im various Gases by 8 Rays on their Velocity. 319 
molecule. Besides, the recoil atom may also help to ionise the 
atoms of the molecule and to break it up, and also to ionise other 
molecules like an a particle. Now all the ions that are made from 
the molecules which give rise to the cathode rays appear as 
direct ionisation since it varies proportionally to the pressure. So 
does all the ionisation produced by the recoil atom, since its path 
would be extremely short in comparison with the diameter of 
chamber even at very low pressures. Thus the ionisation that 
varies directly as the pressure may not be of negligible magnitude 
in comparison with the whole ionisation, especially at low pressures. 
Bragg* has used a method to investigate the point under dis- 
cussion which is entirely different from that used by Crowther 
and Beatty. He concludes that all the ionisation is produced by 
the secondary cathode rays ejected by the X rays. His method 
would not shew as direct ionisation that produced from the 
parent molecules of the cathode particles. Thus a discrepancy 
between his and Crowther’s and Beatty’s experiments may be 
expected. 
Some experimenters have investigated the variation of the 
ionisation by y rays of a gas with its pressure over a great 
range of pressures. But this can furnish little if any reliable 
information about the exact nature of the process of ionisation. 
For the ionisation produced by the secondary 8 rays from the 
walls of the vessel, that of the molecules producing the secondary 
® rays, and the ionisation produced by the recoil atom, if any, 
varies directly as the pressure. The experiment does not isolate 
these different ionisations. The remainder of the ionisation varies 
as the square of the pressure. 
But this subdivision of the ionisation holds only if all the 
B rays end their life in the walls of the vessel. Strictly the 
ionisation produced by the secondary 8 rays from the gas which 
do not cross the chamber or are reflected by the walls of the 
vessel and end their life in the gas, varies approximately pro- 
portionally to the pressure over small ranges of pressure. It 
increases strictly somewhat more rapidly than proportionally to 
the pressure. The ionisation by the @ rays from the walls of the 
vessel which end their life in the gas of the vessel is approximately 
constant for small variations of the pressure, but strictly increases 
with the pressure. 
Thus one part of the ionisation produced by the soft y rays 
from the walls of the vessel varies as the square of the pressure of 
the gas, and the other part approximately directly as the pressure. 
The latter part would increase with increase of the softness of 
the rays and the pressure of the gas. That part of the ionisation 
* Phil. Mag., Sept. 1910, p. 406. 
