ON THE SILVER VOLTAMETER. 587 



In a final experiment a current of 1 ampere was passed through a solution during 

 200 hours until the solution was hlue with copper from the " pure " silver anode. 

 During this interval the mass of silver transferred through the solution amounted to 

 MM It-ss than 820 grammes, or more than 100 times as much as in a normal 

 electrolysis. The values obtained for the electrochemical equivalent were : 



Before electrolysis .... 1 '11827 (solution H,) 



After 100 hours 1-11822 (17a) 



After 200 hours 1 '11832 (23d) 



We were therefore driven to the conclusion that the mere act of electrolysis is not 

 sufficient to produce the effects recorded by RODGER and WATSON and others. 



The only change which we have been able to detect in the properties of the solution 

 after repeated electrolysis is that it gives somewhat more coherent and very slightly 

 striated deposits. KAHLE* has pointed out that the filter paper may have an effect 

 on the electrolyte, and this view is adopted by VAN DiJKf in explanation of the 

 increase which he observed. In our experiments the volume of the electrolyte was 

 exceptionally large, and the mass of filter paper J immersed was small ; contamination 

 of the solution by the paper would therefore not be likely to produce any marked 

 effect in the course of a few electrolyses at ordinary temperatures ; at higher 

 temperatures we have reason to think that important effects may be produced by the 

 action of the filter paper on the electrolyte. No filter paper was used in the 200-hour 

 electrolysis except during the actual determination of the electrochemical equivalent. 



E. The Question of Anodic Impurities. 



It has been generally assumed that the high values obtained on repeated electrolysis 

 were due to the introduction of impurities at the anode during electrolysis. It was 

 to overcome the supposed effects of such impurities that the porous pot voltameter 

 was introduced by RICHARDS. We were at first inclined to agree with this view ; it 

 is well known that when silver nitrate is electrolysed with a platinum anode, crystals 

 are formed of a " peroxynitrate," Ag 7 NO,,. This substance was discovered by RITTER 

 in 1804 ; its composition has been established by repeated analysis,]] and it is known 

 to be decomposed when warmed with water at temperatures from 25 upwards 

 according to the equation 



Ag T NO n = AgNOa+SAgA+O,, 



KAHI.R, 'Zeitechr. Instr.,' 22, p. 155, 1902. 

 t VAN DIJK, 'Ann. der Phys.,' 19, p. 249, 1906. 

 J SCHI.KICHKK and SCHULI., No. 595. 



HITTER, ' GERLEUS Neuea J.,' 3, p. 561, 1804. SULC, ' Zeitechr. Anorg. Chem.,' 12, p. 90, 1896. 

 i| MULDER and HERINT.A, ' Verb. Ron. Ak Wet.,' 3, p. 37, 1896. TANATAR, ' Zeitechr. Anorg. Chem.,' 

 28, p. 331, 1901 WATSON, 'Trans. Chem. Soc.,' 89, p. 578, 1906. 



4*8 



