376 On the Changes of Potential of Voltaic Couples. [June 21, 



The minimum proportion of chlorine in this case lay between 1 in 

 518,587,360 and 550,513,022 parts of water. 



Table XX— Zn + Cd + Chlorine in 1550 grains of Water at 11° C. 



Grains. 



Volts. 



Grains. 



Volts. 



0-3565 

 -05592 

 0-02796 



-2687 

 0-2831 

 -3088 



-02027 

 water 



-32032 

 >> 



Eleven other solutions of different strengths, all weaker than 

 0-02027, each gave the same potential as water. The minimum pro- 

 portion of chlorine required to disturb the balance lay between 1 part 

 in 55,436 and 76,4s67 parts of water. In this case, the addition of 

 chlorine decreased the electromotive force ; a similar effect occurred 

 with a zinc-platinum couple in a solution of potassic iodide (" Influence 

 of the Chemical Energy of Electrolytes, &c," 'Roy. Soc. Proc.,' 

 June 7, 1888). 



The following are the minimum proportions of chlorine which were 

 required to change the potential : — 



Table XXI. 



With an Astatic Galvanometer. 

 Mg + Pt + CI. Between 1 in 17,000,000,000 and 17,612,000,000 

 Zn + Pt + CI, „ 1 „ 1,264,000,000 „ 1,300,000,000 

 Zn + Au+ CI. „ 1 „ 518,587,360 „ 550,513,022 

 Cd + Pt + CI. „ 1 „ 8,733,585 „ 9,270,833 

 Zn + Cd + CI. „ 1 „ 55,436 „ 76,467 



*# With a Reflecting Galvanometer. 



Mg + Pt + 01. Between 1 in 27,062 millions and 32,291 millions. 



The examples contained in this paper are sufficient to show, that 

 the proportion of the same exciting liquid, necessary to disturb the 

 potential of a voltaic couple in water, and the order of variation of 

 potential caused by change of strength of liquid, vary with each 

 different positive or negative* metal. The numbers in Tables IV, VIII, 

 XII and XXI, show that the more positive or more easily corroded 

 the positive metal, or the more negative and less easily corroded the 

 negative one, the smaller usually was the proportion of dissolved 

 s.i'ostance required to change the potential. In the case of chlorine, 



* If the negative metal is not at all corroded, the order of change of potential by 

 change of negative metal is not much aLected. 



