PHOSPHORESCENT MOLECULES 283 



cient length the violence of the collisions becomes 

 correspondingly increased. At first sight, indeed, 

 it does not appear that the molecular attractions 

 can give rise to such marked effects when the 

 corpuscles collide with the aggregates, but we know 

 that if large molecules exist and are highly charged, 

 the molecular attraction would be considerable in 

 deviating the path and attracting corpuscles towards 

 the molecular groups. The effect of a collision is 

 thus a function of the free-path of the corpuscles, 

 and luminosity takes place when this reaches a 

 certain length. The point is whether molecular 

 forces very much larger than those contemplated 

 in the ordinary kinetic theory can ever exist, and 

 our endeavour is to prove, or at any rate to show, 

 that there is considerable evidence for supposing 

 that luminosity results from the existence of such 

 forces. 



On the Existence of Separate Phosphorescent 

 Molecules in Liquids 



When violet or ultra-violet light falls on a 

 fluorescent substance such as a solution of sul- 

 phate of quinine, or glass coloured with oxide of 

 uranium, a violent agitation of the molecules or 

 atoms is set up, analogous to that which occurs 

 when chemical action suddenly takes place, or on 

 crystallisation, concussion, or some other excit- 

 ing cause. So long as the exciting light falls 

 on a fluorescent substance, the agitation of the 



