July i 9) 1883] 



NATURE 



277 



phenomena (sections 221 to 225 of his 1852 paper), the 

 following statement is given: — "But by far the most 

 " striking point of contrast between the two phenomena 

 " consists in the apparently instantaneous commencement 

 " and cessation of the illumination, in the case of internal 

 " dispersion when the active light is admitted and cut 

 " off. There is nothing to create the least suspicion of 

 " any appreciable duration in the effect. When internal 

 " dispersion is exhibited by means of an electric spark, it 

 " appears no less momentary than the illumination of a 

 " landscape by a flash of lightning. I have not attempted 

 " to determine whether any appreciable duration could 

 " be made out by means of a revolving mirror." The 

 investigation here suggested, has been actually made by 

 Edmund Becquerel, and the question — Is there any ap- 

 preciable duration in the glow of fluorescence ? — has been 

 answered affirmatively by this beautiful and simple little 

 machine before you, which he invented for the purpose. 



The experiment giving the answer is most interesting, and 

 I am sure you will see it with pleasure. It consists of a 

 flat circular box, with two holes facing one another in the 

 flat sides near the circumference ; inside are two disks, 

 carried by a rapidly revolving shaft, by which the holes 

 are alternately shut and opened ; one open when the 

 other is closed, and vice versd. 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 turn 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 



Fig. 11. — Diagram illustrating the number of molecules in a space of 1/10,000 of a centimetre square and 1/100,000,000 of a centimetre thick. 



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 cognisant, may 

 have been the tenth of a second. If the uranium 

 glass had continued to glow sensibly for the twen- 

 tieth 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 open 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 preced- 

 ing admission 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 difference in the glow. 



Thus it seems that fluorescence is essentially the same 

 as phosphorescence ; and we may expect that substances 

 will be found, continuously bridging over the difference of 

 quality between this uranium glass, which glows 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 1/10,000 of a centi- 

 metre side and 1/100,000,000 of a centimetre thick. 

 The side of the square you see in the diagram is a metre, 

 and represents 1/10,000 of a centimetre. The diagram 

 shows just 100 molecules, being 1/10,000 of the whole 

 number of particles (io 6 ) in the cube of 1/10,000 eenti- 



