[HENDERSON] DEPOSITS OF RADIUM, THORIUM AND ACTINIUM 153 
Even at this high potential gradient the percentage cathode activity 
seemed to be slowly increasing. The shape of the curve showing 
the relationship between the percentage cathode activity and the 
potential gradient was shown to be very similar to the saturation 
curve (ionization current-voltage curve), when the ionization was due 
to arays. The similarity of these two curves was also pointed out by 
Schmidt.7. This similarity would indicate that the rest-atoms 
are subject to recombination with negative ions, just as are positive 
ions. 
Continuing the investigation Wellisch® found that a limiting 
value was apparently reached for the percentage cathode activity 
with increasing potential gradients. He gave later® a corrected value 
of 88-2% for this limit, when the emanation was in dry air. In 
carbon dioxide and in hydrogen the corresponding limits were 78-9% 
and 88-2%. Wellisch also showed that no appreciable fraction of 
the rest-atoms were negatively charged, and that in ether vapor all 
the rest-atoms were uncharged. He further found the coefficient of 
diffusion of uncharged rest-atoms in dry air to be approximately 
the same as that of gaseous ions. 
Eckmann!? had previously obtained a value for the coefficient 
of diffusion of the rest-atoms in good agreement with that found by 
Wellisch. Eckmann, however, came to the conclusion that in air, 
about 98% of the rest-atoms are positively charged, and the remain- 
ing 2% negatively charged. This 2% was doubtless due to the diffu- 
sion of uncharged rest-atoms to the anode, since Wellisch has clearly 
shown that no negatively charged rest-atoms are present. Eckmann 
used a cylindrical testing vessel, but assumed that the total active 
deposit was the sum of the deposits obtained on the central electrode 
when it was first made cathode and then anode. He thus neglected 
the uncharged rest-atoms which diffused to the comparatively large 
cylindrical case. His value of 98% for the positively charged rest- 
atoms is therefore considerably too large. 
The behaviour of the active deposit of actinium was investigated 
by Walmsley." He came to the conclusion that all the actinium 
rest-atoms are positively charged. As his procedure was similar 
to that of Eckmann, he also failed to take account of the active deposit 
collected on the case. Hence his value of 100 for the percentage 
cathode activity is probably too high. 


7 Schmidt, Phys. Zeit., 9, p. 184, 1908. 
8 Wellisch, Am. Journ. Sci., Oct., 1913. 
9 Wellisch, Am. Journ. Sci., Oct., 1914. 
10 Eckmann, Jahr. der Radioakt, May, 1912. 
11 Walmsley, Phil. Mag., Sept., 1913. 
