36 Dr. H. J. S. Sand on the Measurement of the 



was reduced under varying conditions. Two solutions were 

 experimented upon with different electrodes and current- 

 densities, other conditions remaining unchanged as nearly as 

 possible. Stock solutions of accurately known composition 

 were mixed in known proportion, the composition of the 

 resulting liquids working out as follows : — 



- ■,.. . i n ,. , £ • f 5 o-. nitrobenzene. 



1 litre acid solution used tor experi- I , ( P, ^ ^ 



menfa of figs. 1, 8, 9, 10 (PL I.) I ™ f^fr^. 

 contams (Remainder water. 



!5 g. nitrobenzene. 

 82 g. NaOH. 

 •125 c.c. ethylalcohol. 

 Remainder water. 



The anode for the experiments on acid solutions was, as a 

 rule, one of copper in contact with a solution of 30 g. cuprous 

 chloride in 200 c.c. strong hydrochloric acid and 100 c.c. 

 "95 per cent, alcohol. For the experiments on the alkaline 

 solution the anode was mercury in contact with a solution of 

 100 g. mercurous nitrate and 35 c.c. strong nitric acid in 

 500 c.c. water, the solution interposed (see p. 29) containing 

 125 g. ammonium nitrate in 500 cc. 



The curves obtained show, firstly, the degree of coincidence 



yielded by a repetition of experiments, and, secondly, they 



exhibit both interesting similarities independent of the 



material of the electrode and characteristic differences due to 



it. The abscissas again denote the experimentally found 



values of the potential of the standard electrode, the potential 



of the cathode being taken as zero. A small correction may 



be made for the difference of potential due to the resistance 



overcome by the current between the cathode and the 



junction T (p. 23), by taking the value 3J ohms for this 



resistance in the case of the acid solution, and 6 ohms for 



the alkaline solution. All the curves representing the 



results of experiments on acid solutions exhibit first a rise of 



potential. After this it recedes, or becomes almost stationary, 



then increases once more, and finally becomes stationary 



again. Bubbles of hydrogen may now be observed on the 



cathode. It may perhaps be premature to attempt to explain 



definitely the first retrogression of potential in the curves. 



The most probable cause seems to be a temporary change in 



the nature of the electrode, resulting from electrolysis. 



Such changes have been recently shown to occur frequentlv 



