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Proceedings of the Royal Society 
possibly present in the distilled water, bnt probably tbis number 
should not be taken as being a trustworthy statement of the heat of 
solution of the first few molecules of iodide of zinc, as the reactions 
in the cell are very complicated in this case. The next number 
obtained for a solution of -003 grms. Znl 2 in 1 grm. of water is pro- 
bably safer as a starting-point for a curve. 
Evidently the determination of the heat of solution of the first 
traces of a salt is difficult even by this method, though a much nearer 
approximation can be obtained than that possible in the calorimeter. 
It was noticed, in the case of the stronger solutions, that on first 
placing the zinc rod, damp with distilled water, in the solution, the 
electromotive force of the cell was 2 or 3 per cent, too high, quickly 
falling to a constant value. This was no doubt due to the temporary 
dilution of a layer of liquid next the zinc, and the effect disappeared 
on drying the zinc with filter paper. 
On looking at the table, it is noticeable that the electromotive 
force begins to fall off more rapidly again as the solution approaches 
concentration. This is possibly due to the more concentrated solu- 
tion containing, as is well known, an excess of zinc, and being in 
fact no longer zinc iodide. This part of the table requires, however, 
further investigation. 
(4) The variations in the electromotive force, caused by gradually 
increasing the amount of free iodine in a cell containing a constant 
amount of zinc iodide, are given in the table (6). 
The free iodine present was estimated with standard hyposulphite 
solution in the usual way. 
This table requires no particular remark. 
(5) Three determinations of the variation of the electromotive 
force of the zinc iodine ceil were made, due to varying the strength 
of the iodide of zinc, one in an iodine solution in distilled water, 
one in a zinc iodide solution of medium strength, and one in a 
saturated iodide of zinc solution. The results are to be found in 
the table (c).* 
If these numbers are subtracted from those obtained for correspond- 
ing solutions in the cuprous iodide cell, the numbers obtained show 
that the curve for the zinc iodine cell is slightly different, rising a 
* Very similar results have been obtained for sulphates and chlorides by 
Alder Wright, Phil . Mag., 1884. 
