Potential Differences determined by Bull Solutions, 485 
The reason of the difference of 0*125 volt between this and 
the value found by the KCN null solution is suggested by 
inspection of the electrocapillary curves. 
The maxima for the KCN solutions are lower than those 
for the KC1 solutions of corresponding strength, and the 
" horizontal distance " between the curves after they become 
parallel is approximately 0*125 volt greater than the distance 
between the maxima. Consequently (cf. Phil. Trans. I. c. 
p. 67) the solution at the maximum surface-tension in KCN 
is 0*125 volt more positive to the mercury than it is at the 
maximum in KC1. 
§ 11. Every solution for which the maximum of the electro- 
capillary curve is at the origin is a null solution, but the 
potential-differences between mercury and different null 
solutions will be different if the maxima for these solutions 
are not the same. 
If traces of other substances can be added to a n/4 KC1 
solution in such a way as to move the maximum of the elec- 
trocapillary curve to the origin without appreciably raising 
or depressing it, then such a solution will be about 0*125 
volt less positive to mercury than the null solution of n/4 KCN. 
This null solution when measured against pure w/4 KC1 will 
give an E.M.F. of 0*565 in place of the E.M.F. of 0*69 
given by the rc/4 KCN null solution. Hence, if Palmaer's 
solutions were produced from n/10 KC1 without appreciable 
alteration of the maximum, the difference between his results 
and ours is immediately explained. 
§ 12. Effect of Na 2 S on the p.d. between Hg and KC1. — 
The p.d. between mercury and an electrolyte is controlled, 
according to the theory of Nernst, by the concentration p of 
the mercury ions in solution. If it is the potential rise from 
the solution to the mercury, then 
-bv[bp=mi P i ..... (i.)* 
where T is the absolute temperature and k is a constant. 
The rise can therefore be diminished by decreasing p. From 
Palmaer's experiments and others (<?. g., Behrend, Zeits f. 
phys. Chem. xi. p. 481, 1893), it can be inferred that a large 
diminution of p is produced by saturating a KC1 solution in 
contact with mercury with H 2 S. We therefore thought it 
likely that by addition of Na 2 S to the KC1 solution it would 
be possible to move the electrocapillary curve to the right, 
i. e. so that the maximum approached the origin. It remained 
to determine by experiment the relative amount of Na 2 S 
* An interesting method of deducing- a similar equation was given by 
Professor J. J. Thomson in the Philosophical Magazine for 1895. 
