276 Mr Kleeman, The Unstable Nature of the Ion in a Gas. 
correspond to a distance A between the electrodes in the former 
case and a distance A — in the latter. Since the collision current 
increases rapidly with the distance of separation of the electrodes 
and the value of a, the asymmetry in the leaks obtained by re- 
versing the field should also increase with these quantities. The 
writer* has carried out experiments on the ionisation by the a 
particle involving the foregoing principles, and found that the 
ejected electron has a component of motion in the same direction 
as the ionising @ particle. 
The case when the initial ionisation is produced on the surface 
of one of the electrodes in the field producing ionisation by collision 
requires consideration here. Since all the ions start out as simple 
ions, their condition near the surface is different from that some 
distance away, where there is equilibrium between the clusters 
formed and the simple ions. It is evident that when the breadth 
of the region in which equilibrium is produced between the 
clusters and simple ions is comparable with the distance between 
the electrodes, the value of the collision coefficient a should 
depend on the distance between the electrodes and the pressure 
of the gas. But a would obviously not vary abruptly with the 
distance of separation of the electrodes because the region in 
question has no well defined boundary. 
Now Campbell+ has carried out an investigation on the de- 
pendence of a on the distance of separation between the electrodes. 
The object of the investigation was to distinguish between two 
hypotheses as to the changes an ion undergoes when producing 
ionisation by collision, viz. (1) that the simple ion never forms a - 
cluster during collision, (2) that sometimes a cluster is formed 
which is not broken up subsequently. If the latter hypothesis is 
true a should depend on the distance between the electrodes and 
the pressure of the gas. Campbell concludes from his experiments 
that no clusters are formed. Interpreting these experiments from 
the point of view that the ion cluster is unstable, it follows that 
either the region in which equilibrium between the simple ions 
and clusters is produced is small, or, that the period of life of a 
cluster is small in comparison with that of a simple ion in an 
electric field of intensity sufficiently large to produce ionisation 
by collision. It will be easily seen that in the latter case the 
effect of the region under discussion is negligible for all distances 
of separation of the electrodes. The latter explanation is probably 
the correct one. 
It was found impossible to obtain any definite information 
about the period of life of an elementary positive ion or cluster. 
* Proc. Roy. Soc. A, vol. uxxx. p. 195, 1909, and Phil. Mag. p. 198, July, 
1912. 
+ Phil. Mag. p. 400, March, 1912. 
