Potential Differences determined by Null Solutions. 481 
Paschen type ("Wied. Ann. xli. p. 42, 1890) and the same 
electrolyte (cf. Phil. Trans. I. c. pp. 83 et seq.). We have 
found, in the course of the present experiments, that the 
statement of this relation, which is one of the most important 
in electrocapillarity, requires qualification. The relation is 
true only when the medium through which the mercury falls 
before it enters the solution does not contain a constituent 
which interacts chemically with the mercury and the solution. 
In some cases, for example, the relation does not hold until 
the air surrounding the dropping electrode and the solution 
is replaced by hydrogen, nitrogen, or other non-oxidising 
medium. 
§ 8. Assuming the truth of the above relation, it was 
obvious that any solution for which the maximum of the 
electrocapillary curve lay at the origin (applied E.M.F. zero) 
would be a null solution. If the maximum for a given 
solution were slightly (a) to the right or (b) to the left of 
the origin, then the still electrode for that solution would be 
slightly (a) positive or (b) negative to the dropping electrode. 
The change in sign of the potential of the dropper with 
respect to the still electrode, with a small change in the 
composition of the electrolyte,, would not correspond neces- 
sarily (as assumed by Palmaer) with a change in the sign 
of the potential of the still mercury with respect to the 
solution, but only with a change from one side of the origin 
to the other of the maximum of the electrocapillary curve. 
Since it is almost certain that this maximum does not 
always correspond with zero potential-difference, it seemed 
equally certain that a null solution of the kind studied by 
Palmaer could not always, and need not ever, be one 
exhibiting no contact p.d. with respect to mercury. 
§ 9. Null Solution of KCN. — It has long been known that 
moderately concentrated solutions of KCN give electro- 
capillary curves of which the maxima lie to the left of the 
origin (applied E.M.F. negative), while weaker solutions 
give curves of which the maxima' lie to the right. If a 
solution could be found of which the maximum lay at the 
origin, this would be a null solution of the kind studied by 
Palmaer. A null solution of KCN was apparently found 
approximately by Amelung in a research which Palmaer 
describes (Zeits. f. pliysik. Chemie, lix. p. 164, 1907) but 
regards as unsatisfactory. This solution used according to 
Palmaer's method would give a value of roughly 0*7 volt for 
the contact potential Hg | n/10 KC1. It was obtained by 
