82 



secondary electrons, but that each of the latter set free fewer 

 tertiaries, and so on, so that the ionisation would fall at a 

 far greater rate than the pressure as soon as the free path 

 of the electrons became comparable with the dimensions of 

 the chamber. And, again, «the /S rays differ only in speed 

 from cathode rays, which produce quantities of slow-speed 

 electrons, even where their own velocity is great. 



For these reasons I think it must be concluded that the 

 P particle (and any high-speed secondary) produces slow- 

 speed electrons along its path, in very much the same way 

 as the a particle does, though not in such great numbers. 

 The high-speed secondary rays, studied by McClelland, Allen, 

 and others, are but few in number compared to the slow- 

 speed electrons, though their greater energy puts them more 

 in evidence. McClelland concludes from his experiment that 

 the p rays do not produce any slow-speed electrons, when they 

 strike a metal surface, which are comparable in number with 

 the electrons displaced in the g'as through which they have 

 passed. This is quite consistent with what has been said 

 above. There must be a few, but the number to be ex- 

 pected is quite small, for the (3 electrons dive so deep into 

 the metal which they strike, and ionise so few of the mole- 

 cules through which they pass, that very few of the slow- 

 speed, highly-absorbable electrons can be discharged from 

 the surface of the plate. Even in the case of the a particle 

 these electrons are not readily observed ; in the case of the 

 /5 particle the difficulty must be much greater. 



As regards X-rays, we have no such accurate measure- 

 ments of the velocities of the electrons which are ejected 

 from the molecules of a gas traversed by the rays, as we 

 have in the case of the cathode rays, so far as I am aware. 

 But a very large amount of labour has been spent on the 

 investigation of the secondary radiation caused by the X-rays, 

 from which we may gather much indirect evidence on the 

 point. Perrin (Ann. Chim. Phys., xi., p. 496, 1897), has 

 shown that the rate of production of ions per cc. by rays 

 of given intensity is proportional to the pressure of the gas. 

 Again, we know from the investigations of Curie and Sagnac, 

 Townsend, and Barkla that metals struck by X-rays return 

 a secondary radiation, which, in the case of the low atomic 

 weights, may be considered to consist principally of scattered 

 primary radiation, and in the case of the hisfh atomic weights 

 to contain both X-rays more absorbable than the primary 

 and cathode rays. Dorn has shown that the latter have speeds 

 averaging about 5 x 10^ cm., so that they must produce con- 

 siderable ionisation, consisting of S rays, in the few milli- 

 metres of air close to the metal. The free path of electrons 



