1883.] on the Size of Atoms. 211 



nately shut and opened ; one open when the other is closed, and vice 

 versa. A little piece of uranium glass is fixed inside the box between 

 the two holes, and a beam of light from the electric lamp falls upon 

 one of the holes. You look at the other. 



Now when I tui-u the shaft slowly you see nothing. At this instant 

 the light falls on the uranium glass through the open hole far from 

 you, but you see nothing, because the hole next you is shut. Now the 

 hole next you is open, but you see nothing, because the hole next the 

 light is shut, and the uranium glass shows no perceptible after-glow as 

 arising from its previous illumination. This agrees exactly with what 

 you saw when I held the large slab of uranium glass in the ultra-violet 

 light of the prismatic spectrum. As long as I held the uranium glass 

 there you saw it glowing ; the moment I took it out of the invisible 

 light it ceased to glow. The " moment " of which we were then cog- 

 nisant may have been the tenth of a second. If the uranium glass had 

 continued to glow sensibly for the twentieth or the fiftieth of a second, 

 it would have seemed to our slow-going sense of vision to cease the 

 moment it was taken out. Now I turn the wheel at such a rate that 

 the hole next you is oj^en about a fiftieth of a second after the uranium 

 glass was bathed in light ; still you see nothing. I turn it faster and 

 faster, and it now begins to glow, when the hole next you is open about 

 the two-hundredth of a second after the immediately preceding admis- 

 sion of light by the other hole. I turn it faster and faster, and it 

 glows more and more brightly, till now it is glowing like a red coal ; 

 further augmentation of the speed shows, as you see, but little 

 diti:erence in the glow. 



Thus it seems that fluorescence is essentially the same as phos- 

 phorescence ; and we may expect that substances will be found 

 continuously bridgiug over the difference of quality between this 

 uranium glass, which glow^s only for a few thousandths of a second, 

 and the luminous sulphides which glow for hours or days or weeks 

 after the cessation of the exciting light. 



The most decisive and discriminating method of estimating the 

 size of atoms I have left until my allotted hour is gone — that founded 

 on the kinetic theory of gases. Here is a diagram (Fig. 11) of a 

 crowd of atoms or molecules showing, on a scale of 1,000,000 to 1, all 

 the molecules of air, of which the centres may at any instant be in a 

 space of a square of 1-10, 000th of a centimetre side and 1-1 00,000,000th 

 of a centimetre thick. The side of the square you see in the diagram 

 is a metre, and represents l-10,000th of a centimetre. The diagram 

 shows just 100 molecules, being 1- 10,000th of the whole number of 

 particles (lO*") in the cube of l-10,000th centimetre, or all the mole- 

 cules in a slice of l-10,000th of the thickness of that cube. Think of 

 a cube filled with particles, like these glass balls,* scattered at random 



* The piece of apparatus now exhibited, illustrated the collisioiiri taking place 

 between the molecules of gaseous matter and the diffusion of one gas into 

 another. It cunsijited of a board of about one metre square, perforated with 



