im various Gases by B Rays on their Velocity. 315 
mentioned, the relative ionisations with and without a magnetic 
field were 1:97 and 1:82 respectively. The relation between 
velocity and number of @ rays from radium has been investigated 
by Pashen. It appears from this that in my experimental 
arrangement the velocities of the rays producing the larger 
part of the ionisation probably lay between 1x10" and 2°9 x10" cm. 
per second. It would of course have been preferable to obtain a 
spectrum of the rays by means of a magnetic field and investigate 
the ionisation produced at different parts. If any indications 
had been obtained that the relative ionisation depended on the 
velocity of the 8 ray, an elaborate experiment of this nature 
would have been carried out. It would, however, be desirable 
to carry out experiments on relative ionisation, using the cathode 
rays obtained in a discharge tube, or those ejected from metals 
by X rays, since their velocity is considerably less than the 
majority of the 8 rays from radium. An experiment of this nature 
is about to be carried out. 
The results obtained have an important bearing on the nature 
of the process of the ionisation in a chamber through which 
y rays are allowed to pass. The ionisation in question may be 
divided into three parts, viz. (a) the ionisation produced by the 
B rays given off the walls of the vessel, (b) that by the secondary 
y rays given off the walls, (c) and that due to the absorption of 
the primary y rays by the gas in the vessel. The ionisation 
under (c) consists of the ions produced by the @ rays ejected 
from the molecules of the gas under the action of the primary 
y rays, and possibly of direct ionisation by the primary y rays. 
According to some experiments of C. T. R. Wilson*, however, the 
latter part is comparatively small or non-existent. We shall return 
to this point later. It can be easily shewn that the ionisation 
under (¢) is small in comparison with that under (a). For the total 
absorption of the primary y rays by the gas is very much smaller 
than that by the walls of the vessel. And since the mass of 
matter per cm.” of the walls of the vessel through which a @ ray 
is able to penetrate corresponds to probably hundreds of times 
that of the mass per cm.? of the gas in the vessel, a much larger 
number of 8 rays will cross the chamber which originate in its 
walls than which originate in the gas. The ionisation under 
(b) has usually not been deemed worthy of much or any con- 
sideration by physicists who have worked with y rays. It is 
by no means of negligible magnitude. This will appear from 
a study of the ionisation by y rays of different hardness. The 
writer+ has shewn, using secondary y rays, that the ionisation in 
methy! iodide relative to that in air greatly increases with the 
* Proc. Roy. Society, A, vol. 87, p. 277 (1912). 
+ Ibid., vol. 82, p. 358 (1909). 
