222 STUDIES IN LUMINESCENCE. 



as the temperature rises. Sooner or later a temperature will be reached 

 where groups can no longer exist. At temperatures above this point we 

 should expect no effect from exposure to the infra-red, and all hysteresis 

 effects should disappear. The experiments of Pierce show that hysteresis 

 still persists in Balmain's paint at 133 C. No other data bearing upon 

 this point are available. 



It would be a matter of no great difficulty to develop the theory of ionic 

 groups in much greater detail than has here been attempted, for when 

 uncomplicated by the effects of irregular distribution and diffusion the 

 theory lends itself readily to analytical treatment. But in view of the 

 small amount of experimental data that are available for testing the con- 

 clusions it seems inadvisable to carry the development further at present. 



SUMMARY. 



In the case of a homogeneous substance having only one band in its 

 phosphorescence spectrum and uniformly excited throughout, the dis- 

 sociation theory first suggested by Wiedemann and Schmidt leads to a 

 linear relation between / and I~ j during the decay of phosphorescence. 

 The observed decay curve, however i. c, the curve obtained when Z _J is 

 plotted as a function of / usually shows a more or less sharp curvature in 

 the early stages of decay, and only later becomes straight. Using the dis- 

 sociation theory as a basis, we have discussed first the influence upon the 

 decay curve of various disturbing factors, such as absorption of the exciting 

 and emitted light, diffusion, etc. We have then considered the effect upon 

 the various phenomena of phosphorescence of the formation of complex ions, 

 which are assumed to be produced by the attachment of simple ions to 

 neutral molecules; and after the introduction of certain hypotheses regard- 

 ing the behavior of such ionic groups, an analytical theory is developed. 



Briefly the results of the discussion are as follows: 



While the observations thus far made on the decay of phosphorescence 

 are perhaps not sufficient to definitely disprove Becquerel's explanation of 

 the form of the decay curve, viz, that the curvature is due to the existence of 

 two bands in the phosphorescence spectrum, numerous objections to this 

 explanation are discussed. For example, the fact that the decay curve has 

 the usual form even when precautions are taken to study only a single band; 

 that the phosphorescence spectrum remains unaltered during decay; the 

 fact that the decay curve is of the same type for all substances thus far 

 studied; and the fact that Becquerel's explanation takes no account of 

 hysteresis phenomena, the effect of infra-red rays, etc. 



2. It is shown that lack of transparency in the phosphorescent sub- 

 stance, resulting in the absorption of either the exciting light or the emitted 

 light or both, will cause a slight curvature in the decay curve. But the 

 effect can under no circumstances be sufficient to account for the observed 

 deviation from linearity. 



3. Lack of uniformity in the distribution of the active material through- 

 out the mass of the phosphorescent substance will cause a deviation from 

 linearity corresponding to that actually observed. Under certain con- 



