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NATURE 



\July 12, 1883 



Some not hitherto published experiments with polished 

 silver plates, which I made fifteen years ago, showed me 

 very startlingly, an electric influence from a quite infini- 

 tesimal whiff of iodine vapour. The effect on the con- 

 tact-electricity quality of the surface, seems to go on 

 continuously from the first lodgment, to all other tests quite 

 imperceptible, of a few atoms or molecules of the attack- 

 ing substance (oxygen, or iodine, or sulphur, or chlorine, 

 for example), and to go on increasing until some such 

 thickness as 1/30,000 or 1/40,000 of a centimetre is 

 reached by the film of oxide or iodide, or whatever it 

 may be that is formed. 



The subject is one that deserves much more of careful 

 experimental work and measurement than has hitherto 

 been devoted to it. I allude to it at present to point out 

 to you how it is that by this electric action, we are enabled 

 as it were to sound the depth, of the ocean of molecules 

 attracted to the metallic surface, by the vapour or gas 

 entering into combination with it. 



When we come to thicknesses of considerably less than 

 a wave-length, we find solid metals becoming transparent. 

 Through the kindness of Prof. Dewar, I am able to show 

 you some exceedingly thinfilms, of measured thicknesses of 

 platinum, gold, and silver, placed on glass plates. The 

 platinum is of 19 X 10 thickness, and is quite opaque ; 



Flint Glass 



Fig. 3. — Diagram of Huyghen's construction iV r wave front of refracted 

 light. Drawn for light passing from air to flint glass. 



but here is a gold film of about the same thickness, which 

 is transparent to the electric light, as you see, and trans- 

 mits the beautiful green colour,whichyouseeon the screen. 

 The thickness of this gold (1*9, or nearly 2) is just half the 

 wave-length of violet light in air. This transparent gold, 

 transmitting green light to the screen as you see, at the 

 same time reflects yellow light to the ceiling. Now I 

 will show you the silver. It is thinner, being only 

 i - 5 X io- ! of a centimetre thick, or 3/8 of the air-wave- 

 length of violet light. It is quite opaque to the electric 

 light, so far as our eyes allow us to judge, and reflects all 

 the light up to the ceiling. It is not wonderful that it 

 should be opaque ; we might wonder if it were other- 

 wise ; but there is an invisible ultra-violet light of a small 

 range of wave-lengths, including a zinc-line of air wave- 

 length 3'4 X 10- 5 , which this silver film transmits. For 

 that particular light Lhe silver film of 1 -5 X 10- 5 thickness 

 is transparent. The image which you now see on the 

 screen, is a magic lantern representation of the self- 

 photographed spectrum, of light that actually came 

 through that silver. You see the zinc-line very clear 

 across it near its middle. Here then we have gold and 

 silver transparent. The silver is opaque for all except 



that very definite light of wave-lengths from about 3x7 

 to 3-32. 



The different refrangibility of different colours, is a 

 result of observation, of vital importance in the question 

 of the size of atoms. You now see on the screen before 

 you a prismatic spectrum ; a well-known phenomenon 

 produced by the differences of the refractions of the dif- 

 ferent colours, in traversing the prism. The explanation 

 of it in the undulatory theory of light, has taxed the 

 powers of mathematicians to the utmost. Look first, 

 however, to what is easy, and made clear by that diagram 

 (Fig. 3) before you ; and you will easily understand that 

 refraction depends on difference of velocity of propagation 

 of light, in the two transparent mediums concerned. The 



Fig. 4. Fig. 5. 



Twelve particles in Wave-Length. 



angles in the diagram are approximately correct, for refrac- 

 tion at an interface between air or vacuum and flint glass ; 

 and you see that in this case, the velocity of propagation 

 is less in the denser medium. The more refractive 

 medium (not always the denser) of the two, has the less 

 velocity for light transmitted through it. The " refractive 

 index " of any transparent medium, is the ratio of the 

 velocity of propagation in the ether, to the velocity of 

 propagation in the transparent substance. 



Now, that the velocity of the propagation of light 

 should be different in different mediums, and should in 

 most cases be smaller in the denser than in the less dense 

 medium, is quite what we should, according to dynamical 

 principles, expect from any conceivable constitution of the 



