of Silver and of Copper. 405 



The number -3512 has a note against it in my notebook 

 stating that copper was lost in the washing, and generally 

 that the deposits from solutions of this density are unsatis- 

 factory with this size of plate and strength of current. It 

 would appear, therefore, that the last line of the table should 

 be struck out, but, as there is no note against the first result, 

 I have quoted the complete numbers in both cases. 



Effect of Acidity. — In the first experiments made on the 

 relative value of the electrochemical equivalents of copper and 

 silver the solution was made with pure sulphate of copper 

 which had been supplied by Burgoyne Burbidges and Co. as 

 very free from acid. The results obtained were at first very 

 puzzling, the gain of weight being generally greater than the 

 loss and very irregular. They are given in Table III., from 

 which the general nature of the result will be readily gathered. 



The ratio of the equivalents of copper and silver are 

 not given in the table. They may be calculated from the 

 results given ; but as the early results are of no value for this 

 purpose, and the area of the plate has not been recorded for 

 the later ones, it has been thought unnecessary to give them. 



The results with the nearly neutral copper sulphate were 

 invariably too high, and this excess of w r eight was increased 

 by leaving a copper plate in the solution. Repeated use of 

 a solution which has been supersaturated with copper by 

 leaving a copper plate in it seems to bring the solution back 

 towards its normal state. 



The addition of a very small percentage of acid causes 

 different solutions to give perfectly accordant results. 



One possible explanation of the peculiar action of the 

 neutral solution suggested itself, namely the formation of 

 subsalts of copper when the solution was placed in contact 

 with metallic copper, the subsalt giving twice as much copper 

 as the ordinary sulphate. The real explanation is, however, 

 I believe, the oxidation of the deposit while in the electro- 

 lytic cell. In one or two cases the deposit was hammered so 

 as to separate it from the sheet, and it was then found that 

 the back of the deposit obtained with the neutral solution 

 and the surface of the plate were completely oxidized, while 

 those from the acid solution were bright. 



Some further experiments were made with various speci- 

 mens of copper sulphate, some of them obtained in Glasgow 

 and others in London, with the view of finding whether any 

 difference might arise due to a change from one sample of 

 sulphate to another. These need not be quoted in detail, the 

 result being entirely negative. Precisely the same gain of 

 weight was obtained from a solution of the ordinary com- 



Phil. Mag. S. 5. Vol. 22. No. 138. Nov. 1886. 2 F 



