FORMATION OF AGGREGATES 275 



periods esist during phosphorescence — that is, new 

 molecules. 



In the case of gases there is direct evidence 

 of the existence of such large molecular groups, 

 which may be made to diffuse from one tube to 

 another, through narrow metal tubing earthed ; 

 thus showing that the molecules do not carry a 

 free charge of electricity. Electrical forces, there- 

 fore, cannot be expected to enter, otherwise they 

 would, if the charge on the particles be sufficiently 

 large, influence molecular forces to a very great 

 extent. 



The influence of intra-molecular forces is doubt- 

 less of importance in the phenomenon of phos- 

 phorescence, since solids are so much more 

 phosphorescent than liquids, and these far more 

 so than gases, in which an afterglow is of rather 

 rare occurrence ; and then it is characteristic chiefly 

 of the gases which polymerise, such as oxygen, 

 cyanogen, acetylene, etc. 



These facts indicate that the influence of 

 molecular forces is, however, of importance ; more- 

 over, since the light emitted by fluorescent crystals 

 is polarised, it shows that the emission of light 

 depends upon vibrations in groups of molecules, 

 rather than in the individual molecules themselves. 

 From which it may be inferred that the emission 

 of light depends largely upon the nature of the 

 molecules and their environment. 



The first point we have endeavoured to prove, 



T 2 



