390 Mr. T. Gray on the Electrolysis 



6ionally used by Sir William Thomson as a check on the 

 graduation of galvanometers ; but the question of its reliability 

 had not been systematically attacked before the series of expe- 

 riments here described were taken up. From a number of 

 experiments made in the above laboratory about four years 

 ago, I inferred that copper was capable of giving fairly uniform 

 results ; and I obtained as the electrochemical equivalent of 

 copper '003307, or as the amount of copper deposited by a 

 coulomb of electricity -0003307. This value has since been 

 found to be too high ; and recent experience indicates that 

 there are at least two ways of explaining the discrepance — 

 namely, either that the measurement of the current was in 

 error, or that the solutions were wrongly treated. The first of 

 these needs no remark, it is impossible to tell now whether the 

 current was measured with the needful accuracy or not ; but 

 the second will be referred to at some length in this paper. 

 The solutions were in that case very carefully saturated with 

 copper by shaking them up with, and filtering them through, 

 copper oxide ; and I bring the matter forward now because I 

 believe that treatment was a serious mistake. It is easy to 

 make a solution of copper sulphate which will give as high, 

 and even a much higher, value for the electrochemical equiva- 

 lent than that above stated; but once the cause of this is known, 

 it is equally easy to avoid the error. In order to obtain good 

 results it is necessary that the solution be distinctly acid. 

 The ordinary commercial copper sulphate, or the sulphate 

 ordinarily sold as pure, usually contains enough of acid ; but 

 the same solution cannot be repeatedly used with safety. 



Following the results of the investigations of Kohlrausch, 

 and of Lord Rayleigh and Mrs. Sidgwick, the substances used 

 in the earlier of the present series of experiments were pure 

 silver and pure silver nitrate. Sheet silver was used both for 

 the gain and for the loss plates ; it was supplied by Messrs. 

 Johnson and Mathey as pure silver. The form of the cell 

 used for the silver is illustrated in fig. 1 (Plate VII.); it con- 

 sists of a glass vessel partially filled with a solution of silver 

 nitrate, in which three plates of silver, arranged with their 

 planes parallel, are suspended from spring clips of the form 

 illustrated in figs. 2 and 3. This form of cell offers, in com- 

 parison with the platinum-bowl method recommended by 

 PoggendorfF and adopted by Lord Rayleigh, some advantages 

 which, in my estimation, outweigh its disadvantages. The 

 total weight of the plate is very small, and hence a light and 

 very delicate balance can be used. The thorough polishing 

 and cleaning of the plates is easier than when a bowl is used. 

 The cleaning of the platinum bowls by dissolving off the 



