1883.] on the Size of Atoms. 209 



The principle is mechanically represented by this model (described 

 above with reference to Fig. 2). A simple harmonic motion is, 

 as you now see, sustained by my hand in the uppermost bar, in 

 a period of about four seconds. You see that a regular wave- 

 motion travels down the line of molecules represented by these 

 circular disks on the ends of the bars, and the energy continually 

 given to the top bar, by my hand, is continually consumed in heating 

 the basin of treacle and water at the foot. I now remove my hand and 

 leave the whole system to itself. The very considerable sum of 

 kinetic and potential energies of the large masses and spiral springs, 

 attached to the top bar, is gradually spent in sending the diminishing 



^ series of waves down the line, and is ultimately converted into heat 

 in the treacle and water. You see that about half of the amplitude 



I of vibration, and therefore three-fourths of the energy, is lost in half 

 a minute. 



You will see on quickening the oscillation how very different the 

 result will be. The quick oscillations which I now give to the top 

 bar (the period having been reduced to about one and a half seconds), 

 j is incapable of sending waves along the line of molecules ; and it is 

 that rapid oscillation of the particles which, according to Stokes, con- 

 stitutes latent or stored-up light. 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 accordance with what we see, let me say scarcely any sensible loss). 

 This is a mechanical model correctly illustrating the dynamical 

 principle of Stokes's explanation 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 Kue. (Experi- 

 ment shown.) 



I . _ Now I will show you Stokes's 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 now 

 visible by being illuminated by invisible rays. The rays by which it 

 is illuminated even before it comes into the visible rays are manifestly 

 invisible so far as the screen receiving the spectrum is a test of 

 visibility ; because the uranium glass, and my hands holding it, throw 



l| no shadow on the screen. Also you see the uranium glass which I 

 hold in my hand in the ultra-violet light, while you do not see my 



I j hand. I now bring it nearer the place where you see the air (or 

 rather the dust in it) illuminated by the violet light : still no shadow 

 m the screen, but the uranium glass in my hand glowing more 

 Drilliantly with its green light of very mixed constitution, consistin«» 

 )f waves of longer periods than that of the ultra-violet, which the 

 iicident light, of shorter period than that of violet light, causes the 

 jarticles of the uranium glass to emit. This light is altogether un- 

 )olarised. It was the absolute want of polarisation, and the fact of 



p 2 



