274 Mr Kleeman, The Unstable Nature of the Ion in a Gas. 
small fraction of a second. The order of magnitude of the period 
of life of an elementary negative ion is about ;4, that of a 
cluster. 
The results obtained have an important bearing on the motion 
of a negative ion in a gas under the influence of an electric field. 
The average velocity V of the ion under unit electric field is 
obtained from an equation given at the beginning of the paper 
which may also be written 
y= Coot Care 
Co + Ca 
b) 
Z| 
where cq denotes the concentration of the clusters (considered in — 
a body) and v, the average velocity of a cluster, and ¢ and % these 
quantities corresponding to the elementary ions. The experiments 
of Latty on the velocity of the negative ion in air specially dried 
suggest that v, is of the order 100V. Since cy, we have seen, 
is of the order 
= , the order of magnitude of ¢,% is the same as 
that of cad, and the presence of free negative ions in a gas thus 
affects the average ionic velocity. Thus the various expressions 
for the velocity of an ion through a gas that have been obtained 
by different physicists on the supposition that the ion consists of 
a definite unchanging cluster of molecules cannot be regarded as 
representing what actually happens in the gas, but have value 
only as useful empirical formula. It is conceivable that the effect _ 
of the passage of the ion through cycles of clustering on its 
velocity might be the same as that of a cluster of average mass. 
It should also be noticed that these formulae usually rest on one 
or more additional assumptions, which are not likely to be 
realized in practice. One of them is that the motion of an ion 
due to the electric field is reduced to zero on each collision with 
a neutral molecule. 
The results obtained have also an interesting bearing on the 
interpretation of the results obtained on ionisation by collision im 
the usual (Townsend’s) experimental arrangement. We have seen 
that Be ps (=) , where ¢, (=) increases with an increase of the 
ve Pp Pp 
field and consequently decreases with an increase of the pressure. 
But it probably varies only appreciably with the field when its 
strength is comparable with that necessary to produce ionisation 
by collision. But even when ionisation by collision occurs 4, may 
not be zero, for sometimes the collision of an elementary ion with 
a molecule must be favourable for the formation of a cluster. But 
the life of such a cluster will of course be much smaller than 
when the field is weak, Thus the current when ionisation by 
collision takes place may be divided into two parts, a current 
