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Mr Wilson, On Radio-active Rain. 
On Radio-active Rain. By C. T. R. Wilson, M.A., F.R.S., 
Sidney Sussex College. 
[Read 5 May 1902.] 
As the experiments of Elster and Geitel ( Physik . Zeitschr. II. 
p. 590 and ill. p. 76 and p. 308) and of Rutherford and Allen 
{Physik. Zeitschr. 1st March 1902) have shown, a negatively 
charged body exposed in the atmosphere becomes radio-active, 
apparently indicating the presence of some radio-active substance 
in the atmosphere ; it occurred to me to test whether any of this 
radio-active substance is carried down in rain. 
For this purpose I have recently on several occasions boiled 
down freshly fallen rain to dryness and tested the residue for 
radio-activity. 
Rain collected both at Peebles and at Cambridge has been 
found to impart radio-activity to the vessel in which it has been 
evaporated. 
The radio-activity was detected by means of the increase in 
the ionisation of the air within a small vessel of which the top, 
or in other experiments the bottom, was of thin aluminium or 
of gold leaf, the other walls being of brass. The ionisation within 
this vessel was measured by the same method as was used in 
experiments on the spontaneous ionisation in air and other gases, 
and described in the Roy. Soc. Proc. Vol. LXVin. p. 151. 
In the Peebles experiments the vessel was cubical, the length 
of each edge being 5 cms., and the top was of thin aluminium 
•00032 cms. in thickness. The apparatus is shown in the figure. 
The brass rod passing through the vertical tube was insulated 
from it by a sulphur plug and was kept at constant potential. Fixed 
on its upper end by means of a small sulphur bead was a thin brass 
wire with a narrow clean-cut gold leaf attached. This brass wire 
and gold leaf formed a leaking system of very small capacity. It 
could be brought to the same potential as the supporting rod 
by means of a contact-maker, consisting of a piece of the balance 
spring of a watch, soldered to the supporting rod and bent at the 
top so that it might make contact with the brass wire of the 
leaking system without touching the sulphur. The contact-maker 
was worked from outside by a magnet. 
