KAYS. 



691 



FIG. 1. SHADOW CAST BY CA- 

 THODE RAY-;. 



the pressure reaches one millionth of an atmosphere 

 no proper discharge takes place any longer. The 

 purple glow around the negative tcrmin. 

 as the exhaustion ] iving a dark space. 



which finally fills the whole tube. The residual ga- 

 in the tube now has curious properties, and \va- 

 considered by Crookes to lie in what he called "the 

 fourth state of matter." Rays, now generally known 

 athode rays." arise from the cathode, and pro- 

 ceed from it in straight lines, causing phosphores- 

 cence in the glass where they strike, and on any 

 phosphorescent substances that may be placed in 

 their path. These rays, which are capable of reflec- 

 tion, exert mechanical effects on objects that they 

 encounter, and can thus be made to drive smal'l 

 mills, etc., that are introduced into the tube before 

 its exhaustion. These phenomena were explained 

 by their discoverer as due to streams of electrified 

 us molecules, and he has generally been fol- 

 lowed by English and American physicists. The 

 Germans, on the contrary, who claim Hittorf as the 

 discoverer of the rays, have been inclined to see in 

 them a phenomenon of the luminiferous ether. 

 Crookes concluded that with increasing exhaustion 

 of the tube the " mean free path " of the molecules 

 that is. the average distance that they move between 

 collisions with other molecules increases until it 

 becomes so great that 

 the molecules can 

 move without hin- 

 drance in straight 

 lines from the cathode 

 to the sides of the 

 tube ; and that in the 

 "dark space." which 

 with decreasing pres- 

 sure expands to fill the 

 whole tube, the molecules do not bombard each 

 other, and hence do not produce the effect of light. 

 His experiments, all of which supported this theory, 

 showed that the cathode rays are rectilinear, ra- 

 diate normally from the surface of the cathode, cast 

 shadows, or their analogues (Fig. D. when ob- 

 jects interposed in their path interfere with the 

 phosphorescent action on the opposite glass, exert 

 mechanical force as noted above, mutually repel 

 each other when rays from two cathodes are brought 

 together, and are deflected by magnets (Fig. 2). In 

 support of the contention that the rays are merely a 

 form of ultra-violet light, Hertz found in 18!)2 that 

 they pass through thin metal plates within the dis- 

 charge tube, but support was given to the other side 

 by the experiments of Perrin. who found that the 

 cathode rays are charged with negative electricity, 

 corresponding positive charges being propagated 

 in the reverse direction and precipitated on the 

 cathode. J. J. Thomson, too, measured the ve- 

 locity of the 



ray-, and found _ -j- 



that it corre- <gB^MBHHHBHBB^ 

 sponded more i%^ 



closely to that ^ffiBBy _ 

 of a stream of _ "sS^ 



minute electri- 

 fied bodies than 

 to that of light. 

 Up to this 

 time the Eng- 

 lish school 

 seemed to have 



the best of the argument, but in 1894 Lenard, 

 acting on a hint from Hertz, found that the rays 

 would apparently pass through an aluminium disk in 

 the side of the tube, into the open air (Fig. 3). The 

 properties of the rays outside the tube were practically 

 the same as within it, but they vanished at about 5 

 centimetres from the tube. Within this distance 



FIG. 2. DEFLECTION OF CATHODE RAYS 

 BY A MAGNET. 



FIG. 3. LONARD'S FXPERIMFNT : PAS- 

 SAGE OF CATHODE RAYS THROUGH AN 



ALfMINTM WINDOW. 



they produced phosphorescence, conld pass through 

 metals, cast "shadows" with gla.-s and mica, and 

 exerted photographic action. They appeared 

 scattered by the aluminium " window ' in all direc- 

 tions. The an- 

 swer of the 

 English physic- 

 ists to this dem- _*; -V, 

 onstration was 

 that the rays 

 observed out- 

 side the tube 

 were probably 

 not transmitted 

 from within, 

 but were a 

 new phenome- 

 non, generated in open air at the outside surface 

 of the "window." The rays certainly had differ- 

 ent properties without and within the tube, but 

 that there was no fresh generation of rays was 

 shown, as claimed by Lenard, by using a tube with 

 two compartments, in one of which the rays are 

 generated, while they are observed in the "other. 

 The rays passed through the metal partition into 

 the observing tube when the vacuum in the latter 

 was too high to admit of their generation. Lenard 

 also showed that there seemed to be several kinds 

 of cathode rays, having different properties. For 

 instance, rays generated at high pressures are dif- 

 fused more easily than those that arise at low pres- 

 sures. This is analogous to the fact that light of 

 short wave-length is diffused more easily in certain 

 turbid media. He also proved that charged bodies 

 are discharged by cathode rays, as they are by ultra- 

 violet light. These experiments, whi'le confirming 

 the German physicists in their opinion that the rays 

 are propagated by a disturbance in the ether, and 

 are similar to light, if not identical with it, have 

 not converted the English physicists, and the two 

 opposed opinions are still held'by the rival parties, 

 and have had a marked effect oil the interest that 

 is taken in experimentation on this subject and the 

 closely related one of Rontgen rays. 



In January, ls'16. Prof. Wilhelm Conrad Ront- 

 gen (see ROXTGEN), of the University of Wiirzburg, 

 Austria, announced at a meeting of 'the local scien- 

 tific society that he had discovered a new form of 

 radiation. The discovery was accidental, and was 

 made by observing that" a highly fluorescent sub- 

 stance with which he was experimenting gave out 

 light whenever a neighboring Crookes tube was ex- 

 cited, though this tube was covered with an opaque 

 cloth. The effect was not due to cathode rays, be- 

 cause cathode rays can not pass through the' glass 

 walls of the tube, but the rays that" caused the 

 fluorescence appeared to take their rise at the point 

 where the cathode rays impinged against these 

 walls. Further investigation showed that the new- 

 rays, like cathode rays, passed easily through many 

 substances that are opaque to light, and appeared 

 to be capable neither of reflection nor of refraction, 

 but that they could not be deflected by a magnet. 

 and that they passed easily through the atmosphere 

 at the ordinary pressure. This similarity to the 

 cathode rays in action on a sensitive plate and in 

 transmission through opaque substances made it 

 possible to repeat on a large scale in open air pho- 

 tographic experiments that could before have been 

 le only within the exhausted tube. In par- 

 ticular it was found that when the human hand 

 was interposed between the tube and a photo- 

 graphic plate the difference in opacity of the flesh 

 and bones to the new rays caused a marked shadow 

 picture of the skeleton to appear on the plate. 

 Nothing but a shadow picture was possible, owing 

 to the fact that the rays are not reflected ; and the 



