ELECTRIC RADIATIONS—BRAGG. 197% 
sure. If the chamber is first exhausted and air gradually admitted, 
it is found that the number of ions produced by the 6 rays is pro- 
portional to the pressure. The paths of the 8 rays will not be 
appreciably affected by the introduction of the air; and so the ex- 
perimental results are consistent with the simple hypothesis that 
the B particle (primary or secondary) makes slow-speed ions in 
proportion to the number of gas atoms traversed. Nor does any 
other hypothesis seem to be consistent with the facts. It can not 
be supposed that the bulk of the ionization which is caused in the 
ionization chamber consists of high-speed secondary rays, though, 
of course, these are originated when the primary rays strike the 
metal surface of the chamber, and to a small extent when they strike 
gas molecules. For if all the negative electrons set free by the B 
rays were of high velocity we should expect certain effects, as may 
be seen from the following considerations, and none of these effects 
have been observed. 
Rutherford has shown (“ Radioactivity,” 2d edition, p. 434) that 
the a particle of Ra makes about: 86,000 ions in air; that one B 
particle is emitted from Ra for every four a particles; and that the 
ionization due to £ particles is of the order of 1 per cent of that due 
to a particles in the case of Ra in equilibrium. Thus the @ particle 
of Ra produces some thousands of ions. This is also evident from 
the experiments of Durack (Phil. Mag., May, 1903), who has shown 
that the B particle produces about 130 ions per cm. in air at atmos- 
pheric pressure. Now, the @ 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 8 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 8 rays consists of 
electrons moving with a speed approximating to that of the primary. 
We can not 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 £ 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 
