Natures of various Electric Radiations. 435 



In my own experiments on the a, rays (Phil. Mag., March 

 1907)., I have brought forward evidence to show that the 

 amount of ionization produced in an atom is proportional to 

 the volume of the atom approximately. Taking this in 

 conjunction with the rule that the ionization produced in a 

 gas is nearly proportional to the inverse of the speed, we 

 have the very simple, if approximate, law, that the ionization 

 produced by an a particle in any atom under any circum- 

 stances is inversely proportional to the time spent inside the 

 atom. This appears to point to the ionization as purely a 

 trigger effect. Not that the a particle spends no energy in 

 the atom ; it is clear it must do so, since its speed is gradually 

 reduced, but there is not a direct connexion between the 

 energy spent and the number of ions produced. Bat what- 

 ever energy the ionizing agent may spend, or in whatever 

 way it spends it, it seems likely that the issue of the S particle 

 is the result of some disruption in the atom, or sub-atom, 

 which is the same for all atoms and under all circumstances. 



If we turn our attention now to all secondary radiation 

 other than the 8 rays, it seems to >be, in general, a rough 

 reflexion or scattering of the primary. Allen has shown 

 that there is only a little less velocity in the secondary rays 

 than in the primary j3 rays, or in the tertiary than in the 

 secondary. McClelland has measured the total ionization 

 produced by the secondary as compared with the primary 

 ]3 radiation ; and since he used a small ionization-chamber 

 with which he explored the whole space traversed by the 

 secondary rays, which chamber the secondary rays would, as 

 a rule, completely cross if they entered it, it may be taken 

 that he really compared the number of ft particles in the 

 secondary beam with the number of those in the primary. 

 The numbers which he obtained varied from 15 per cent, to 

 50 per cent., according to the substance, which is the order of 

 things we should expect if the secondary were simply scat- 

 tered primary radiation. Again, the loss of velocity of the 

 cathode particles, which is found to occur on scattering 

 at a plate, presuming the secondary radiation to be scattered 

 primary, is just what we should expect. In the case of the 

 a rays no secondary radiation other than 8 rays has been 

 found ; but a small reflexion of canal rays has been observed, 

 e.g., by Fuchtbauer (PKys. Zeit. March 1, 1906). Barkla 

 has shown that the secondary radiation produced by X rays 

 consists in part of scattered primary radiation, especially 

 when the surface struck is of material whose atomic weight 

 is low. The only cases in which a secondary radiation appears 



