May 26, 1898] 



NATURE 



89 



true mechanism of energy. It is, in fact, for the reason that 

 within the Crookes' radiant matter tube it is possible to deal, 

 not as in every-day life with aggregates of matter, but perhaps 

 individually with single molecules and single atoms floating 

 apart in space, that so much attention is at present being 

 ■devoted to this particular branch of physics. 



Every one is now acquainted with what has become the quite 

 ordinary phenomenon of the kathode rays. These excite 

 luminescence in the glass upon which they fall, and cast a sharp 

 shadow of any obstacle interposed in their path. When the 

 tube is suitably placed in a magnetic field the shadow rotates 

 and becomes at the same time smaller, the magnetic field having 

 thus the property of concentrating the rays, and at the same 

 time giving them a twist. This concentration of the kathode 

 rays by means of a magnetic field, which has been studied by 

 Birkeland and by Fleming, can be employed to show the 

 intensely heating effect and erosive properties of the rays. 

 Indeed, by suspending a tube over one pole of a straight electro- 

 magnet, and thus concentrating the rays to a point, it is possible 

 by moving the tube or the magnet to actually engrave on the 

 interior surface of the glass a figure of any desired form. 



The more ordinary method of producing a concentrated 

 kathode discharge is by employing as kathode a spherical 

 aluminium cup, from the concave side of which the rays are 

 given off normally to the surface. By employing two such cups, 

 connected to the two secondary terminals of an induction coil 

 supplied with alternating electric current, and giving at the 

 secondary terminals a pressure of about 20,000 volts, the intense 

 heating effect of the kathode rays can readily be shown by 

 allowing them to fall upon a small fragment of quicklime. In 

 this manner a brilliant and beautiful light is produced, and it is 

 not at all improbable that it may eventually be found possible 

 to obtain in this way, commercially and practically, high voltage 

 electric lamps of much higher efficiency than the ordinary 

 incandescent filament lamps, and possibly even rivalling arc 

 lamps. In both these latter it is necessary that the incandescent 

 substance should be a fairly good electrical conductor ; whereas 

 in the kathode ray lamp there is no such limitation, and con- 

 sequently there is a much wider range of available refractory 

 substances. It is also quite conceivable that in the future an 

 electric furnace of this nature may be found of service in some of 

 the more delicate of chemical investigations, where it is necessary 

 to obtain in isolated substances exceedingly high temperatures. 

 Indeed, already, Crookes and Moissan have employed this means 

 for turning into graphite the surface of diamonds. 



It is now becoming more and more generally believed that 

 Sir William Crookes' original theory as to the nature of these 

 kathode rays is correct. According to this theory they consist 

 of material particles of residual gas, which, being similarly 

 electrified by contact with the kathode, are violently repelled by 

 the latter. This has been the view held for a long time by most 

 English physicists, and the chief point of diflference now appears 

 to be whether these material particles are single atoms, single 

 molecules, or larger aggregations of matter. This theory is 

 supported by the erosive action of the rays, which are found 

 after a short time to bore straight and very minute holes right 

 through the block of quicklime in the kathode ray lamp. A 

 model, consisting of a gilded pith ball suspended between two 

 metal plates connected to a Wimshurst machine, may be used 

 to roughly illustrate what is supposed to occur. The ball 

 obtains an electrical charge from whichever plate it starts in 

 contact with, and is violently repelled into contact with the 

 other plate, and so on backwards and forwards. In a Crookes' 

 tube, however, the velocity of the negative stream is undoubtedly 

 much higher than that of the positive stream. This may be 

 connected with the fact that the positive discharge is much more 

 dispersive than the negative. Indeed, a tube while in action 

 appears to be filled almost entirely with positively electrified 

 atoms, while it is only behind the kathode and in the kathode 

 stream itself that any negatively electrified atoms are to be 

 found. It is, however, possible to show experimentally that 

 something, at any rate, producing the same effect as a positive 

 stream does exist at very high exhaustions. For this purpose a 

 radiometer tube, as shown in Fig. i, containing a small mill 

 wheel with mica vanes, similar to those employed by Crookes, 

 may be used. The wheel is mounted upon a sliding carrier, so 

 that it can be moved bodily either out into the centre of the 

 tube, when the kathode stream impinges directly upon the 

 vanes, or back into an annex, when the vanes are quite outside 

 the kathode line of fire. In the former position, as discovered 



NO. 1 49 1, VOL. 58] 



by Crookes, the wheel rotates with great rapidity in a direction 

 indicating an atomic stream from the kathode to the anode. In 

 the latter position, with sufficiently high exhaustion, the wheel 

 is found by the author always to rotate in an opposite direction, 

 indicating a returning stream of atoms from the anode to the 

 kathode, the anode stream passing outside of the kathode stream. 

 As suggested by Prof. G. F. Fitzgerald, some action of this 

 nature will perhaps explain the curious effects obtained by the 

 author, and already noticed in Nature for April 15, 1897, from 

 which it appears that both the convergent and divergent cones 

 of kathode rays in a focus tube are usually hollow, it seeming 

 likely that if the supply of atoms to the active kathode surface 

 is from all round the edge of the latter, the atoms may be all 

 shot off again from the kathode in the form of a hollow cone, 

 before they get further than a certain distance towards the 

 centre. 



Birkeland has shown that if a thin kathode stream, obtained 

 by passing the rays from a flat kathode disc through a narrow 

 slit in a piece of platinum serving as the anode, is deflected by a 

 suitable magnetic field, it is split up into bundles of rays ; and if 

 allowed to fall upon the glass walls of the tube, it gives fluor- 



escent bands of alternate brightness and darkness. The author 

 has been able to photograph these bands by simply binding a 

 strip of sensitive photographic film round that part of the bulb 

 upon which the bands are formed, and making a single discharge 

 by a single break of the contact breaker of the induction coil. 

 Further, by inserting between the glass and the photographic 

 film a piece of very thin black paper, so placed as to cover only 

 one half of the image, it is possible to obtain a photograph of 

 the bands, one halt of which is due to the visible fluorescent 

 luminosity of the glass, and the other half to the invisible 

 Rontgen rays. Photographs produced in this manner show that 

 the Rontgen rays are also under these conditions given off in 

 bands, which are co-terminous with the fluorescent bands, though 

 photographically fainter than the latter. It is important to note 

 that in the Rontgen ray photograph the greatest effect is always 

 produced by the least deflected of the kathode-ray streams ; that 

 IS to say, by that stream which is presumably travelling at the 

 greatest velocity. Here we have a probable explanation of the 

 existence of the bands, which are most likely due to the atoms 

 of the kathode rays having from the first different velocities im- 

 parted to them, due to the oscillatory character of the induction 

 coil discharge, and from their gathering into groups travelling at 



