Potential Differences determined bjj Null Solutions. 
487 
the electrocapillary curve to the left of the origin. Further, 
when the value of y was about '00037, an extremely small 
variation produced a very large change in the position of the 
maximum. This is represented in fig. 3, which shows how 
the displacement of the descending branch of the electro- 
capillary curve depends on the percentage composition of 
the n/10 solution. Since the concentration in kations was 
the same for all solutions, this curve represents (Phil. Trans. 
/. c. p. 80) how the potential of the solution with respect 
to the mercury rises with increase in the amount of the 
sulphide. 
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§ 13. The extremely rapid concentration variation of the 
potential near y =-00037 was very inconvenient for our 
purpose, for it happened that the solution which we sought, 
i. e. one having its electrocapillary maximum at the origin, 
would contain an amount of sulphide corresponding approxi- 
mately to this value of y. The experiments of Behrend 
(/. c. p. 481) suggested that the rapid variation of potential 
at a critical value of y occurred near the completion of the 
conversion of feebly soluble chloride of mercury into still 
