RICHARDS. — NOTE CONCERNING THE SILVER COULOMETER. 93 



"We conclude, therefore, that whilst the abnormally low values 

 which are observed from time to time can only be explained by the 

 presence of acid, it may be very difficult in practice to add nitric acid 

 without at the same time introducing other impurities which may 

 more than counterbalance the effects produced by the acid itself." 5 



To this supposed influence of nitric acid they ascribe the fact that 

 on thirteen occasions they found less silver in the experiments where a 

 porous cup was interposed between the anode and the cathode than 

 where the cup was absent. They infer that the porous cup was not 

 adequately washed from nitric acid. This is possible, although it 

 seems more probable that, as they suggest, cyanide, which is notori- 

 ously difficult to wash out of porous material, was the real cause of the 

 deficiency, as indeed they suggest on the third line of page 564. They 

 obtained good results when their porous cups were ignited for some 

 time in an electric furnace. This treatment would drive off not only 

 nitric but also hydrocyanic acid, and might oxidize any remaining 

 cyanide. 



To sum up the last paragraph : it may be pointed out that there is 

 little evidence presented that nitric acid, if present in traces, would 

 have produced a deficiency in the silver deposited, and some doubt as 

 to whether nitric acid was present in the experiments of Smith, Mather 

 and Lowry with porous cups. Hence the conclusions of those gentle- 

 men concerning the unsatisfactory behavior of their insufficiently 

 washed cups are of doubtful value. Nevertheless it would obviously 

 be well in future work to make sure that nitric acid is wholly absent, 

 and they have done a service by calling attention to the danger of incom- 

 plete washing of the porous cell if that is used. It is not probable that 

 this difficulty affected the determinations made at Harvard, because 

 cyanide was not used for washing the cells, and, as is stated in one of 

 the papers,6 the solution around the cathode in our cases remained 

 wholly neutral. Moreover, in the Harvard experiments the porous 

 cup method was compared with another method free from any possible 

 defect of this kind, and found to give the same result.7 



One other point may be mentioned in which the results of Messrs. 

 Smith, Mather and Lowry differ from the Harvard results. The Eng- 

 lish experimenters were unable to find that freshly formed anode liquid 



5 Professor Smith, in a letter kindly written after he had seen the manu- 

 script of the present note, explains that there was some doubt as to the purity 

 of some of their nitric acid. This doubt may have applied equally to that 

 used in washing their porous cells, however. 



6 These Proceedings, 35, 141 (1899). 



7 These Proceedings, 37, 420 (1902). 



