of Edinburgh, Session 1881 - 82 . 
725 
the cell is exhausted, the whole of the iodine has combined with 
the zinc to form iodide of zinc. If an electric current is passed 
into this exhausted cell, the iodide of zinc is decomposed, zinc being 
deposited on the zinc plate, and iodine being set free at the carbon 
plate. The cell is now ready for use, and will give a steady current, 
having an EMF of T2 volts until the iodine is again combined 
with the zinc. I tried several experiments on this cell in New- 
castle, with a view to testing its applicability to electric lighting as 
a storage battery. 
In using a secondary battery for practical purposes, it is neces- 
sary to get a powerful battery of considerable storage into as small 
a sjDace and weight as possible. The plates must therefore be 
brought close together. This makes it impossible to employ the 
usual methods for protecting the zinc plate by means of porous cells. 
A solution of iodine acts upon zinc to form zinc iodide, the rate of 
this action depending very much on the strength of the iodine 
solution. I directed my attention to finding out the amount of waste 
in the cell from this cause, and to finding out ways of checking it. 
I charged the cells with both battery and 1 dynamo/ measuring the 
current put in and taken out on a voltameter, or charging two cells 
and using one at once, the other some hours afterwards. I had great 
difficulty in getting results from these experiments, and was at first 
deceived by the small waste in a cell partially charged. I ultimately 
found that in a fully charged cell I lost about § of the full charge 
in a few hours. This enormous loss was due to the closeness of the 
plates, the strength of the iodine solution, and the porosity of the 
zinc deposit. The cell was also hot from the charging and sub- 
sequent running down. The great rapidity of the loss up to a 
certain point prevented me from discovering it at first. I used 
every method I could devise for checking the diffusion of the iodine 
to the zinc plate. The carbon plate was covered with powdered 
carbon to make a large absorbent surface for the iodine. It was 
then wrapped in a double layer of parchment paper, and placed in 
a narrow zinc box forming the other pole of the cell. The space 
between the carbon and the zinc ( J inch) was stuffed with paper 
pulp from the paper works, which pulp had been steeped in iodide 
of zinc. All this was done to check the diffusion of the iodine 
but it was useless. 
