developed during the Combination of Zinc and Iodine. 297 



mine whether my strong solutions contained more complex 

 compounds than Znl 2 . 



In the next place, the electromotive force of an iodine cell 

 containing an almost saturated solution of zinc iodide should 

 correspond pretty closely with the calorimetric number for 

 the heat of combination of zinc and iodine, as in this parti- 

 cular case the electromotive force due to the heat of solution 

 of zinc iodide is eliminated. 



Disregarding the portion of the iodine curve where the 

 E.M.F. of solution of the iodine is considerable, as being too 

 small a portion to affect materially the heat set free in the 

 calorimeter, and taking the portion between 1*268 volt and 

 1*287 volt as being a straight line, we get the average E.M.F. 

 of a cell containing *33 grm. of Znl 2 in 1 grm. of water as 

 being 1*277 volt. This gives the E.M.F. of a cell with nearly 

 saturated zinc iodide as 1*104 volt, a number probably too 

 high, since the falling portion of the iodine curve has not been 

 considered. 



The value in volts of the number given by Naumann is 

 1*085 volt. 



A more exact comparison is impossible as the strength of 

 the iodine solution used in the calorimeter experiments is not 

 given by Naumann. 



The calculation of the heat set free during the solution of a 

 given quantity of zinc iodide in a given quantity of water can 

 be easily made from the curve of E.M.F. If a cuprous - 

 iodide cell be started with distilled water and allowed to run 

 down, during the passage of the current zinc iodide will 

 accumulate in the cell, and the E.M.F. will fall along the 

 curve plotted in the diagram. At the same time the amount 

 of zinc iodide dissolved during any time is a measure of 

 the current that has passed through the cell. 



We have only then to state percentages of zinc iodide in 

 terms of unit current to calculate, by the integration of the 

 curve, the quantity of heat set free during the solution of the 

 zinc. It is necessary of course to take some value of E.M.F. 

 as representing y = 0, where y is the E.M.F. due to heat of 

 solution only. It is probably best, as explained above, to 

 take *415 volt as the value corresponding to y = 0. 



If then y=$oc is the equation of this curve, then the heat 

 of solution of *044 grm. of zinc iodide in 1 grm. of water 



no 

 (ZnI 2 ,400H 2 O) can be calculated from \$x.dx. Plotting this 



J '044 



portion of the curve very carefully, and finding this area 

 arithmetically, the value for the heat of solution (ZnI 2 ,400H 2 O) 



