200 Dr. C. R. A. Wright on the Determination of 



copper-silver cells and lowered in the other two kinds : the 

 alteration in the E.M.F. varied between + -005 and —'003, 

 averaging +'0005. 



126. On allowing newly set-up cells to stand for several 

 hours, different results were brought about in each of the 

 three cases according as the silver was opposed to zinc, cad- 

 mium, or copper. In the first case the E.M.F. invariably fell; 

 the maximum value was observed immediately after the cell 

 was set up, and continued sensibly steady for a variable period 

 of time, a distinct diminution becoming perceptible sometimes 

 after half an hour, sometimes only after two or three hours. 

 With cadmium the value after several hours was somewhat 

 greater than that set up at first and during the subsequent 

 hour or so ; and with copper the value attained after several 

 hours was still greater than that exhibited during the first hour 

 or two. The following numbers represent the average altera- 

 tions thus observed, being the differences between the average 

 readings during the first hour and during a period of from 

 3 to 5 hours after setting up : — 



Amalgamated Zinc opposed. 

 Bright silver. Electro-silver. Amalgamated silver. Mean. 



-•010 --014 --012 --012 



Bright Cadmium opposed. 

 + •003 +-001 +-003 +-002 



Electro-Cadmium opposed. 

 + •003 +-004 +-003 +-003 



Electro-Copper opposed. 

 + •009 +-007 +-007 +-008 



Amalgamated Copper opposed. 

 + •011 +-009 +-005 +-008 



These alterations were traced to the variations in the nature 

 of the surfaces of the plates opposed to the silver ; for on 

 taking out, for instance, an amalgamated zinc plate after 5 

 hours, and replacing by a freshly amalgamated plate, the 

 E.M.F. was restored to sensibly the same value as at first ; 

 and similarly with the other metals. On the other hand, on 

 taking out from two cells, for instance, a zinc and a copper 

 plate after 5 hours, and replacing them respectively in two 

 beakers containing zinc and copper sulphate solutions of the 

 same molecular strengths, and connected by a siphon tube, the 

 E.M.F. of the cell thus formed was found to fall short of the 



