RONTGEN RAYS 321 



the primary cathode rays in the Rontgen tube has been pointed 

 out by Bragg. He inclines to the view that when the primary 

 cathode particle strikes the anticathode it picks up a positive 

 there, becomes neutral and constitutes an X ray : subsequently 

 it is stripped of the positive and ends up as a secondary cathode 

 ray ; thus the identity of the negative remains the same 

 throughout and its speed invariable or nearly so. The fact that 

 a hard X ray is generated by a fast cathode ray is of course in 

 keeping with all this. A similar argument holds in the case 

 of the 7 rays. 



The close relationship between the cathode and Rontgen 

 rays would provide an explanation of the mechanism of absorp- 

 tion ; for a Rontgen ray which had been deprived of its positive 

 and become a secondary cathode ray would be lost to measure- 

 ment as an X ray and be absorbed or scattered in the manner 

 of cathode rays. 



Experiment has not yet succeeded in isolating the positive 

 electron. Bragg's theory does not, however, postulate its free 

 existence, but only that it can be torn from its attachment in 

 some atom and carried away by a passing corpuscle, and again 

 that it can be left behind in some atom which the pair sub- 

 sequently traverses. 



Rontgen and 7 rays. — Before summing up the position, a 

 word as to Rontgen and 7 rays. So many points of resem- 

 blance exist between the two that their identity of nature can 

 scarcely be doubted. But it is remarkable that, while the 

 impact of cathode rays gives rise to X rays, experiment has not 

 yet been able to show that the impact of fi rays occasions the 

 generation of 7 rays (Stark, March 1908 ; Eve, August 1908). 

 Evidence as to the existence of polarisation in primary or 

 secondary 7 rays would be most valuable in establishing their 

 kinship to Rontgen rays. As matters stand, probably the most 

 noteworthy difference between X and 7 rays, other than that of 

 degree, is that the former arise where the cathode rays are 

 arrested, and the latter when the /3 rays originate. 



The present position. — The "discontinuous pulse " and " neutral 

 pair" theories attribute structures to the Rontgen rays which 

 are so similar in behaviour that present experiment cannot be 

 said to discriminate between them. Both theories account for 

 the high penetrating power, the absence of deviation in electric 

 and magnetic fields, and the lack of any refraction. The pre- 



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