320 Mr Kleeman, Dependence of Relative Lonisation, ete. 
varies as the square of the pressure does not violate the con-— 
clusions drawn by the writer from the experiments ou soft 
secondary y rays. It was found that the ionisation was pro- 
portional to the pressure over the range of pressures used. Any 
deviation from this at other pressures could not seriously affect 
the results, since the ionisation in a gas was compared with 
approximately the same mass of air and then reduced to standard 
pressure. Thus the scattering and absorption of the @ rays by 
the gas was approximately the same in the two cases. The 
principal reason for doing this at the time was that it rendered 
the ionisations in most cases nearly equal to one another. : 
I have carried out some experiments with the object of 
detecting the ionisation that is probably produced by the recoil 
atom or molecule when a8 ray is ejected. They depended on 
the principle that the ions produced by the recoil atom would 
probably shew initial recombination like those made by the 
a particle. The difficulty is to get rid of the ionisation by @ rays 
from extraneous sources. The experiments were not a. success, 
probably owing to this difficulty not having been overcome to 
a sufficient extent. A shallow cylindrical ionisation chamber was 
used whose walls consisted of tightly stretched tissue-paper over 
a skeleton chamber of thin aluminium wire. A mixed beam of 
8 and y rays from a quantity of radium was passed through 
the chamber. The current was measured for fields of different 
strengths when the beam was largely freed from 6 rays by means . 
of a strong magnetic field, and when it was not so modified. The 
current saturation curves obtained were the same in the two 
cases. It could be shewn from energy considerations that when 
the beam was free from @ rays the ionisation due to the recoil 
atom must have been less than 10°/, of the whole ionisation. From 
the outset, therefore, the experiment had not much chance of 
success. Another method is about to be tried. 
