51 



mula from which the second and third properties in the above 

 summary may be derived. Even then his theory fails to ex- 

 plain the first and fourth properties. It seems to us to be 

 clear that the application of Planck's theory is not justified. 

 And, generally, we conclude that the energy of the secondary 

 ^ ray does not come from the atom. 



We now come to the second case. In this there is no sug- 

 gestion of trigger action ; the energy of the j8 radiation is sup- 

 posed to be entirely derived from that of the ether pulses. 

 As already mentioned, this theory has lately been maintained 

 by J. J. Thomson (Proc. Camb. Phil. Soc, vol. xiv,, pt. iv., p. 

 417). It is also discussed by N. R. Campbell ("Modern Elec- 

 trical Theory"). Since an ether pulse of the orthodox form 

 spreads its energy over wider and wider surfaces as it radiates 

 from its origin, and since the energy of the ejected secondary 

 particle is immensely greater than can be imparted to it 

 during the passage of the weak, and always weakening, pulse, 

 it becomes a necessity on this hypothesis to concentrate the 

 energy of the pulse along radial lines, having their centre at 

 the place where the primary cathode particle is suddenly 

 stopped or accelerated. Thomson speaks of ''bundles of 

 energy" occupying only a very small portion of the wave- 

 front, the rest of the front being blank. Of course this at 

 once suggests explanations of some of the difiiculties of the 

 y and X-rays, such as the ionization of only a few of the 

 atoms swept over by the wave, and the absence of relation be- 

 tween the velocity of the secondary electron, on the one hand, 

 and, on the other, the intensity of the radiation and the 

 nature of the atom. Also, it makes provision for a concentration 

 of momentum. This theory, however, postulates a very special 

 and complicated structure of the ether. And, in the second 

 place, it does not even then offer an explanation of all the 

 phenomena. 



If a ''bundle of energy" provides the energy with which 

 the secondary cathode particle leaves the atom, then the 

 energy-content of the bundle must be greater than the energy 

 of the particle. If, on the other hand, as in the case of the X. 

 rays, the energy of the bundle is derived from that of the 

 arrested cathode particle, the former must be less than the 

 latter. Now, it seems quite clear that the energy of the 

 secondary electron is at least nearly as great as that of the 

 primary cathode particle. For in the case of the X-rays the 

 velocity of the secondary electron is nearly 10^°, and is there- 

 fore much the same as that of the cathode rays in the bulb. 

 And we have shown above, in the analogous case of y rays, that 

 the velocity of the secondary ray, produced by the y ray, is 

 practically the same as that of the primary $ ray, which is- 

 sues with the y ray. We must, therefore, conclude on this 



