CHAPTER XI. 
RADIO-ACTIVITY OF THE ATMOSPHERE AND OF 
ORDINARY MATERIALS. 
209. Radio-activity ofthe atmosphere. The experiments 
of Geitel! and C. T. R. Wilson? in 1900 had shown that a positively 
or negatively charged conductor placed inside a closed vessel gradu- 
ally lost its charge. This loss of charge was shown to be due to a 
small ionization of the air inside the vessel. In addition, Elster 
and Geitel had found that a charged body exposed in the open 
air lost its charge rapidly, and that the rate of discharge was 
dependent on the locality and on atmospheric conditions. A more 
detailed description and discussion of these results will be given 
later in section 218. 
In the course of these experiments Geitel had observed that 
the rate of discharge increased slightly for some time after the 
vessel had been closed. He considered that this might possibly 
be due to the existence of some radio-active substances in the air, 
which produced excited activity in the walls of the vessel and so 
increased the rate of dissipation of the charge. In 1901 Elster 
and Geitel® tried the bold experiment of seeing if 1t were possible 
to extract a radio-active substance from the air, The experiments 
of the writer had shown that the excited radio-activity from the 
thorium emanation could be concentrated on the negative electrode 
in a strong electric field. This result indicated that the carriers 
of the radio-activity had a positive charge of electricity. Elster 
and Geitel therefore tried an experiment to see if positively charged 
1 Phys. Zeit. 2, p. 116, 1900. 
2 Proc, Camb. Phil. Soc. 11, p. 32,1900. Proc. Roy. Soc. 68, p. 151, 1901. 
3 Phys. Zeit. 2, p. 590, 1901. 
