THE EXTRACTION OF GOLD, ETC. 315 



cipitated by zinc in twenty-four hours, and, under precisely 

 similar conditions, 98*67 per cent, of the gold was precipi- 

 tated by the copper-zinc couple, the residual liquor containing 

 only two grains of gold per ton. 



Apart from the zinc process, the only other method in 

 extensive use of recovering the gold is that of Siemens and 

 Halske, in which a current of electricity is passed from 

 iron anodes to sheet-lead cathodes through the solution. 

 The electrodes are placed from i|- to 3 inches apart and a 

 current of four volts, and from 0*03 to 0*05 ampere is used. 

 The gold is precipitated on the lead, which, after a run of 

 one or two months, is removed, melted and cupelled. The 

 chief advantage derived is that precipitation is possible 

 from very dilute solutions containing from o*ooi to cvi per 

 cent. KCy, and as these are almost, if not quite as effective 

 as stronger solutions in dissolving gold from its ores, while 

 suffering less loss from decomposition of the cyanide, some 

 saving is effected by their use, estimated by Bettel to be 

 about ^ lb. of cyanide per ton of tailings. One of the 

 main difficulties in electrical precipitation lies in the great 

 resistance offered to the passage of the current by the 

 dilute solutions employed. In order to reduce this as far as 

 possible the electrodes must be of large size and the distance 

 between them small, so that the use of mercury for the 

 cathode, otherwise marked out as the most suitable substance, 

 is not practicable. 



The ions of aurocyanide of potassium are K and 

 AuCy 2 . As the result of electrolysis, therefore, potassium 

 is set free at the lead cathode, where it attacks the water 

 forming potash and hydrogen ; at the same time the gold 

 in the double cyanide is displaced and precipitated, both by 

 the potassium and the nascent hydrogen. Such hydrocyanic 

 acid as may be formed is neutralised by the potash. Mean- 

 while the anion, AuCy 2 , is set free at the anode, but is at 

 once split up into AuCy and Cy ; the latter unites with the 

 iron, forming cyanides which become converted into Prussian 

 blue, and are also oxidised in part, forming ferric oxide, and 

 the cyanide of gold is partly precipitated in this substance, 

 although if enough free potassium cyanide is present it may 



