Weights of Silver and Copper. 139 



sulphate, and, if possible, to establish some formula of correc- 

 tion by which the deposit of copper at any current-density 

 could be reduced to that at a standard density. The results 

 show that the amount of copper deposited tends to a maximum 

 value when the current-density reaches the highest limit with 

 which it will give coherent deposits, this limit being about 

 •13 ampere per square centimetre. The amount of deposit 

 does not, however, vary appreciably from this until the 

 current-density is below *025 ampere per square centimetre. 

 For current-densities less than *02 and down to *0014 the 

 amount of deposit can, with fair concordance between the 

 results of different experiments, be reduced to that at the 



higher current- densities by multiplying by a factor 1 H j — , 



where d is the current-density ; although it is possible that a 

 slightly different value may have to be assigned to the nume- 

 rator of the fraction when all the circumstances of the state 

 of the solution, temperature, and other variables are taken 

 into account. 



The fact that the amount of deposit tends to a limiting 

 value for high current- densities, but not for low ones, has led 

 me to examine the results of adopting this as a method of 

 determining the atomic weight of copper from that of silver. 

 And the first result that appears is that the ratio of the che- 

 mical equivalents of silver and copper is given, as the mean 

 of fifteen final experiments with current-densities above '025, 

 as 3*39983 ; which differs so little from 3*4000 that this value 

 may be adopted as the true ratio within the limits of error of 

 the experiments. The amounts of copper deposited were 

 generally speaking, about 1 gramme, and were weighed to 

 0*1 milligramme. The greatest error from the mean of the 

 fifteen experiments is *0058, or nearly *2 per cent. ; while the 

 mean error is '00175, or less than '06 per cent. 



The ratio has been determined experimentally by Lord 

 Rayleigh in three experiments with platinum bowls, quoted 

 in the paper by himself and Mrs. Sidgwick in the Philoso- 

 phical Transactions, Part II. 1884, p. 458. His results are 



\ 3-408 \ w ^k curren t-density about '012 ampere per square 



centimetre, and 3*404 with a current-density about *026. 

 And the subject was brought to the notice of readers of the 

 Philosophical Magazine by the paper by Mr. T. Gray, last 

 November. Mr. Gray's number for the ratio is 3*4013, when 

 the current-density is *02 ampere per square centimetre. The 

 number that I have here given is derived from experiments 

 with platinum-wire cathodes, generallv with higher current- 

 ly 



