THE SIZE OF ATOMS. 207 



line of molecules ; and it is that rapid oscillation of 

 the particles which, according to Stokes, constitutes 

 latent or stored-up light (see Fig. 42). Remark 

 now that when I remove my hand from the top bar, 

 as no waves travel down the line, no energy is 

 spent in the treacle ; and the vibration goes on for 

 ever (or, to be more exact, say for one minute) 

 as you see, with no loss (or, to be quite in accord- 

 ance with what we see, let me say scarcely any 

 sensible loss). This is a mechanical model, correctly 

 illustrating the dynamical principle of Stokes' ex- 

 planation of phosphorescence or stored-up light, 

 stored as in the now well-known luminous paint, 

 of which you see the action in this specimen, and 

 in the phosphorescent sulphides of lime in these 

 glass tubes kindly lent by Mr. De La Rue. 

 (Experiment shown.) 



Now I will show you Stokes' phenomenon of 

 fluorescence in a piece of uranium glass. I hold 

 it in the beam from the electric lamp dispersed 

 by the prism as you see. You see the uranium 

 glass made visible being illuminated by invisible 

 rays. The rays by which it is illuminated, even 



