually bubbled through it. 

 Much evidence was afforded by him in support of the 

 correctness of the equation : — 



4Au + 8KCy + 2 + 2H0O = 4KAuCy 2 + 4KOH. 

 Nevertheless this equation does not represent the whole of 

 the chemical action, as a substance reacting like hydroxyl 

 seems to be produced. To explain this, G. Bodlander of 

 Clausthal puts forward the equation : — 



1 Roy. Inst. Proc, vol. ii., p. 308. 



2 Jour. Chem. Soc. (1893), v °l* 63, p. 724. 



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