136 PROCEEDINGS OP THE AMERICAN ACADEMY, 



is to be noticed that the value (Go. 54) obtained from Gray's observa- 

 tions by our method of correction is near these figures.* 



If the absolute accuracy of Faraday's law is assumed, these results 

 show either that the silver voltameter must yield nearly 0.1 per cent too 

 much silver, or else that the value of the atomic weight of copper found 

 chemically in this Laboratory (63.604) must be 0.1 per cent too high. 



Convincing as these conclusions seemed, the attempt was made to ob- 

 tain yet further light upon the copper voltameter by varying the conditions. 

 For example, several experiments were made with a mercury cathode, 

 in the hope that the amalgam of copper might be less easily influenced 

 by side reactions than the metal itself; but the difficulties in the way of 

 determining accurately the weight of the large volume of metal which 

 was needed to contain the copper, soon led to the abandonment of this 

 attempt. Moreover other chemical difficulties, due to the possible dis- 

 solving of mercury, added complications. 



It seemed now worth while to make a few determinations of the 

 amount of copper deposited from a solution saturated with cuprous salt, 

 in order to fix the highest possible limit for the electrochemical equiva- 

 lent in a fashion wholly free from any correction, as well as to test 

 experimentally our criticism of Foerster and Seidel's remarks about 

 such a solution. In order to saturate the liquid with cuprous ions, the 

 weighed platinum cathode was I'aised above the solution by means of its 

 sliding support, and the current was run backward and forward for an 

 hour or more between the two coils of copper (wrapped in filter jjaper, 

 B and C, Fig. 4) which were to serve as anodes. Instead of the large 

 bottle of the earlier experiments, a test tube just large enough to contain 

 the electrodes was used, so that the small amount of necessary solution 

 could be more easily saturated. In due time the cathode was lowered, 

 and the quantitative electrolysis commenced. Of course the solution had 

 been boiled, and was protected by a current of hydrogen throughout 

 the experiment. The silver voltameter used was an improved arrange- 

 ment, but its results are reduced to the standard used in the earlier 

 experiments for the sake of ready comparison. This matter will be 

 fully explained in the sequel. 



This value of the atomic weight, 63.573, is 0.06 per cent higher 



* Gray in liis first paper reports that with plates of 240 sq. cm. area he obtained 

 0.29303 gram of copper for every gram of silver, while with plates of 50 sq. cm. area 

 he obtained 0.29407 gram. Hence with plates of zero area he would have obtained 

 0.29434, corresponding to the atomic weight given above. (Phil. Mag., [5], 22, 407 

 [1886]. Last three determinations in table.) ^ 



