» 
im various Gases by B Rays on their Velocity. 317 
emerged. Since they got no further increase in ionisation after 
a thickness of material giving the maximum 8 radiation had 
been reached, they concluded that the secondary y rays produce 
no appreciable ionisation. But it must be taken into account 
that since the energy of the secondary y radiation is_pro- 
bably smaller than that of the @ radiation, the absorbability of 
the secondary y rays must be of the same order of magnitude 
as that of the @ rays to produce an appreciable ionising effect. 
This secondary y radiation would thus reach a maximum for 
a thickness of material certainly not much greater than that 
giving the maximum 8 radiation. An increase in the ionisation 
with the thickness of material cannot therefore be expected, and 
Bragg and Madsen’s conclusions do not therefore definitely 
follow from these experimental results. It should be pointed 
out that the soft secondary yrays, whose coefficient of absorption 
is the same as that of the @ rays, are eliminated in measurements 
of the coefficient of absorption of secondary y rays. For the 
thickness of the wall of the ionisation chamber is usually such as 
to absorb all the @ rays falling upon it and consequently all the 
secondary. y rays of the same absorbability. Measurements by 
various observers have shewn that the secondary y rays as a 
whole are much more absorbable than the primary y rays. Thus if 
carbon is taken as the radiating and lead as the absorbing material 
the writer* has shewn that the coefficient of absorption for the 
secondary y rays is about 10 times the magnitude of that of 
the primary rays. Much softer rays are thus likely to exist 
which are eliminated in the way explained. However, taking all 
the evidence into account, there is no doubt that the ionisation 
produced by the secondary y rays from the walls of the ionisation 
chamber is less than that produced by the 8 rays when a gas 
containing atoms of low atomic weight such as air is in the 
chamber. Probably in that case it is of the order of 20 per cent. 
of the totai ionisation. The percentage probably increases with the 
softness of the primary y rays. When, however, a heavy gas like 
methyl iodide is in the chamber it may be very much greater, 
since the ionisation of heavy gases relative to air increases greatly 
with the softness of the rays; thus the ionisation of methyl iodide 
relative to that of air is about 6 for the primary y rays of radium 
calculated from absorption data, while for X rays it is about 80. 
Braggt has given a formula which expresses the ionisation in 
a chamber through which y rays are allowed to pass in terms of 
other quantities. This formula does not take into account the 
production of secondary y rays in the walls of the vessel. It is 
necessary to add two terms to the formula, one term expressing 
* Phil. Mag., May 1908, p. 652. 
+ Ibid., Sept. 1910, p. 402. 
