Mr Kleeman, The Unstable Nature of the Ion in a Gas. 275 
of elementary ions and a current of clusters. But the elementary 
ions only produce further ions by collision. Now the quantity a 
in the ordinary theory of ionisation by collision is calculated on 
the supposition that the whole current consists of elementary ions. 
The quantity a when it refers to the partial current of elementary 
ions is however the important one, it will in this case be denoted 
by a. The a’s are connected by the equation C,a,=a(C, + C,), 
where C, denotes the current of elementary ions and C, that of 
the clusters. Since C, increases while C, decreases with increase 
of electric field, the value of a, differs most from that of « when 
ionisation by collision begins to come in, being always greater 
than a,, while for much greater fields they may be practically 
equal to one another. The curves connecting - and 3 would 
therefore be less concave towards the D axis near the origin than 
the curves connecting > and ; obtained by Townsend. We can- 
not obtain any information about the relative values of C, and C, 
from measurements of the ionisation current, and it does not seem 
possible to devise a simple experiment by means of which they 
could be directly measured. 
The velocity with which an electron is ejected from its parent 
atom has little influence on the ionisation by collision. It is 
practically only the first collision of the electron with a neutral 
molecule that would get the benefit of the initial velocity of the 
former, if the motion of the electron is in the same direction as 
that given to it by the electric field. On the average the electron 
undergoes hundreds of collisions before it reaches one of the 
electrodes, and the first collision is therefore comparatively of no 
importance. 
But the ionisation by collision should be considerably affected 
by the direction of motion of the ejected electrons relative to that 
given to them by the electric field. Consider the two cases when 
the electric field acts in the same direction as the ejected electron 
is moving, and when it acts in the opposite direction. In the 
latter case the electron must be reduced to rest before a velocity 
can be given to it sufficiently large to produce ionisation by 
collision. The chance of a cluster being formed is therefore much 
greater in the latter case than in the former. On a cluster being 
formed it must run a distance « depending on its period of life 
and velocity before it becomes an elementary ion again, after 
which it can be used by the electric field for the production of 
new ions by collision. Thus in the latter case the collision current 
is smaller than in the former. The effect may roughly be said to 
