Certain Metals in Dissolved Iodine. 221 



rate of diffusion of the triiodide molecules, which in turn must 

 vary with the nature of the positive ion. Hence the rate of 

 solution of a metal in the liquid should be different in the 

 presence of different iodides. 



These differences in the rate of solution furnish the basis for 

 an interesting and valuable test of the validity of the diffusion 

 theory, and we have therefore carried out a series of parallel 

 experiments with cadmium in the presence of a number of dif- 

 ferent iodides in equivalent concentration. These experiments 

 were very carefully performed, and the results, given in detail 

 in Table VI, are very concordant. The greatest fluctuation in 

 the constants occurred with cadmium iodide, and is probably 

 due to the low iodine concentrations which had to be employed, 

 which magnified the errors of titration. Corrections for rate 

 of stirring were applied in the manner already described. The 

 magnesium iodide used contained a small amount, perhaps one 

 or two per cent, of calcium iodide, but the effect of this impur- 

 ity must have been negligible as the constants for these two 

 iodides are practically equal. Values for potassium iodide, 

 comparable with Table VI, are furnished by experiments 2, 3, 

 and 4 of Table II. The agreement between experiment 1 of 

 Table VI and experiment 7 of Table II shows that hydriodic 

 acid alone has practically the same effect as sulphuric acid of 

 the same concentration in the presence of an equivalent amount 

 of potassium iodide. 



Discussion. 



A summary of all the results of previous tables with the 

 exception of those obtained with impure metals is given in 

 Table VII. Most of the values represent the mean of two or 

 more experiments under the stated conditions. 



Under A, are the mean values of the velocity constants for 

 different metals in the presence of potassium iodide, for each 

 degree of acidity tested. To facilitate comparison results 

 obtained before the readjustment of the apparatus referred to 

 on page 211 have been multiplied by the factor 6 - 86/6*72, 

 thus reducing them to the later standard. Such values are 

 indicated by stars. 



The agreement between the metals cadmium, iron, nickel, 

 and cobalt is very striking, and clearly proves that under like 

 conditions these metals dissolve in iodine at the same rate. In 

 the earlier investigation a like result was obtained with the five 

 metals, mercury, copper, silver, zinc and cadmium. Eight 

 metals in all have, therefore, been shown to possess the same 

 rate of solution in iodine, a result for which there seems to be 

 no satisfactory explanation other than that furnished by the 

 diffusion theory. 



