ROSA AND VIXAL: THE SILVER VOLTAMETER 455 



regulating the current. As a further check, a potentiometer 

 was also included in the circuit. Our reference standards were 

 frequently compared with the standards of the Bureau, and suit- 

 able corrections made. 



The porous cups were of Pukal ware made by the Koniglich 

 Porzellan Manufaktur of Berlin. They were prepared by filtering 

 dilute HX0 3 thru the pores, and, after eliminating the acid by 

 distilled water, the cups were soaked in pure electrolyte. Be- 

 tween experiments they were kept submerged in electrolyte. 

 After washing the deposits and replacing any loose silver, they 

 were dried in an electric oven at 150°C. After cooling, they 

 were placed in the balance case for several hours before weighing. 



The first experiments were with the Eayleigh voltameter. As 

 a mean of 37 experiments, the value 1.01866 volts at 20 : C. for 

 the Weston normal cell was obtained, assuming the electrochemi- 

 cal equivalent of silver as 1.11800 mg. per coulomb, as defined 

 by the London Conference of 1908. A difference of about 40 

 parts in 100,000 was found between this and the porous cup 

 form, the latter giving the lower value. It occurred to the authors 

 that if this excess were due to the filter paper letting thru a com- 

 plex ion. as supposed by Richards, doubling or trebling the thick- 

 ness of filter paper ought to decrease the discrepancy. But it 

 was found that using two or three sheets of filter paper had just 

 the opposite effect, and also increased the striatums which were 

 soon found to be a sure indication of a heavy deposit. It was 

 found that the excess of deposit in the filter paper form over that 

 in the porous cup form depended on the amount of filter paper 

 present, and that the porous cup voltameter gave the same high 

 values as the filter paper form if filter paper were wrapped around 

 the outside of the porous cup. or if the electrolyte had previously 

 been contaminated by soaking filter paper in it. 



When solutions were repeatedly used in a filter paper voltame- 

 ter, the deposits were increasingly heavy with each electrolysis, 

 but a similar experiment with the porous cup voltameter always 

 gave the same weight or slightly fighter. To further test the 

 complex ion theory, we tried re-electrolyzing some of the elec- 



