448 Prof. W. H. Bragg on the Properties and 



5 X 10° per sec, i. e., a speed due to thousands o£ volts, and 

 are scattered in all directions from the surface on which the 

 rays fall. Neither the weak impulse applied to B by the 

 wave coming along the wire, DB, nor the peculiar form of 

 the surface, B, could have any sensible effect in the way of 

 guiding these fast-moving electrons into the cylinder, F. 

 Only slow-moving electrons or 8 rays could be guided by 

 such means. It is no doubt true that X rays do liberate a 

 certain number of 8 rays, but it is clear that the experiment 

 of Marx is quite consistent with the hypothesis that the 

 X rays are complex, and consist in part of sether pulses tra- 

 velling with the velocity of light, and producing 8 rays, and 

 in part of material particles, or pairs, travelling at a speed as 

 yet undetermined, and exciting high-speed cathode rays. It 

 would be reasonable to expect that a stream of pairs should 

 be accompanied by sether pulses which had their origin at the 

 time and place where the pairs broke away. 



It is possible that the example of the a particle shows that 

 a pair cannot possess a velocity greater than 10 9 , since at a 

 higher speed it would be stripped of an electron, and become 

 an u particle. J. J. Thomson has suggested that at this 

 critical speed the a particle becomes electrically neutralized 

 by the attachment of an electron. Presumably such a pair 

 would then go on as a 7 ray. No such consequence has been 

 observed ; and on the present hypothesis it would be better 

 to suppose that the a particle ends its career by being taken 

 up by an atom, as Rutherford has suggested. There is no 

 reason to suppose the y ray or X ray to possess any great 

 speed, so as to give it enough penetrating power. The latter 

 might depend rather on the limitation of the field of the 

 pair ; and a sufficient range for the velocity can be found 

 between the minimum speed of the a particle and the maxi- 

 mum speed necessary for penetration, which appears to be 

 about 10 8 for a charged particle, but may be less for one 

 without charge. A moderate speed would account for the 

 reflexion or scattering of the X ray, and would indeed be 

 necessary for this purpose. 



To sum up, it is clear that a stream of X rays contains 

 some sether pulses, but it is not easy to explain all the pro- 

 perties of X rays on the aether-pulse theory. The explanations 

 are easier if the rays are supposed to consist mainly of neutral 

 pairs ; and the existence of such pairs is not improbable 

 a priori. 



