XI] AND OF ORDINARY MATERIALS 353 
to the excited activity produced by the emanations of radium and 
thorium. No investigators have contributed more to our know- 
ledge of the radio-activity and ionization of the atmosphere than 
Elster and Geitel. The experiments here described have been the 
starting-point of a series of researches by Elster and Geitel and 
others on the radio-active properties of the atmosphere which have 
led to a great extension of our knowledge of that important subject. 
Rutherford and Allan! determined the rate of decay of the 
excited activity produced on a negatively charged wire exposed in 
the open air. A wire about 15 metres long was exposed in the 
open air, and kept charged by an influence machine to a potential 
of about —10000 volts. An hour's exposure was sufficient to obtain 
a large amount of excited activity on the wire. The wire was 
then rapidly removed and wound on a framework which formed 
the central electrode in a large cylindrical metal vessel. The 
lonization current for a saturation voltage was measured by 
means of a sensitive Dolezalek electrometer. The current, which 
is a measure of the activity of the wire, was found to diminish 
according to an exponential law with the time, falling to half value 
in about 45 minutes. The rate of decay was independent of the 
material of the wire, of the time of exposure, and of the potential 
of the wire. 
An examination was also made of the nature of the rays emitted 
by the radio-active wire. For this purpose a lead wire was made 
radio-active in the manner described, and then rapidly wound into 
the form of a flat spiral. The penetrating power of the rays was 
tested in a vessel similar to that shown in Fig. 16. Most of the 
ionization was found to be due to some very easily absorbed rays, 
which were of a slightly more penetrating character than the a 
rays emitted from a wire made active by the radium or thorium 
emanations. The intensity of the rays was cut down to half value 
by about 0:001 cm. of aluminium. The photographic action ob- 
served by Elster and Geitel through 0°1 mm. of aluminium showed 
that some penetrating rays were also present. This was afterwards 
confirmed by Allan, using the electric method. These penetrating 
rays are probably similar in character to the 8 rays from the 
radio-elements, 
1 Phil. Mag. Dec. 1902. 
R. B.-A. 23 
