428 



NA TURE 



[Sei'temiskr 3, 1896 



nf the tube (I will suppose the tube, as it is called, to be made 

 of German glass) it produces a greenish yellow fluorescence, or 

 phosphorescence of very brief duration. I need hardly say that 

 if you do not limit what comes from the negative electrode by 

 the screen with a hole in it, you get a broader beam which 

 atTects the glass wall over a larger sjiace. 



Now what is it that proceeds from the negative electrode 

 towards the glass, and, when it gets there, produces this phos- 

 phorescence? Is it light, or is it matter? 



One remarkable circumstance connected with this something 

 is, that you can deflect it in its course by a magnet. If you 

 present a magnet to a ray of light it does not deflect it at all ; 

 but this soiiict/iiiii; is easily deflected by a magnet, even by a 

 tolerably weak magnet. Mr. Crookes found that in addition to 

 that property, if this discharge of a something fell upon one 

 side of a very light fan, formed of thin, split mica, and delicately 

 mounted so as to enable it to spin readily, it sent it spinning 

 lound ; and he believed that the 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 diff^erence of opinion as to whether the matter 

 propelled is of the essence of the phenomenon, or whether it is 

 something 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 — something 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, Prof. 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 ihan 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 rnore 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. Riintgen. 



In (jermany. Prof. Lenard made a very remarkable series of 

 experiments in what the Germans call, and what we may call, the 

 kathodic rays, and which he believed to be actual rays, and not 

 streams of molecules sent from the kathode. 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 per- 

 fect 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 approach- 

 ing to a perfect vacuum. But it is a very remarkable thing, 

 though Lenard, I believe, was not the first to discover it, but 

 Hittorft', that these kathodic 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 

 kathodic rays at one side and something of the same kind at the 

 other. Lenard constructed an apparatus commencing with a 

 Crookes tube, in which there was very high, though not too high, 

 exhaustion, with a kathode which was either flat or cup-shaped at 

 one end, and opposite to that, in the part where the kathodic rays 

 would strike the glass if it were there, instead of glass it was 

 closed by a thin plate of aluminium foil, so thin that it would 

 support the atmospheric pressure although it was impervious to 

 air. But as a continu.ation of that tube he had another tube, 

 which was also capable of exhaustion. The two tubes had glass 

 tubes leading from them to the same air-pump. There was 

 communication with the air-pump and communication between 

 the two tubes, and you could exhaust them together, and the 

 pressure would be so far reduced that the aluminiiim plate was 

 strong enough to sustain the reduced pressure. They were both 

 exhausted together until a suitable exhaustion was produced for 

 the production of the kathodic rays in the first tube, and then 

 the connection between the two tubes was intercepted, and 

 the exhaustion of the second tube, which was kept con- 

 nected with the air-pump, was continued for several days, until, 

 as near as he could get it, there was nothing at all, in the way 

 of gas, left in it. What was the result ? In the first tube the 

 kathoilic r.ays were produced by the electric discharge. They fell 



NO. 1 40 1, VOL. 54] 



on the aluminium foil at the end, and then there was a continua- 

 tion of kathodic rays in the highly exhausted tube— the vacuum 

 tube I will call it — and these went on as if they had been rays of 

 light. They were deflected by the magnet just like the original 

 kathodic rays. 



Now at first sight that looks very much as if you had to deal 

 with actual rays, which passed through the aluminium foil, just 

 as rays of light would pass through a plate of glass. But I think 

 the real explanation of it is altogether diflerent. I believe it to 

 be of this nature. First I will use rather a gross illustration, in 

 order that you may the better apprehend the nature of the other 

 explanation that I am aliout to bring before you. Suppose that 

 I have a row of ivory balls in contact, such as billiard balls, and 

 that another similar ball strikes the first of these. The result is 

 that the last of the balls is sent oft", and the striking ball and the 

 intermediate balls remain approximately at rest. Now it is con- 

 ceivable that something analogous to that may take place as 

 regar<ls these so-called kathodic rays, supposing they are not rays 

 at all, but streams of molecules. It is conceivable thai the 

 molecules proceeding from the kathode or negative electrode of 

 the first tube, be they of residual gas, or aluminium, or platinum, 

 might fall upon the thin aluminium plate which forms a wall 

 between the two tubes, separating the one from the other, and 

 that that would give rise to molecular discharge in the second 

 space, although the actual moving molecules never pas.sed 

 through the wall. As I say that is a rough illustration — 

 rather a gross and material illustration — to enable you to 

 understand more clearly the view I have to bring before 

 you. 



I have said that the so-called kathodic rays are easily deflected 

 by a magnet. Now we know from other experiments that if a 

 body sufficiently charged with electricity is in rapid motion, and 

 that motion takes place in a magnetic field, the body tends to be 

 deflected. This looks, therefore, very much as if these kathodic 

 rays are actually streams of molecules, which being highly 

 charged electrically, and of almost inconceivable minuteness, 

 would be deflected by a slight magnetic force. Now, if these 

 highly-charged molecules come to strike on the aluminium wall 

 which separates the two tubes (which are end to end) from one 

 another, it may be that an electrical action goes on which 

 resembles very much what electrolysis is sujiposed to be 

 according to the views of Grotthuss. I shall not have lime to 

 enter into an explanation of that now, for it would lead me too 

 far from the subject ; but several present will no doubt under- 

 stand what I mean when I refer to the views of Grotthuss. 

 The molecules then impinge on the wall, and give rise to a 

 projection of molecules from the second side of the wall, but the 

 latter are not the same molecules which imjiinged on the first 

 side of it. Whether the molecules projected in the second tube 

 come from a very minute quantity of residual gas, or whether 

 they are derived from the aluminium wall itself, from which 

 they are torn, as it were, does not signify for my purpose. We 

 have here, you see, a conceivable mode of emitting these so- 

 called rays in this way, simulating the transmissiim of a ray of 

 light through a plate of glass, though it is no ray at all that we 

 are dealing with. I confess I think that that is the true view 

 of the action which takes place. But Lenard himself believed 

 that the kathodic rays were, as he said, processes in the ether. 

 By means of the first tube used alone, as was done in the first 

 instance, but closed with a ' ' window " of somewhat thicker 

 aluminium foil, so as to sustain the atmospheric pressure, he 

 was able to receive the kathodic rays which came from the 

 second surface of the aluminium foil in air, where he could 

 examine them at pleasure, using for their detection sometimes a 

 phosphorescent or fluorescent screen, sometimes a photographic 

 plate. He found that under these conditions they were 

 quickly deflected from their original direction and dispersed, 

 so that they could not be traced far, just like rays of 

 light in a turbid medium, such as water to which a little milk 

 has been added ; whereas in a subsequent series of experiments, 

 to which reference has already been made, in which the kathodic 

 rays were received into a second tube, the dispersion became 

 less and less .as the exhaustion proceeded, until at the highest 

 attainable approach to a perfect vacuum the dispersion almost 

 disappeared, and the rays were traced right onwards for a metre 

 and more, and that, without being enlarged by diffraction, as 

 would be the case with rays of light. 



Lenard mentioned incidentally that these kathodic rays, as 

 he sujiposed they were, were able to pass through the hand 

 even. He missed the discovery of the X rays because he had. 



