316 Mr Kleeman, On the Dependence of the Relatwe Ionisation 
softness of the rays. This fact has recently been used by | 
Rutherford to isolate the different groups of the y rays of radium. ~ 
Now according to the results obtained in this paper this could | 
not be due to differences in the nature of the @ radiation given off — 
the walls of the vessel. It could not have been due to some of the 
soft 8 radiation from the walls being totally absorbed by the gas, 
since the total ionisation depends little on the nature of the gas*. 
Therefore since the ionisation under (c) is small in comparison 
with that under (a), this increase must be due to an increase of — 
the ionisation under (b). 
It appears from the experiments by C. T. R. Wilson mentioned 
that ® rays produce secondary 8 rays of small velocity only, 
ie. S rays. Bragg+ and the writer} have shewn that the velocity 
of the secondary £ rays produced by y rays decreases with the 
absorbability of the latter. It follows from this that if the 6 and 
y rays of radium are allowed to fall upon a plate of material, 
and soft y radiation is produced in it, soft 8 radiation should 
emerge from the surface of the plate due to this soft y radiation. 
The writer§ has shewn the existence of this soft 8 radiation, some 
of which is totally absorbed in about 2 cm. of air at atmospheric 
pressure. The energy of the soft* y radiation producing the 
ionisation under (b) is probably much less than that of the primary 
y rays. But since it is much more absorbable the ionisation it 
produces could easily be greater. It is to be expected then that 
if a narrow beam of vy rays is sent through an ionisation chamber _ 
and the 8 rays curled up by means of a magnetic field, an experi- 
ment the writer|| has carried out, there should still be con- 
siderable ionisation left in the chamber. Bragg has objected to 
this experiment, because it is difficult to curl up all the 6 rays 
to the same extent owing to their cutting the lines of magnetic 
force at different angles. Undoubtedly this objection might be 
raised, but still I think that the diminution with increase of 
magnetic field should have been greater under the particular 
circumstances than that obtained. Bragg and Madsen{ have 
carried out some experiments which they thought incidentally 
shewed that the ionisation produced by secondary y rays in the 
vicinity of material on which primary y rays are allowed to fall is 
small in comparison with that produced by the secondary 8 rays. 
They placed plates of material of different thicknesses in in- 
creasing order of magnitude across a beam of y rays and measured 
the ionisation produced on the side of the plate where the y rays 
* Proc. Roy. Society, A, vol. 84, p. 17 (1910). 
+ Phil. Mag., Dec. 1908. 
+ Proc. Roy. Society, A, vol. 82, p. 128 (1909). 
§ loco. cit. 
|| Proc. Camb. Phil. Society, vol. xv. Pt 11. p. 169 (1909). 
4] Phil. Mag., Dec. 1908, p. 932. 
