436 PROCEEDINGS OF THE AMERICAN ACADEMY. 



alloy, — although several times this could not he prevented. For heat- 

 ing the deposits on platinum gauze (see Tahle IX), a small oven was 

 constructed from a large porcelain crucible, covered by a platinum fun- 

 nel. The platinum disc was supported by a wire reaching through the 

 tube of the funnel. In this case, the silver in the platinum crucibles 

 with which that on the gauze was to be compared, was heated in the 

 oven also, in order to expose both to the same temperature. Since the 

 figures of this comparison are given in Table IX, it is necessary only to 

 tabulate here the loss observed in crucibles when heated directly. Of 

 course ^allowance has been made for the very slight hygroscopic loss 

 (0.10 milligram) which a platinum crucible without silver deposit would 

 have undergone. The silver films were usually those remaining from 

 some of the preceding determinations. 



This percentage loss is slightly higher than that given by Lord Rny- 

 leigh, and still larger than that determined indirectly by Richards and 

 Collins. It is evident that the amount of included mother liquor varies 

 according to the rate and mode of deposition, and it is quite possible that 

 different average amounts were really included in the several investiga- 

 tions. The inclusion is probably chiefly in recesses in the platinum 

 kathode. The differences in included liquid given in the above table are 

 of the same order as the differences in the uncorrected weights of silver 

 given at first ; * hence we may ascribe at least a part Of the differences 

 in the early table to inclusion of mother liquor. 



All this evidence unites in indicating that even under the best condi- 

 tions the silver does not exceed a purity of 99.99 per cent ; and in apply- 

 ing a correction, one should obviously use the value found in the particular 

 investigation under review. 



IV. The Atomic Weight of Copper. 



Having thus clear light upon the various errors of the silver voltam- 

 eter, it became a matter of great interest to recur to the original ques- 

 tion which started the whole investigation, namely, the quantitative 

 accuracy of Faraday's law. 



Accordingly, a voltameter like that used by Richards and Collins f — 

 a modified form of Lord Rayleigh's instrument — was compared with 

 a standard porous cup voltameter, neither precipitate being ignited. The 



* See page 418. 



t These Proceedings, 35, 133 (1899). 



