3 i4 SCIENCE PROGRESS. 



ragged edges of the shavings would assist the gas to form 

 into bubbles and become detached, so that less " polarisa- 

 tion " would occur, but it must be admitted that the 

 well-known evolution of hydrogen in the zinc boxes may be 

 explained in other ways. The layer of ZnCy 2 is at once 

 dissolved if potassium cyanide is present, and in its absence 

 potash has a similar but less active effect, potassium zincate 

 being formed. 



The view, however, formed too hastily without sufficient 

 experiment, that if the strength of free cyanide falls below 

 about CV2 per cent, precipitation isimpracticable, has now been 

 abandoned except by the advocates of rival precipitation 

 processes. McBride obtained satisfactory precipitation by 

 zinc at the Glencairn Mine 1 with solutions containing as 

 little as 0*025 to 0*03 per cent. KCy, and James showed 

 the conditions to be observed in such cases, one of them 

 being- that the solution must be in contact with the zinc for 

 at least an hour, so that 1 30 cubic feet of shavings (weigh- 

 ing 1 lb. per cubic foot) are required to treat 100 tons 

 of solution in twenty-four hours. 



It must, of course, be borne in mind that the zinc used 

 is not pure, and that the small percentage of lead usually 

 contained in it has a powerful effect in increasing its action, 

 a couple being formed with the zinc as the positive element. 

 The gold-zinc couple, produced after precipitation has pro- 

 ceeded for some time, is probably also of value, but on the 

 other hand Goyder has shown 2 that the presence of iron 

 in contact with the zinc is injurious, checking the rate of 

 precipitation of the gold and increasing the waste of zinc 

 and the tendency for the formation of ZnCy 2 on the threads. 

 Iron screws should therefore not be used in the precipita- 

 tion boxes. Ehrmann more recently 3 tried the copper-zinc 

 couple and found it most efficacious in hot solutions, but 

 even in the cold his results were striking. For example, a 

 cold solution containing 0*017 per cent. KCy and seven 

 dwts. of gold per ton had 61 per cent, of the latter pre- 



1 Proc. of the Chem. and Met. Soc. of S. Africa, vol. i., p. 289, 1897. 



2 Chem. News, vol. lxxiii. (1896), p. 272. 



3 Chem. and Met. Soc. of S Africa, 17th April, 1897. 



