SIR G. G. STOKES, BART., ON THE RONTGEN RAYS. 17 



nature of that which we have here to do with is, that it is 

 a stream of molecules. Nobody, I suppose, denies that there 

 is matter propelled; but there has been a considerable 

 difference of opinion as to whether the matter propelled is 

 of the essence of the phenomenon, or whether it is some- 

 thing merely accidental. Mr. Crookes held that it was of 

 the essence of the phenomenon, and that we had here, really, 

 a stream of molecules, and I must say, for my own part, I 

 believe he was right. But some foreign men of science 

 hold that the projection of matter is altogether a secondary 

 phenomenon, and that what comes through this small hole 

 is really only a process which goes on in the ether — some- 

 thing so far of the nature of light, but yet differing from 

 ordinary light most markedly in the property of being 

 deflected by a magnet. To illustrate what I mean by saying 

 something secondary. Professor Wiedemann, who holds the 

 opinion that it is of the nature of light, or a process going on 

 in the ether, imagines that the projection of matter has no 

 more to do with the phenomenon than the path of a cannon 

 ball has to do with your hearing the sound of the cannon. 

 I think, myself, that it has a great deal more to do with it 

 than that. However, I will leave that matter for the present, 

 to pass on to some researches which led up to the remarkable 

 discovery by Dr. Rontgen. 



In Germany, Professor Lenard made a very remarkable 

 series of experiments in what the Germans call, and Avhat we 

 may call, the cathodic rays, and which he believed to be 

 actual rays, and not streams of molecules sent from the 

 ■cathode. In order to produce these rays, as I will call them, 

 you want a very high vacuum. If, however, you make your 

 vacuum too high and too nearly perfect, you cannot get the 

 ■electric discharge to pass through it. A perfect vacuum, 

 appears to ,be a non-conductor, and if you attempted to 

 make the electric discharge pass through it, it would go, by 

 preference, on the outside from one electrode to the other, 

 so that you cannot work directly with anything too nearly 

 approaching to a perfect vacuum. But it is a very remark- 

 able thing, though Lenard, I believe, was not the first to 

 discover it, but HittorfF, that these cathodic rays pass or 

 appear to pass through a plate of aluminium which is 

 perfectly impervious to light, or even to the ultra-violet 

 rays, which we know by their effects, though we do not 

 see them directly ; so that you may have these cathodic rays 

 <it one side and somethins: of the same kind at the other. 



