494 
Messrs. Smith and Moss on the Contact 
Palmaer if the measurements are interpreted in the way he 
describes. If, on the other hand, the curve for the null- 
solution electrolyte has a higher maximum than the corre- 
sponding curve for KCl, then the p.d. Hg | KCl found by 
Palmaer's method will be less than that which he gives. 
We have not attempted to find a null solution exactly of this 
last kind, although it is known that the electrocapillary 
maximum of a saturated KOH solution is near the origin 
and is considerably higher (Phil. Trans. I. c, p. 68) than that 
of concentrated KCL 
We have, however, obtained null solutions by the addition 
of small quantities of Na 2 S (and acetic acid) to n/lOKI 
solution, which has a very depressed maximum, and to 
n/10 KOH solution, which has a higher maximum than 
n/10 KCl. The results of these experiments exhibit the 
truth of the conclusions we have drawn. 
§ 19. Null Solution of KI. — The attempt to obtain a null 
solution from nj 10 KT succeeded approximately when the 
composition of the solution was about , 001?tNa 2 S + *099^KI. 
The maximum surface-tension occurred when the applied 
E.M.F. was — *01 volt. It was unnecessary for our purpose 
to obtain a nearer approximation to a null solution. The 
E.M.F. between a dropping electrode falling through 
hydrogen into this solution and a still electrode was at first 
imperceptible. After some time it was *006 volt. Mercury 
in this solution measured against mercury in n/10 KCl gave 
an E.M.F. of -793 volt. A fresh null solution of KCl, made 
up by trial for comparison, measured against n/10 KCl in 
the same way gave *565 volt. The two solutions measured 
against each other gave *228 volt. The results of the 
measurements are tabulated below: — 
1. 
9 # 
3. 
4. 
5. 6. 
7. 
Null Solution. 
Max.S.T. 
(scale- 
reading;. 
E.M.F. for 
max. S.T. 
volt. 
Horizontal 
distance to 
11 
Yq KCl curve. 
volt. 
1 : n, II 1 
1 sol. i jqKC1|+! ! sol. | + 
volt. | volt. 
I null : null 1 
1 KI i KCl 1 i " 
volt. 
io KI +rooo Na 3 s 
n 
joKCl+*Na 2 S... 
28-85 
30o 
-•01 -789 
-•01 569 
•793 
•565 

-01 
•228 
