122 Ycm Name and Brown — Tri-Ioclide and 



extrapolation rather iiiicertain. The resnlthig vahies for the 

 active fractiou, as summarized in Table XYII, are therefore to 

 be regarded only as rough approximations. 





Table XVII. 







Total Halide Concentration. 



Active 



Salt. 



(grin. 



equiv. per liter.) 



Fraction ^, 



CdI,.2KI 





0-92 



15 



CdBr„.2KBr 





0-55 



47 



a ' a 





0-27 



61-5 



HgI^.2KI 





0-5 



8-5 



HgBr^.2KBr 





0-5 



13-2 



U ii 





0-2 



17 



HoBr 





0-029 



4 



In considering these results for solutions of double or mixed 

 halides it should be remembered that in the case of a mixture 

 of zinc and potassium iodides, in the proportions Znl^.KI, 

 the value of K^ was found to be normal, and the active frac- 

 tion, therefore, 100 per cent. In other words, both iodides, 

 which are normal when alone, act entirely inde23endently in 

 the mixture, a result which is probably true in general. 



This, however, is not the case when one of the halides is 

 abnormal. Let us take, for example, the case of Cdl2.2KI, 

 where the active fraction was found to be 15 per cent. Since 

 the potassium iodide made up 50 per cent of the total iodide, it 

 follows that even assuming that the cadmium iodide united 

 with no iodine, only 30 per cent of the potassium iodide was 

 active. It is evident that a large part of the latter must have 

 been taken up in the formation of complexes. The same 

 effect is visible with CdBr,.2KBr to a slight extent, while with 

 Ho^I„.2KI and Ho:Br,.2KBr it is even laro;er than in the first 

 example. 



As already stated, it is very difficult to obtain any trust- 

 worthy results with pure mercuric bromide. Considering the 

 low concentration of the mercury salt, the result in the table 

 indicates that the tendency to complex formation in mercuric 

 bromide solutions is very great. 



An entirely different hypothesis to account for the proper- 

 ties of the iodine-iodide mixture has been proposed by Parsons, 

 who ascribes the phenomena to simple solubility of the iodine 

 in the dissolved iodide — "solution in a dissolved solid." 

 The acceptance of this view, however, would leave the constancy 

 of K^ entirely without explanation, and, indeed, would involve 

 the complete rejection of evidence of the kind we have been 

 considering. 



Parsons'^ likens the case to that of iodine (or camphor) dis- 



*Jonr. plivs. Cbem.. xi, 664. 1907; also Parsons and Corliss, Jour. Am. 

 Chem. Soc. xxxii, 1374, 1910. 



