1888.] upon the Potential of Voltaic Couples. 



371 



The electromotive force in the seven strongest solutions rose quickly 

 after immersion; this was due to an extremely thin solid coating 

 forming upon the magnesium. 



Table YL— Ditto at 19° C. 



Grains. 



Volts. 



Grains. 



Yolts. 



0-00099 



1 -7018 



0-000723 



1-5588 



-00089 



1 -7089 



-00066 





-00088 



1 -6589 



-00033 



>> 



-000805 



1 -6446 



water 





With the astatic galvanometer the minimum proportion of iodine 

 required to alter the potential lay between 1 in 577,711 and 643,153 

 parts of water. If the magnesium was merely wiped between each 

 measurement, instead of being cleaned with emery cloth, the electro- 

 motive forces on first immersion were 0'18 volt higher in Tables Y 

 and YI. 



The smallest proportion of iodine necessary to upset the balance of 

 a zinc-platinum couple in water has already been published 

 (" Influence of the Chemical Energy of Electrolytes, &c," ' Roy, 

 Soc. Proc.,' June 7, 1888) ; it lay between 1 part in 3,100,000 and 

 3,521,970. 



Table YII.— Cd + Pt + Iodine in 465 grains of Water at 19° C. 



Grains. 



Volts. 



Grains. 



Volts. 



Grains. 



Volts. 



0-132 



0-9884 



-0675 



1 -0027 



-0030 



0-8311 



0-119 



0*9741 



0546 



0-9854 



0-002625 



-8028 



0-1062 



-9884 



-0417 



1 -0198 



0-002325 



-7882 



0-0933 



-0288 



0-9854 



-002079 



0-747 



0-0804 



0-9827 



0-0159 



-9741 



water 



n 



The minimum proportion of iodine required to change the poten- 

 tial lay between 1 part in 200,431 and 224,637 parts of water. 



The curves of variation of electromotive force by uniform change 

 of strength of liquid with zinc-platinum and cadmium-platinum, 

 presented certain similarities, but that with magnesium-platinum was 

 considerably different, probably in consequence of insoluble films 

 forming upon the magnesium^ 



