PHENOMENA OF PHOSPHORESCENCE. 223 



ditions the effect is the same as though the substance possessed two or 

 more bands, coincident as regards wave-length, but differing in rate of 

 decay. It is pointed out that such a lack of homogeneity is to be expected 

 from the method of preparation of phosphorescent substances, and has a 

 great deal to do with the complexity of the phenomena. 



4. Diffusion of the ions will produce effects similar to those caused by 

 irregularities in the distribution of the active material without diffusion, 

 but the effects will be more marked. An explanation of hysteresis effects 

 may also be based upon the presence of diffusion. 



5. The assumption of complex or grouped ions is shown to lead to a law 

 of decay which agrees closely with experiment. Ionic grouping also 

 accounts satisfactorily for the dependence of the decay curve upon the 

 duration of excitation and the previous history of the substance, and for 

 the effect of exposure to the infra-red. 



The preceding discussion appears to justify the conclusion that the dis- 

 sociation theory proposed by Wiedemann and Schmidt accounts satisfac- 

 torily for all the phenomena of phosphorescence thus far studied. Addi- 

 tional quantitative data are greatly needed, however, in order to make pos- 

 sible the further development of the theory. The present difficulty is not 

 so much in accounting for the observed facts as in discriminating between 

 different hypotheses that are at present equally plausible, and in deciding to 

 what extent various recognized sources of disturbance are of importance. 

 The form of the decay curve, for example, may be accounted for equally well 

 by the assumption of two bands, by an irregular distribution of the active 

 substance, by diffusion, or by ionic grouping, and is unquestionably modi- 

 fied by absorption. No quantitative test of either explanation is possible 

 until the relative importance of the other factors is known. It may be, for 

 instance, that diffusion does not occur at all; and it is possible, but not 

 probable, that the disturbances due to lack of homogeneity are of no signifi- 

 cance. Experiments that will furnish a definite answer to questions of this 

 sort are of the greatest importance in the development of this or any other 

 theory of phosphorescence. 



