462 Va7i Name and Brown — Ionization and 



tions, an apparatus was finally devised which satisfactorily met 

 the needs of the case. The construction and manipulation of 

 this apparatus have been fully described in a former paper,* 

 so that only the principal points in the procedure need be 

 given here. 



The different cadmium iodide solutions used were kept in 

 thoroughly steamed bottles of Jena glass. The freezing point 

 of the water was first taken. The water was then replaced by 

 the cadmium iodide solution, which had been prepared by dis- 

 solving the carefully dried and weighed salt in a portion of the 

 same water, and diluting to an exact volume. At least three 

 determinations of the freezing point were made, using varying 

 degrees of undercooling, and no result was accepted unless the 

 final temperature held constant within 0'001° (the maximum 

 sensitiveness of the temperature reading) for at least five min- 

 utes. The volume of solution used in the freezing point 

 apparatus was either 200 or 250^"^\ 



The pure cadmium iodide solution was next replaced by a 

 portion of the same solution which had been shaken at room 

 temperature with an excess of iodine in a Jena glass bottle for 

 some hours. At 0° this solution was supersaturated with 

 iodine. To insure equilibrium with both ice and solid iodine 

 the liquid was allowed to partially freeze and was then kept at 

 its freezing point with continual stirring for several hours. 

 Equilibrium was assumed to have been reached when succes- 

 sive determinations of the freezing point agreed, and also suc- 

 cessive titrations of the dissolved iodine. In this way the 

 freezing points were obtained for solutions saturated with 

 iodine, and for those containing no dissolved iodine. Data for 

 the intermediate iodine concentrations were obtained by mix- 

 ing varying amounts of the saturated solution with the original 

 pure cadmium iodide solution, determining the freezing point 

 as before, and, finally, estimating the iodine by titration. 



In all these measurements care was taken to maintain the 

 room temperature as constant as possible, and in a few cases 

 where appreciable variation occurred the thermometer readings 

 were corrected for change in the length of the projecting nier- 

 cury thread with the aid of the usual formula, using the value 

 0*000156 for the apparent expansion coefficient for mercury 

 in glass. This correction was applied only in working with 

 the more dilute solutions where its importance was obviously 

 greatest. 



The results of these experiments are given in Table lY. 

 The four concentrations of cadmium iodide studied are the 

 same as in the measurements of electromotive force at 25°. 

 Iodine concentrations are given in the second column, the 

 starred values being those for solutions approximately saturated 

 with iodine, having iodine present as a solid phase. In the 

 *This Journal (4), xliii, 110, 1917. 



