ON THE CONTACT THEORY OF VOLTAIC ACTION. 
25 
Date. 
The following— 
Has to the following— 
At— 
A contact 
difference 
of 
potentials, 
in volts. 
Remarks. 
°c. 
15/3/78 . 
Saturated copper sulphate . 
1 of distilled water with 3 
16 
-0 102 
Each of the numbers in the 
of saturated solution of 
preceding column is the 
zinc sulphate. 
average of about ten obser- 
V ,, 
1 of distilled water with 1 
16 
-0-093 
vations. 
of saturated solution of 
zinc sulphate. 
Zinc. 
Saturated zinc sulphate . . 
16 
-0-140 
Copper.. . 
Alum, saturated .... 
15 
-0-109 
As a small leakage was now observed between the needle of the electrometer and the quadrants this instrument 
was taken to pieces and cleaned. 
18/3/78 . 
Copper. 
Mercury. 
13-5 
+ 0-351 
19/3/78 . 
Iron. 
. 
13'5 
+ 0-473 
Copper. 
Alum, saturated .... 
15 
-0-145 
Platinum. 
Strong sulphuric acid . . 
15 
+ 1-587 
Lead. 
V 
14-5 
+ 1-252 
Copper. 
}1 )> 
15 
+ 1170 
20/3/78 . 
Platinum. 
Mercury. 
15-5 
+ 0-198 
Brass. 
Distilled water. 
15 
+ 0-256 
26/3/78 . 
Sal-ammoniac, saturated at 
17 
-0-361 
Prom the 26th March to the 
15°'5 C. 
8th April occupied with 
8/4/78 . 
Zinc. 
15-7 
-0-697 
experiments on the con- 
fact difference of poten- 
tials of hot and cold mer- 
cury. 
IV. 
We now endeavoured to measure the contact differences of potential of each pair of 
constituents in a Latimer Clark’s mercurous sulphate cell. First we tried mercury 
and mercurous sulphate paste, the mercury being put into one of our porcelain dishes 
and the mercurous sulphate paste into the other, the connexion between the two 
being made by a small glass siphon filled with the mercurous sulphate paste and the 
earth connexion by a platinum wire dipping into the paste in the porcelain dish. 
Many days were spent unsuccessfully at this experiment, and at similar ones which 
follow, on account of the thin layer of water which floated on the top of the mercurous 
sulphate, even after great care had been taken to remove it with a pipette. Often, 
therefore, instead of measuring the contact difference of potentials between a substance 
A and the mercurous sulphate paste, we obtained that of A with the paste plus that 
of the paste with water. At length, however, we obtained the following, each number 
given being, as usual, the mean of many experiments. Very great care was taken to 
have the materials chemically pure, the mercury being frequently redistilled; also to 
prevent any discrepancies in these results being caused by any possible electrification 
of the porcelain vessels themselves, they were thoroughly de-electrified after cleaning 
and before using. 
It may be noticed that this difficulty experienced by the thin layer of water which 
floated on the mercurous sulphate paste is a very good example of the inaccuracies 
MDCCCLXXX. 
E 
