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



Table (c). — Variations in Electromotive Force caused by 

 varying the strength of the Iodide of Zinc in a cell con- 

 taining '014 grm. of Iodine in 1 c.c. 



Electromotive Force in Volts. Grms. of Znl 2 in 1 grm. of Water. 

 1-075 3-870 



1-303 -118 



1-594 D.W.* 



On subtracting these from 

 the cuprous-iodide numbers, 

 we get 



•686 



•667 



•719 



Having now described the experiments, and given the 

 results, I wish to make a few remarks upon them. 



In the first place there are one or two points worth referring 

 to with regard to this zinc-iodide curve. It will be noticed 

 that the E.M.F. of solution for the first few traces of the salt 

 is very high. This agrees with Prof. HelmhohVs experiments 

 on zinc chloride, given in his second paper on the " Thermo- 

 dynamics of Chemical Combination/' In connection with that 

 paper I may say that I propose to make measurements of the 

 vapour- tension of zinc-iodide solutions, with the view of com- 

 paring them with the curve of E.M.F., as Prof. Helmholtz 

 has done for zinc chloride. 



One very curious point is the drop in E.M.F. for a satu- 

 rated solution. This is no doubt due to the different physical 

 conditions in the cell at this point, owing to the separation of 

 solid zinc iodide ; but it requires further investigation, as it 

 is difficult to understand why the formation of the solid salt 

 should lower the E.M.F. In all the calculations given below 

 I have taken the value above *390 volt, namely *415 volt, as a 

 starting-point for the values of y. With reference to this part 

 of the curve, the two numbers, 3*9 grms., require explanation. 

 The analysis of the very strong solutions with accuracy was 

 difficult, and therefore, although the second solution was 

 certainly just off the saturation-point, and the first solution 

 contained solid zinc iodide (which was settled out before 

 analysis), it seemed better to give the strength of the two 

 solutions as the same, rather than to give numbers which lay 

 within the errors of experiment. 



I must also state that I have made no experiments to deter- 



* Iodine calculated to -014 grm. 



