Percentage Cathode Activity. 
90 
85 
80 
75 
70 
56 
[HENDERSON] DEPOSITS OF RADIUM, THORIUM AND ACTINIUM 161 
with those of thorium rest-atoms and is also in accord with the ideas 
advanced in the latter portion of this paper. 
DISCUSSION AND EXPLANATION OF CURVES. 
The results for dry air, as given in Table I, are shown graphically 
in Fig. 2. If this curve be examined, it can readily be divided into 

2000 400 600 800 1000 1200 1400 1600 1800 2000 
Potential Gradient. Volts per cm. 
Figure 2. 
three parts, A. B, and C, each of which has a distinctly separate 
slope. The part Ais very steep. It represents that condition in which 
the potential gradient is not sufficient to prevent the rest-atoms from 
combining with negative ions in the body of the gas. If the field 
were entirely absent, this recombination would go on till all the rest- 
atoms were neutral. The rest-atoms would diffuse equally to both 
plates and the curve would cut the axis of ordinates at 50%. The 
application of a weak field easily prevents this volume recombination 
and, as is evident from the curve, at a potential gradient of about 
75 volts per cm. the volume recombination has been practically 
prevented. It might be remarked that the transition from the part 
A to B is more abrupt when the experiments are carried out with 
parallel plate vessels than when carried out with cylindrical vessels, 
