333 
of Edinburgh, Session 1884 - 85 . 
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 electro- 
motive force would have appeared, as is shown by Alder Wright’s 
results with modifications of the Daniell cell.* 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, showing that the 
electromotive force does not vary - 0001 in 1° C. 
Consequently the correction, if any, to be applied before calculat- 
ing the heat of combination from the electromotive force is less than 
2 per cent. 
One iodine cell, made up with a saturated solution of iodide of 
zinc, was raised from 17° C. to 70° CL No variation of electro- 
motive force could be detected. 
(3) A series of measurements of the electromotive force of the zinc- 
cuprous iodide cell, with varying strengths of iodide of zinc, were 
made. The amount of zinc iodide in each solution used was esti- 
mated by determining volumetrically the amount of combined iodine. 
The temperature of the room during the experiments varied a 
little above and below 20° C. 
The results of the observations are given in the table (a) at the 
end of the paper. 
The lowest number given is that obtained in a saturated zinc 
iodide solution; the highest is that obtained with only distilled 
water in the cell. 
In every case zinc corroded in iodine solution, and then washed, 
was used. Some very minute traces of zinc iodide were therefore 
* Phil. Mag., Jan. 1885. 
