480 Messrs. Smith and Moss on the Contact 
the other H 2 S and a small quantity of acetic acid, for which 
the potential-difference between the still and dropping elec- 
trodes was zero. It is of course well known that a concen- 
trated solution of a mercury salt behaves like a Palmaer 
null solution in the respect that there is practically no p.d. 
between still and dropping mercury electrodes immersed in 
it; but small variations in the composition do not here 
produce effects of the kind observed by Palmaer. Moreover, 
the absence of a p.d. in this case has been explained in 
a way which is not applicable when the amount of dissolved 
mercury is very small. 
§ 5. Experiments of the kind performed by Palmaer were 
suggested by Nernst (Ann, d. Pliysik^ lviii. p. 11, 1896), 
and it is obvious that the results can be explained, either 
according to the Helmholtz theory of dropping-electrodes or 
to that of Nernst, if we assume that when the observed E.M.F. 
is zero a solution has been found which exhibits no potential- 
difference with respect to mercury. In such a case there 
would be no tendency to produce an E.M.F., whether, in the 
general case, the double-layer potential is altered by extension 
of the mercury surface (Helmholtz), or whether the double- 
layer forms practically instantaneously but is accompanied 
by concentration changes in the solution (Nernst). 
§ 6. Connecting each of his null solutions in turn to a 
solution of ft/10 KC1, Palmaer measured the p.d. between 
mercury in the null solution and mercury in the ft/10 KG. 
Allowing for small contact potential-differences between the 
electrolytes, he found practically identical values (of which 
the mean is the number already quoted) for the p.d. 
Hg | n/10 KOI. Strictly, these measurements prove only 
that the p.d. between mercury and each of the null solutions 
was the same. 
Palmaer found only two satisfactory null solutions; but 
experiments with several others are described below. From 
these experiments it will be seen that the p.d. between 
mercury and a null solution of the type described by Palmaer 
is not always the same, and hence is never necessarily zero 
as Palmaer assumes. 
§ 7. A method of search for other Null Solutions. — In 
searching for other null solutions we were guided by the 
relation deducible from Paschen's experiments, that although 
the p.d. between Hg and an electrolyte when the surface- 
tension is a maximum need not be zero, it is nevertheless 
always equal to the p.d. between a dropping electrode of the 
