[HENDERSON] DEPOSITS OF RADIUM, THORIUM AND ACTINIUM 157 
circular brass plates, 7-00 cm. in diameter, supported in guard rings 
C and D by small lugs e fixed to A and D. The guard rings were of 
7-10 cm. internal and 10-5 cm. external diameter and were also of 
brass. The guard rings C and D were held parallel at about 3 cm. 
from one another by three glass rods f. The glass legs g supported 
the whole on the plate of an air pump, to which the lower plate was 
electrically connected. A bell jar of internal diameter 11-5 cm. 
fitted over the plates. To prevent diffusion of rest-atoms from 
above or below the plates cotton wadding was placed over C and under 
D to close up the small space between the rim of the guard ring and 
the bell jar. Connection to the upper plate was made by a wire 
passing through the top of the bell jar. In filling the bell jar with the 
required gas, the bell jar was exhausted to 1 or 2 mm. pressure and the 
gas was then allowed to pass in through a tube of Ca Cl anda tube of 
P: O;. The bell jar was again exhausted and the gas allowed to enter 
a second time. The radium preparation and two small dishes con- 
taining P.O; were always kept in the bell jar, being placed above A. 
In this condition the apparatus was allowed to stand from 12 to 24 
hours to allow sufficient emanation to collect. The rods f and the 
outside edges of the guard rings were coated with paraffin to prevent 
electrical discharge, but no insulating material was near the field 
between A and B. The active deposits on both plates were measured 
in an «@ ray electroscope under identical conditions.” - 
EXPERIMENTS WITH THORIUM. 
The active deposit of thorium, which had received the least 
attention, was first investigated by the writer.# Experiments were 
carried out not only in dry air but also in air mixed with vapors 
and in some pure gases and vapors. It was found possible in dry 
air to concentrate all the rest-atoms on the cathode. The behaviour 
of thorium is in this regard in striking contrast with that previously 
found in the case of radium by Wellisch and others. 
The effect of the water vapor in the air was to lower the fraction 
of the rest-atoms which was concentrated on the cathode in a given 
field. A similar result was obtained by Wellisch in the case of 
radium. Experiments with different relative amounts of air and 
water vapor seemed to indicate that in an atmosphere of water vapor 
alone all the rest-atoms of thorium would be uncharged. 
In ether vapor all the active deposit particles were found to be 
uncharged. An entirely similar result was found by Wellisch in the 

*For further particulars as to apparatus and manipulation, see a recent paper 
by the writer, Trans. N.S. Inst. Sci. vol. XIV., pt. 1, p. 123. 
4 Henderson, Trans. N.S., Inst. Sci., XIV pt. 1, p. 1. 
