294 Mr. A. P. Laurie on the Electromotive Forces 



showing that the electromotive force does not vary *0001 in 

 1°C. 



Consequently the correction, if any, to be applied before 

 calculating 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° C. No variation of 

 electromotive 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, was made. The amount of zinc iodide in each solu- 

 tion used was estimated 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 Table (a). 



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 probably present in the distilled water, but 

 probably this number should not be taken as being a trust- 

 worthy statement of the heat of solution of the first few 

 molecules of iodide of zinc, until further experiments have 

 been made. The next number obtained for a solution of -003 

 grm. Znl 2 in 1 grm. of water is probably 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 a filter paper. 



On looking at the table, it is noticeable that the electro- 

 motive force falls off suddenly in a saturated solution. This 

 is probably due to the separation of solid zinc iodide during 

 the passage of the current. 



(4) The variations in the - electromotive force, caused by 

 gradually increasing the amount of free iodine in a cell con- 

 taining a constant amount of zinc iodide, are given in the 

 Table (b). 



