I98 STUDIES IN LUMINESCENCE. 



Upon the basis of the dissociation theory of luminescence it is clear also 

 that we should expect no change in the form of the phosphorescence 

 spectrum during decay. 1 The intensity of phosphorescence will depend 

 at each instant upon the number of recombinations that occur per second, 

 and will therefore diminish as the number of free ions becomes less. But 

 it seems probable that the number of recombinations that occur under 

 such conditions as to give light of a certain wave-length will still remain 

 the same fraction of the whole number. The case has some resemblance 

 to that of the distribution of velocities among the molecules of a gas ; if the 

 total number of molecules in a given volume is diminished, the number 

 having a given velocity will also diminish ; but this number will still be the 

 same fraction of the whole. 



Probably the most serious objection to the form of the theory of 

 Wiedemann and Schmidt that is here considered is the absence of any 

 direct evidence of electrolytic dissociation during the excitation of lumines- 

 cence. The experiments of Howe, 2 although carried out with apparatus of 

 high sensibility, fail to show any ionization in anthracene vapor when 

 excited to fluorescence. Ionization of gases by the direct action of ultra- 

 violet light has indeed been observed in numerous instances, but not under 

 conditions which indicate any connection with luminescence. No change 

 in electrical conductivity during excitation, such as might be expected to 

 result from electrolytic dissociation, could be detected in fluorescent liquids 

 by Cunningham, Regner, or Camichel. Our own experiments with 

 alcoholic solutions of the fluorescent dyes seemed at first to indicate an 

 increase in conductivity during fluorescence in some cases as great as 1 per 

 cent. But the work of Hodge and of Goldman has shown that these results 

 were in reality due to a change in polarization or to the establishment of a 

 photo-electric E.M.F. 2 



On the other hand, Lenard has found indirect evidence of an increase in 

 the conducting power of certain phosphorescent sulphides during lumines- 

 cence. The fact that luminescent substances usually exhibit strong photo- 

 electric activity, as was first pointed out by Elster and Geitel, also points 

 to a close connection between ionization and luminescence. Even the 

 absence of increased conductivity in fluorescent liquids is not so important 

 as would at first appear, for the solutions tested were those in which the 

 property of fluorescence resides in the ion. If a negative ion is dissociated 

 by the separation of an electron no change in the number of ions will result, 

 and the only change that could be expected in the conductivity is that 

 which might come from a change in the mobility of the ions. Whether the 

 mobility would be increased or decreased we have no means of telling; but 

 that the change would not be great seems reasonably certain. 



While the failure of experiments to detect increased conductivity in 

 solutions thus appears to be without great significance, the fact that cells 

 containing fluorescent liquids give an E.M.F. , when one electrode is illumi- 

 nated, which is far greater than that of other photo-active cells, gives another 

 instance of a close connection between fluorescence and electrical effects. 



! The hypothesis of molecular groups developed in Chapter XV in discussing the form of the decay curve 

 calls for a slight change in the form of the spectrum. But it seems probable that this change is so small 

 ;js to be difficult of detection. 



s See Chapter X. 



