of Edinburgh, Session 1881-82. 
831 
paper. As these experiments were all made with modifications of 
the “iodine cell,” or of the “cuprous iodide cell,” we shall begin with 
a brief description of these two cells. 
The iodine cell consists of a carbon rod and a zinc rod immersed 
in a solution of iodine in iodide of zinc. The cuprous iodide cell 
consists of a zinc plate and a copper plate immersed in a solution of 
iodide of zinc, the copper plate being covered with a paste of cup- 
rous iodide, and then wrapped in parchment paper. 
The E.M.Fs. of the cells were measured on a Thomson Quadrant 
Electrometer, the deflection corresponding to 1 volt, being found 
by measuring the deflection given by a Daniell cell. The Daniell 
cell was assumed to have an E.M.F. of 1T2 volts. This is 
not a strictly accurate assumption, but was sufficient for these pre- 
liminary experiments. The deflection given by the Daniell cell was 
measured at the beginning and end of each series of experiments. 
The zero of the instrument was usually noted after each deflection. 
The formula used has been already mentioned. It is 
E E 
0 = f~t oy @~th — tty: — (in C.G.S. units) : 
Je 4-2 x 10 7 x e ' ' 
e is equal to -003411 grm. for zinc. 
On connecting an iodine cell as above described with the electro- 
meter, zinc as the active metal gave a deflection of sixty divisions 
on the electrometer scale. (The deflection given by a Daniell cell 
was 52 divisions.) 
Copper, as the positive metal, gave no deflection. This was due 
to the coating of iodide formed on it by the iodine solution. It 
seems under these circumstances to be in a similar state to negative 
iron. 
We then constructed an iodine cell in the following manner : — 
In the centre of a glass vessel containing a solution of an iodide 
is placed a porous pot, containing the iodine solution and the 
carbon rod. Closely surrounding the porous pot is a cylinder 
of perforated zinc with which the iodine combines as fast as it 
diffuses through the porous pot. In this way the iodide solution 
in the outer vessel is quite free from iodine. 
Experiments with this cell give as an average of five experiments — 
Deflection with zinc, 63, in terms of the electrometer scale. As 
