developed during the Combination of Zinc and Iodine. 293 



similarly constructed cells, especially to note the effects of 

 temperature. No correction has been made for the new 

 determination of the B.A. unit, as this affects both the electro- 

 motive force and the zinc dissolved by unit current. Conse- 

 quently the error introduced in calculating from electromotive 

 force to heat of combination is very slight*. 



With the above apparatus the following experiments were 

 made : — 



1. Experiments to find the effect, on the electromotive force 

 ' of the cell, of varying the nature of the zinc. 



2. Experiments to find the effect, on the electromotive force 

 of the cell, of varying the temperature of the cell. 



3. Experiments to find the effect of varying the strength 

 of the iodide of zinc solution. For these the cuprous-iodide 

 cell was used, as simplifying the changes taking place in the 

 cell. 



4. Experiments on varying the strength of the free iodine 

 present. 



5. Experiments on varying the strength of the iodide of 

 zinc solution in the iodine cell. 



(1) In making measurements with the electrometer, the 

 surface condition of the metals used is evidently of great 

 importance, as so little current is drawn from the cell. If 

 wet zinc is exposed to the air, it becomes coated with a film 

 of hydrate, which perceptibly lowers the electromotive force 

 of the cell. 



I could find no difference between the deflections given by 

 zinc electro-deposited from pure zinc, zinc " free from 

 arsenic," zinc cleaned with sand-paper, zinc corroded by the 

 iodine solution, and zinc with a freshly-broken crystalline 

 surface. Probably, if two or three cells in series had been 

 used, slight differences in electromotive force would have 

 appeared, as is shown by Alder Wright's results with modi- 

 fications of the Daniell cellf. It is well known that when a 

 crystalline metal is treated with a weak acid, the crystalline 

 structure is brought out by the dissolving of the crystals, 

 which have been cut through in smoothing the surface of the 

 metal, and there is, therefore, probably a slight difference of 

 electromotive force between a smooth-filed zinc surface and a 

 broken crystalline zinc surface. This difference, however, is 

 too small to be shown on the electrometer with one cell. 



(2) Two iodine cells in series, containing a dilute solution 

 of iodine and of iodide of zinc, were raised from 10° C. to 

 50° C. No variation of electromotive force could be detected, 



* See Alder Wright's recent papers in the Phil. Mag. for 1885. 

 t Phil. Mag. Jan. 1885. 



