Mr Kleeman, The Unstable Nature of the Ion in a Gas. 273 
It is of interest to obtain a value for the average number of 
collisions a negative cluster undergoes before becoming a simple 
ion. From the kinetic theory of gases we find that the number 
of collisions an air molecule undergoes with other molecules at a 
temperature of 30° is 1:6 x 10" per second. A negative ion cluster 
in air at standard pressure therefore undergoes on the average a 
number of collisions of the order 10° per second before becoming 
a simple ion, and an elementary ion undergoes a number of 
collisions of the order 10° per second before it successfully forms 
a cluster. 
The above calculations cannot be carried out with success in 
the case of CO,; very likely y = is small in comparison with 7. 
Townsend’s experiments on diffusion suggest that 7 is probably 
smaller in CO, than in other gases. Better results might be 
obtained after making some improvements in the experimental 
arrangement. These would consist in having the distance be- 
tween gauze and plate as large as is convenient, and using as 
large a potential difference as possible, the pressure of the gas 
being regulated to suit these conditions. 
The numerical results obtained for air can only be approxi- 
mately correct because the calculations are subject to considerable 
errors, as small errors in the data may appear as large errors in 
the ultimate results. Moreover, we have seen that the period of 
life of a cluster and elementary ion depends greatly on the dryness 
of the air. Latty’s experiments suggest that the period of life of 
an elementary ion in air specially dried is much greater than that 
obtained by the writer. It appears also that under these circum- 
stances the values of « would be very different from those obtained 
by Townsend. The dryness of the air in Townsend’s and the 
author's experiments was however probably approximately the 
same, and as the velocity of an ion and other quantities are 
approximately independent of the dryness when it is not ex- 
ceptionally great, the use of Townsend’s values of « was not 
objectionable. Further the values of 7 and ¢, are probably also 
seriously affected by the electric field when it is so large that new 
ions are produced by collision. However, the results obtained 
give one some idea of the processes going on in an ionised gas, 
and the order of magnitude of the quantities involved. 
The conclusions of a general nature that can be drawn from 
the author’s experiments and those of other investigators are as 
follows. The period of life of an elementary negative ion or a 
eluster depends very much on the nature of the gas in which it 
is formed. It depends also very much on slight admixtures of a 
different gas. The period of life of a cluster in a gas at standard 
pressure may have a value lying between a few seconds and a 
