CORRELATION OF PHENOMENA 71 



therefore of the intensity of the fluorescent light 

 itself; so that the absorption ought to be the same 

 whether this fluorescent body is fluorescing or inactive. 

 This, however, is by no means true. 



The absorption depends upon the fluorescence, and 

 can be explained, as we shall endeavour to show, only 

 on the assumption of a discontinuity in the structure 

 of the body, so that new periods are produced which 

 last merely during the time of fluorescence. These 

 new periods appear to be due to the formation of new 

 molecules, which last during the fluorescence and to 

 which the fluorescence is due. Thus the luminosity 

 may be said to be but the manifestation of a molecular 

 process, of building up and breaking down of 

 aggregates or groups of molecules. And this applies 

 not merely to fluorescence and phosphorescence, but 

 to the phenomena of luminosity generally. A process 

 analogous to metabolism and essentially catalytic in 

 its nature being that which forms the basis of the 

 facts of light as well as of life around us. 



For the time being the luminous body may be said 

 to be different from its non-luminous preceding state, 

 to which it gradually returns when the source of 

 energy which influences it is once more removed. 



Thus these various kinds of luminosity, in the 

 many fields of physics which we shall traverse, 

 indicate that molecular interactions underlie the 

 more elementary, as well as the more complicated, 

 facts of observation ; and that the " molecular 

 shufflings" and "unstable aggregations," whether 

 chemical or semi-chemical changes of an ephemeral 



