202 
It is hardly surprising that initial recombination should 
be more effective in a complex gas than in air. For the 
molecule contains many atoms, each one of which is just as 
likely to lose an electron as if it were not associated with 
other atoms. Perhaps, therefore, the molecule as a whole 
loses two or three electrons, and its electric field is the more 
intense. Recombination of this kind must also be easier, tne 
shorter the free path. 
It will be clear from the foregoing that certain effects 
occur which are characteristic of a j^rocess of initial recombina- 
tion, a process which is a priori not improbable. The question 
now arises as to whether any other cause could j^roduce the 
same effects. 
When we consider the great increase of current in a 
complex gas which is caused by an increase in the electric 
force ajDplied, we cannot but ask whether any of it is due to 
the production of other ions by those actually made by the a 
particle. Could the electric force aid the freed electron to 
gather speed sufficient to ionise by collision 1 A process of 
this kind would be similar in its results to initial recombina- 
tion, in that it would be independent of strength of current 
and depth of ionisation chamber. It seems probable, how- 
ever, that its effectiveness would rather be increased than 
diminished by lowering the pressure : and also it would be 
difficult to account for the existence of a saturation value of 
the current. Nevertheless, it does not seem safe as yet to say 
that no such process occurs. Probably further light could be 
thrown on the subject by an investigation into the total num- 
ber of ions produced in different gases under varying condi- 
tions. Some initial experiments of this kind will be described 
presently. 
Rutherford has recently shown that the a particle of 
RaC has only lost 40% of its velocity when it ceases to ionise. 
If this fact is considered in conjunction with our investiga- 
tions into the form of the ionisation curves for gaseous mix- 
tures, it is at once clear that the a jDarticle stops ionising in 
every gas when its speed has fallen to precisely the same 
value- For, if not, the ionisation curve for a mixture would 
show a superposition of simple curves, of which effect there is 
no trace. This and other considerations seem to show, as we 
have already said ('Thilosophical Magazine," September, 
1905), that the a particle performs the same number of acts 
of ionisation in every gas. If, then, we find the total satu- 
ration current to be different in different gases, we must come 
to the conclusion that either the ions in the gases of higher 
conductivity produce others by the help of the electric f\eld, 
or that in the gases of lower conductivity some of the ions 
made by the a particle do not get free, even under conditions 
