[HENDERSON] DEPOSITS OF RADIUM, THORIUM AND ACTINIUM 163 
and more complex the molecules of the gas, e.g., COs, the greater 
will be the percentage of rest-atoms which fall within the influences 
which determine the third part of the curve. This would explain 
why different apparent limiting values for the percentage cathode 
activity in different gases were found by Wellisch and others when 
they used potential gradients too low to detect the slope of the third 
part of the curve. 
This type of recombination closely resembles the initial recom- 
bination, first conceived by Bragg to account for the lack of satura- 
tion of the ionization current produced by @ rays. Accordingly the 
same term will be used to designate the type of recombination repre- 
sented by the third part of the curve. 
It is well known that the behaviour of radium rest-atoms in air 
under the action of an electric field is very similar to that of ions. 
The curve in Fig. 2 is made up of three parts, A, B and C and the 
explanation has been offered that these parts are due respectively to 
volume, columnar and initial recombination. It is well known that 
parts corresponding to A and B are found on the ionization current— 
voltage curve for air ionized by « rays. These have been previously 
shown to be due to volume and columnar recombination. It seems 
reasonable to suppose that initial recombination between ions also 
takes place. Attempts have been made by previous investigators, 
as Wheelock, to show that initial recombination exists but little 
evidence that such was the case could be obtained from their curves, 
although initial recombination between ions should make itself 
evident by the existence of a third part (corresponding to C) on the 
saturation curve for ionization due to « rays. That is the familiar 
“saturation” curve does not represent saturation at all; the supposedly 
flat portion of the curve having really a small slope. That such has 
not been found may be due to two causes. The effect may be obscured 
by ionization by collision, before potential gradients high enough to 
even partially prevent initial recombination have been reached. Also 
it is impossible to tell what is the true value of the saturation current 
and hence there is no way of telling how far the ionization current 
for any particular potential gradient falls below that value. 
THE BEHAVIOUR OF ACTINIUM. 
Some evidence as regards the behaviour of actinium can be ob- 
tained from the results of Kennedy,® obtained using parallel plates as 


5 Bragg, Phil. Mag., XI, p. 466, 1906. 
16 Wheelock, Am. Jour. Sci., Oct., 1910. 
