INAUGURAL ADDRESS. 21 



in some other transit across an atom, and the Rontgeu ray become 

 once more a cathode particle, probably with less energy than at first, 

 since some loss of energy might well occur at each change. 



And, again, the neutral pair has the further properties of the 

 Rontgen ray in that it is incapable of regular i-eflection, refraction, or 

 polarisation of the ordinary kind. Yet, as I have pointed out else- 

 where, it can be made to exhibit the peculiar polarisation which 

 Barkla lias shown to be a property of the Rontgen rays. 



It is true that it can hardly be supposed to move with the velocity 

 of light, and that a famous experiment by Marx seems to have shown 

 that Rontgen rays and light rays do actually move with the same 

 speed. But I have pointed out that the quantitative conclusions of 

 Marx" experiment are incorrect ; and the whole experiment has 

 recently been seriously questioned by Franck and Pohl. Again, a very 

 small diffraction effect is said to have been found by Haga and Windt 

 as the result of a most difficult and delicate experiment. An effect of 

 this kind might be expected if the rays were sether pulses. But this 

 experiment has also been questioned by Walter and Pohl, who have 

 been unable to confirm it. 



It is often said that the X rays, like alpha and beta rays, cause 

 electrons to be shed by the atoms through which they pass; but, the 

 statement is not logically correct. The X rays disappear as they pass 

 tlnough matter whether sold, liquid, or gaseous, and cathode rays 

 appear in their place: the latter certainly cause electrons to be set 

 free. Unless it can be shown that the actual number liberated is more 

 than can be accounted for as the result of the action of the secondary 

 cathode rays, it is not right to assert that any of them are due to 

 tlie direct action of the X rays themselves. The question may be 

 settled by experiment : but I have not, so far, found it easy to obtain 

 a decisive result. All that can be asserted is that the niunber of 

 electrons .set free when Rontgen rays pass through a gas is so near 

 to the number which would be set free by the cathode rays formed by 

 the gas as the result of the passage of the Rontgen rays, that the 

 burden of proof rests with those who would say that any are due to 

 tlie Rontgen rays themselves. As a final result we have, therefore, a 

 fairly simple picture of the progress associated with the Rontgen ray 

 tube. The cathode particles impinge on the metal anti-cathode; some 

 oi' these pick up tlie positive necessary to neutralise them, and so 

 l>ecome X rays. In this form they cross the glass walls of the tube, 

 the air outside, and any otlier siibstances which they may meet. Tlie 

 stream is continually. weakened, since pairs are always dropping out 

 0*. it ; because they are broken up into the negative electron and its 



