322 SCIENCE PROGRESS 



ponderance of secondary electrons in the direction of propagation 

 of the rays would be expected on either view. Polarisation is 

 a natural consequence of the pulse theory, but Prof. Bragg has 

 shown that it is not impossible to account for it on the neutral 

 pair theory. Prof. Thomson's units or bundles of energy 

 possess momentum, so that they have all the properties of 

 material particles, except they cannot move at any other speed 

 than that of light. Indication of diffraction would be looked for 

 by the supporters of the pulse theory. Thus reliable experi- 

 mental evidence (at present wanting) of velocity and diffraction 

 would be especially valuable. 



The resemblance between the electrical behaviour of ultra- 

 violet light and that of Rontgen rays is insisted upon by 

 Prof. Thomson in advocating the discontinuous pulse and wave 

 front theory ; but Prof. Bragg finds it hard to believe that a 

 diminution of the width of the pulse, no matter how extreme, 

 can increase the energy of the secondary electron by anything 

 from twenty thousand to about two million times. He regards 

 the action of Rontgen and 7 rays as something entirely different 

 from that of ultra-violet light. 



As regards the radioactive or explosion theory, we have seen 

 that it is unable to satisfy all the results of experiment. We 

 may, however, note that Wien (1907), from whom this theory has 

 received active support, has, by the application of a theory of 

 Planck's, derived a result which states that the velocity of the 

 ejected secondary electron varies inversely as the square root of 

 the thickness of the incident pulse. This removes one of the 

 failures of the theory — that the speed of the secondary cathode 

 ray depends on the hardness of the X ray — but the other 

 difficulties remain as great as ever. 



It is not easy to establish argument for or against either 

 " pulse " or " pair " theory from the standpoint of the relative 

 energies of the primary cathode ray, the Rontgen ray, and the 

 secondary cathode ray ; for if in any transformation the energies 

 do not balance, the difficulty may be avoided by invoking the 

 aid of the internal energy of the atom concerned. 



Having regard to the heat developed at an anticathode, it 

 does not seem credible that the whole of the energy of a cathode 

 ray can be handed over either to a pair or a bundle ; that this 

 latter, encountering some molecule, can, independently of the 

 heat liberated there also, send out a secondary ray with almost 



