PHOSPHORESCENT MOLECULES 285 



ever, does not aflfect the freedom of the negative 

 corpuscles to the same extent. 



We may gather from these facts once more that 

 the phenomenon of fluorescence is dependent upon 

 the formation of distinct molecules, and that the 

 fixing of these molecules by gelatin appears to be 

 of a similar nature to that with which we are 

 already familiar in " a photographic plate : with 

 this exception, that the change in the latter case 

 is more permanent. 



Again, it is indeed remarkable with what per- 

 tinacity a particular mode of fluorescence attaches itself 

 to a particular substance. The fluorescence of uranium 

 nitrate, for instance, is the same in crystals which 

 are doubly refracting, as in solution, and Stokes 

 was led long ago to believe that in all proba- 

 bility the vibrations by which the fluorescent 

 light is produced are vibrations of the constituent 

 parts of the molecules themselves, and not vibra- 

 tions in which the molecules move about amongst 

 one another. It is the constituent parts that 

 move about amongst one another, and it is the 

 large positive aggregate that seems to be the 

 chief factor in the performance of the process 

 by which the luminosity is produced. 



Existence of Separate Luminous Molecules in Gases 



Under the influence of the exciting cause a 

 certain amount of energy is stored up in a 

 phosphorescent body and subsequently radiated in 

 the form of light. 



