TRANSACTIONS OF SECTION B. 369 
At the Cape grapes always become perfectly ripe, and when ripe contain more 
sugar and less acid than the ripe grapes on the Rhine and in other wine-producing 
countries of Europe. Notwithstanding this there is a standing reproach against 
Cape wines in the assertion that they are more acid than European wines. The 
author has examined some three hundred samples of Cape wines from wine mer- 
chants and wine farmers, and found that this allegation cannot be upheld. 
4. Report of the Committee on the Transformation of Aromatic Nitro- 
amines and Allied Substances, and its Relation to Substitution in 
Benzene Derivatives,—See Reports, p, 103, 
JOHANNESBURG, 
TUESDAY, AUGUST 29. 
After the President had delivered his Address (see p. 351) the following 
Papers were read :— 
1, How Oxygen Assists and Retards the Dissolution of Gold in Cyanide 
Solutions. By H. Forses Juuian. 
A doubt has for some time existed as to the accuracy of the generally accepted 
idea that free oxygen is primarily essential for the dissolution of gold in cyanide 
solutions, according to the equation— 
4AKCy + 2Au + 0+ H,O=2KAuCy, + 2KOH. 
Experiments are described which go to show (1) that free oxygen plays no 
primary part in the reaction, (2) that any assistance given by free oxygen is of a 
secondary nature, and (5) that free oxygen exerts a retarding influence. 
The experiments show that the galvanometer points to the presence of free 
oxygen as having a retarding influence on the dissolution of the gold, whereas the 
balance points to it being of material assistance. The cause of the two instruments 
not agreeing is discussed, and is attributed to the formation of local voltaic circuits. 
These, in the first instance, deposit hydrogen and oxygen, which, it may be as- 
sumed, become occluded at their respective electrodes until the systems are in 
equilibrium. It is pointed out that cyanogen leaves the solution to combine with 
the gold rather than that gold particles pass into the solution, and it is shown that 
cyanogen does not leave the solution until the deposited oxygen has been occluded 
to a certain degree of concentration. The reason for this is that the expenditure 
of energy necessary to remove oxygen from the solution is less than that neces- 
sary to remove cyanogen ; but when oxygen is occluded to a certain concentration, 
the expenditure of energy then necessary to cause the metal to occlude a further 
amount becomes as great as that necessary to begin to remove cyanogen from the 
solution. The available energy is obtained from the metal and solution, and it 
follows that when the solution is very dilute the available energy is too small to . 
remoye cyanogen, oxygen being then alone deposited. From this it may be con- 
jectured that no metal actually combines with cyanogen until the solution has 
a certain minimum strength. 
The presence of dissolved oxygen in the solution has a secondary effect in the 
process of dissolution, by oxidising the occluded hydrogen produced through the 
action in the local voltaic circuits. This results in upsetting the equilibrium, and 
introducing into the circuits concentration gas cells, which soon bring about equili- 
brium again, but this time with oxygen at both electrodes at different concentra- 
tions, instead of hydrogen and oxygen. If, now, excess of dissolved oxygen diffuses 
to either of the electrodes the equilibrium is again upset, and an E.M.F. is gene- 
rated by the gas cell in opposition to the E.M.F. generated by the metal couple; 
1905, - BB 
