432 Prof. W. H. Bragg on the Properties and 



that the ft particle produces about 130 ions per cm. in air at 

 atmospheric pressure. Now, the ft particle runs a course in 

 the open air of.' an average length of 100 cm. This leads to 

 an estimate of its ionization even greater than that obtained 

 by Rutherford. If all the electrons, so liberated, had a high 

 velocity, the energy set free would be out of all proportion 

 to that of the original ft particle. Yet if we are to ascribe a 

 high velocity to the electrons set free, it must be a very 

 high one, for it has been shown by Allen (Phys. Review, 

 August 1906), that the secondary radiation of ft rays consists 

 of electrons moving with a speed approximating to that of 

 the primary. We cannot suppose that all these electrons 

 are of this high-speed type. Moreover, if this were the 

 •case, the free path of such electrons would become comparable 

 with the dimensions of the ionization-chamber, when the air 

 pressure was only moderately reduced, and the electrons 

 would then be beyond the control of the electric field. 

 Thus the ionization would not be proportional to the air 

 pressure, as was found by Durack and McClelland. The 

 difficulty as to the energy is not obviated by supposing each 

 primary ft particle to set free only a few secondary electrons 

 of high speed, each of these to become in turn the originator 

 of a few more, and so on. For if that were the case, a 

 reduction of gas pressure would imply, not only that each 

 primary electron set free fewer secondary electrons, but that 

 ^ach of the latter set free fewer tertiaries, and so on, so that 

 the ionization 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 ft 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 

 ft 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 ft 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 gas 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 

 -expected is quite small, for the ft electrons dive so deep into 



