Potential Differences determined by Null Solutions. 479 
capillaiy curves (7. c. pp. 68 & 82), while not conclusive, 
favours the view that it is not so in any o£ the moderately 
dilute solutions of KC1 and KI (giving " depressed" maxima) 
referred to above. 
Palmaer has, however, published experiments recently 
from which it would seem that in the case of n/10 KC1, the 
potential-difference at the maximum surface-tension is, if 
not zero, at least very small. This result, if conclusive, 
would be of great theoretical value, and we have therefore 
performed the experiments described below to test the 
validity of Palmaer's deductions. 
§ 3. Palmaer published in 1903 (Zeitsch. f. Elektrochemie, 
ix. pp. 754-757) a summary of certain experiments with 
drop electrodes, by means of which he sought to show that 
the true contact potential-difference between mercury and 
n/10 KC1 solution is about 0*57 volt, and he has quite re- 
cently {Zeitsch. f. pliysik. Chemie, lix. pp. 129-191, 1907) 
given a full account of these experiments and of others, from 
which his final conclusion is that the potential-difference in 
question is 0*5732 ± '0003 volt at 18° C. 
One of us found that the maximum surface-tension between 
mercury and n/10 KOI solution was produced by a polarizing 
electromotive force of 0'568 + "01 volt (I. c. p. 71). It 
would thus appear that, after all, in the case of n/10 KOI, 
the potential- difference at the maximum surface-tension is 
practically zero, for it is represented by the difference 
between the two numbers just given. We shall attempt to 
show however that, although of much interest in connexion 
with electrocapillarity, Palmaer's deduction from his experi- 
ments is not necessarily true, and that his results leave the 
knowledge of the contact potential-difference between Hg 
and n/10 KOI substantially in the same state of uncertainty 
as before. 
§ 4. Palmaer s Nidi Solutions. — When certain conditions, 
first fully discussed by Paschen, are fulfilled, a mercury 
electrode immersed in n/10 KOI shows a potential about 
0*57 volt higher than that of an electrode of mercury streaming 
into the same solution. Palmaer found that by adding cer- 
tain substances to this «/10 KOI solution, he could not only 
reduce the observed potential-difference between the still and 
dropping mercury until it became very small, but could cause 
it, passing through the value zero, to change in sign. In 
this way he found two different " null solutions/' one con- 
taining small quantities of KCN, KOH, and Hg(CN) 9 , and 
2K2 
