ON THE CONTACT THEORY OF VOLTAIC ACTION. 
27 
Now the mean of the low set of values obtained previously with zinc sulphate and 
mercurous sulphate is + 0T00, and of the high values + 0"787 ; the mean of the 
values previously obtained with mercury and mercurous sulphate is + 0*414, and a 
single value — 0"092. In order to obtain the electro-motive force of the incomplete 
cell we must subtract the two latter from the two former, giving us 
-0-314 or +0-879, 
which are practically equal to the extreme values obtained directly for the electro¬ 
motive force of the incomplete cell. It therefore seems to be tolerably certain that 
the variable results obtained when experimenting with mercurous sulphate paste are 
due to the uncertainty of the nature of the bounding surface. 
The mercurous sulphate paste and the saturated zinc sulphate solution were now 
boiled together, as is done in preparing Mr. Latimer Clark’s constant element. 
Using this mixture in one porcelain vessel and mercury in the other, and joining 
them with a small glass siphon filled with the mixture, we obtained 
Volts. Mean of. 
9/5/78 Mercury has to Mixture, 22° C. —0"210 10 
11/5/78 „ „ „ +0-072 10 
Wetting the surface of the mixture with a little saturated zinc sulphate solution 
brings up the answer to +0"340, the latter being the mean of ten observations all 
giving nearly equal results. The following contact difference of potentials was now 
measured. 
11/5/78 Amalgamated zinc has to Mixture 
3 3 3 3 3 3 
33 33 33 
A little saturated zinc sulphate solution being poured over the mixture and taken 
out again carefully with a pipette altered the result to +0"520. 
The following incomplete cell consisting of mercury and amalgamated zinc joined by 
the mixture of saturated zinc sulphate and mercurous sulphate paste previously 
referred to was now made, and the mean electro-motive force was found to be about 
+ 0 •200 volt, the mercury being positive to the amalgamated zinc; this is roughly 
equal to the first value obtained with mercury and the mixture added to the last 
value obtained with the mixture and amalgamated zinc. 
o 
It was found that 
Volts. 
26/4/78 Amalgamated zinc had to Copper 18 C. +0"894 
From which it follows that 
Amalgamated zinc had to Platinum ,, = + lT25 
e 2 
Volts. 
— 0-840 
— 0-930 
— 0-740 
