3 i2 SCIENCE PROGRESS. 



and cyanide of bromine added, when potassium bromide 

 instead of the hydrate is produced, the action being ex- 

 pressed by the equation 



CyBr + aKCy + 2Au = 2 KAuCy 2 + KBr. 



As already observed, the same direct removal of 

 potassium is probably effected when the halogens are added 

 to cyanide solutions. But, on the other hand, Clennel has 

 advanced some evidence 1 of the formation of considerable 

 quantities of hydrocyanic acid, when bromide of cyanogen 

 is added to potassium cyanide, and a corresponding amount 

 of oxygen would thus be rendered available either to form 

 cyanates or potash, so that the action of bromine would 

 after all be merely one of oxidation. 



Although the alkali metals are positive to gold in cyanide 

 solutions there are others which can be directly displaced 

 by both gold and silver. Mercury is one of these, the 

 solvent action of the double cyanide of potassium and mer- 

 cury being independent of the presence of any substance 

 having an affinity for potassium, and this double salt is 

 sometimes used on refractory ores in which the gold is 

 difficult to get into solution. Other elements, such as iron, 

 lead and carbon, which are negative to gold in cyanide 

 solutions, also increase the rate at which it is dissolved if in 

 contact with it, but the difficulty of obtaining contact is too 

 great for these substances to be of any practical value. 



In general the various quickeners of the action of cyanide 

 are unnecessary with simple ores, as for example in the 

 Transvaal, where the leaching of the tailings occupies two or 

 three days owing to mechanical difficulties, so that nothing 

 is gained by reducing by a few hours the time necessary to 

 dissolve the gold. On the other hand an abundant supply 

 of oxygen in almost any form increases the destruction of 

 the cyanide in various ways, so that the cost may become 

 almost prohibitive. 



The consumption of cyanide has been undergoing 

 reduction by repeated steps during the last few years by 

 means of decrease in the strength of the solution. Beginning 



1 Proc. Chem. and Met. Soc. of S. Africa, vol. i., p. 127. 



