370 REPORT—1905. 
the net result being, of course, a current in the direction of the greater E.M.F. As 
the strength of the solution increases after a certain point, the E.M.F. due to the 
metal couple increases rapidly, whereas that due to the oxygen concentration cell 
remains constant or increases only slowly. 
The increase in the E.M.F. of the metal couple appears to be largely due to the 
formation of AuCy—a compound having a high potential, which acts as an 
electrode. This deposits in films, varying in density or thickness to a maximum 
with the strength of the solution. A couple results of Au—AuCy. After this 
stage of the process, when AuCy is formed, oxygen ceases to exert an influence: 
That is to say, the metal passes into solution by the AuCy dissolving in the potas- 
sium cyanide solution, as one salt dissolves in the solution of another. 
The effect of the gas cell is best observed in highly dilute solutions at ordinary 
or low temperatures. After a certain strength is attained, dependent on tempera- 
ture, the effect of the gas cell is entirely masked. At the higher temperatures 
the E.M.F. of the gas cell diminishes, with a corresponding increase in the E.M.F. 
of the Au—AuCy couple. At boiling-point the retarding oxygen effect of the gas 
cell on the dissolution of the metal practically disappears. 
2. The Solubility of Gold in Thiosulphates and Thiocyanates. 
By H. A. Wuite. 
Attention was called to the presence of the above salts in ordinary working 
cyanide solutions, and to the fact that in presence of certain oxidising agents gold 
is to some extent soluble in both cases. 
The presence of gold in mine reservoirs and in soil under residue dumps is con- 
nected most probably with the presence of such salts, since cyanides as such would 
be speedily attacked and destroyed by the oxidation products of pyrites always 
present. It was shown that the thiocyanates in presence of such oxidising agents 
as ferric salts attack gold with considerable ease, and that thiosulphates; which are 
less stable, exert a less powerful action. Other experiments adduced indicate that 
in well-exposed dumps cyanide and thiosulphates are not directly concerned with 
the observed solution of gold, but that thiocyanates alone are of significance in 
this respect. Results of experiments made on solutions of different strengths with 
pieces of pure gold show that the rate of attack diminishes rapidly after a few 
days, and this is traced to ageing of the solution, with consequent loss of available 
oxygen. 
Rata BES was made to H. 8. Stark’s process of residue re-treatment, which is 
resulting in the profitable extraction of a large portion of the gold in certain of 
the residue dumps on the Rand. 
WEDNESDAY, AUGUST 30. 
The following Papers were read :— 
1. The Law Governing the Solubility of Zinc Hydrate in Alkalis. 
By James Morr, M.A., D.Sc. 
It is a fact well known in analytical chemistry that when salts of zine are 
treated with caustic potash or soda the precipitate of Zn(OH), dissolves in excess of 
the alkali. It is rather extraordinary that no quantitative investigation of this 
phenomenon seems to have been made, and this is the razson d’étre of this paper. 
The phenomenon is essentially an equilibrium between alkali and zincic acid, 
and this equilibrium may be reached from both sides, ¢.e., (1) addition of excess of 
zine hydrate to caustic alkali, and (2) dilution with water of a strong solution pre- 
viously saturated with zine hydrate. In the latter case some of the Zn(OH), is 
re-precipitated, and on standing an equilibrium is reached which, as in the first 
case, depends solely on the concentration of the free alkali. 
Obseryations haye been made on both lines over the range from 7N to ‘OIN 
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