329 
of Edinburgh, Session 1884-85. 
the method to the study of the heats of combination of iodine and 
zinc in presence of water. 
The heats of combination are in each case obtained from the 
electromotive force of either the zinc-iodine cell, or of the zinc- 
cuprous iodide cell. 
The zinc-iodine cell consists of a zinc rod and a platinum wire 
dipped in a water solution of iodine and iodide of zinc. 
The zinc-cuprous iodide cell consists of a zinc rod, and a copper 
wire coated with cuprous iodide, dipped in a solution of iodide of zinc. 
When a current passes through a given zinc-iodine cell, various 
changes take place requiring either the production or absorption of 
heat. In the first place, there is the combination of the zinc and 
iodine, producing heat; in the second place, there is the solution of 
the zinc iodide formed, producing heat. On the other hand, the 
disintegration of the zinc, the decomposition of the iodine molecule, 
and the removal of the free iodine from its solution in zinc iodide, 
absorb heat. 
If now a series of cells are made up differing only in the strength 
of the zinc iodide solution, the heat of solution of the zinc 
iodide formed in the cell during the passage of a current will vary 
with the strength of the iodide of zinc solution, and that probably 
in a way familiar to chemists from the behaviour of sulphuric acid 
when added in larger and larger quantities to water. 
On the other hand, the heat of combination of the zinc and 
iodine, and the heat absorbed in decomposing the iodine molecule 
and disintegrating the zinc, will probably remain the same. We 
shall, in fact, find that the infinitely small quantity of zinc iodide 
dissolved on passing a current has a definite heat of solution deter- 
mined by the amount of zinc iodide already dissolved in the water 
contained in the cell. 
Further, if a series of cells are made up differing only in the 
amount of free iodine dissolved in a given iodide of zinc solution, 
we shall find the heat absorbed in removing the iodine from its 
solution in zinc iodide diminish as the amount of free iodine 
present is increased. 
It is also probable that the heat of solution of a constant quantity 
of iodine will vary with variations in the strength of the iodide of 
zinc. 
