422 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Average deviation of two porous cup voltameters in series = ± y^^^ 

 Difference caused by sealing bottom of cup = — jq^^^ 



Difference caused by use of zinc anode = ± ioo]ooo 



Difference (corrected) between gauze voltameter and cup voltameter ±0 



The agreement of these results is as close as could be expected, since 

 the discrepancies do not exceed the possible experimental error. With 

 Lord Rayleigh's method, when two precisely similar voltameters are 

 compared, Kahle * and Rodger and Watson f have shown that an ac- 

 curacy of 6 or 7 parts in 100,000 can be obtained. On the other hand, 

 the least variation of size of kathode or anode, or of any other condition, 

 causes large deviations which may amount to ten times as large an error. 

 In our experiments given above, the most radical changes of method were 

 introduced, without affecting the results. 



Among the efficient forms of apparatus described above, the porous 

 cup voltameter with a silver anode is the most convenient. Hence for 

 the further purposes of this paper it will be chosen as the standard 

 method. 



II. The Separate Effect of each Anode Irregularity. 



It is obvious from the study of earlier work that more than one irreg- 

 ularity exists at the anode in a silver cell ; and the separation and iden- 

 tification of the individual effect of each irregularity became a matter of 

 considerable interest. The outcome was instructive as an example of 

 the multitude of hidden minor influences which so often modify the ob- 

 vious outcome of chemical experiment. 



Qualitative testing revealed not only acid, but also nitrite, in the anode 

 liquid ; and in those cases where the anode is very small, some experi- 

 menters have indicated the formation of highly oxidized compounds of 

 silver. Moreover, the singular crystalline silver dust which forms 

 around the anode demands an explanation. In order to solve the prob- 

 lem, of course an obvious available method was to introduce artificially 

 each impurity in turn into the pure liquid around the kathode in the 

 porous cup voltameter, and study its effect on the gain in weight of the 

 kathode. 



The first impurity to be investigated was the nitrite. In order to pre- 



* Wied. Ann. N. F., 67, 22 (1809). 

 t Phil. Trans., 186 A, G33 (1895). 



