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Proceedings of the Royal Society of Edinburgh. [Sess. 
in order to completely investigate Jakowkin’s equation. Dawson’s investi- 
gation is made on the assumption that KI a is the only polyiodide present. 
The presence of higher polyiodides would evidently complicate the matter 
still further, but it has been proved by Jakowkin and Dawson’s results that 
such higher poly iodides can at most be present in very small quantities in 
dilute solutions. 
If we then consider a dilute solution of potassium iodide saturated with 
iodine, and if we determine the total amount of iodine present, then this 
amount less the solubility of iodine in water (’00134) will give us the total 
amount of combined iodine, which in a dilute solution may be regarded as 
wholly present as ionised or unionised KI 3 . 
The number of iodide ions present having been determined as described 
above, we can evidently determine the total number of uncombined KI 
molecules by applying the ionisation factor for that strength of potassium 
iodide. Further, if our methods of investigation are correct, the total 
amount of uncombined KI molecules (determined from the measurement of 
the number of iodide ions by means of the E.M.F.) added to the total 
number of combined KI molecules (determined by the titration of the 
saturated iodine solution) should equal the total number of KI molecules 
added to the solution, as long as we are justified in assuming that the only 
polyiodide present in appreciable quantity is KI S . If, however, an appreci- 
able amount of polyiodides higher than KI S are present, this would no 
longer be true ; for the number of combined KI molecules present, as 
determined by the E.M.F. , would be less than the total number of combined 
iodine molecules determined by titration. 
The actual measurements of the E.M.F. of the iodine concentration cells 
were made with a Dolazelek electrometer, the solutions being enclosed 
in the small stoppered electrodes which are figured in the paper above 
referred to.* 
The correction for the contact E.M.F. was found by transferring the 
stoppered electrodes from the potassium iodide solution in which they were 
immersed to the ammonium nitrate solution of the same temperature, and 
taking readings after the electrodes had had an hour or two to settle down. 
The results obtained were a little irregular, but I consider the error as 
probably not more than a millivolt in any single case. 
The iodine saturation -point was determined as follows. A solution of 
potassium iodide of the required strength, and containing the small quantity 
of iodine required for the one electrode, was first prepared. This electrode 
was then filled with this solution and stoppered, and the same solution 
■* Laurie, Proc. Roy. Soc. Edin ., vol. xxviii. (1908), pt. 5, p. 382 ; Zeit. Phys. Chem ., lxiv. 5. 
