430 
Mr Wilson , On Radio-active Rain. 
rain, snow or hail has fallen in the course of half an hour, and 
in which this has been immediately evaporated to dryness, quite 
a marked indication of radio-activity has been shown by the 
residue. From about 50 c.c. of rain-water a radio-active residue 
was in all these cases obtained strong enough, after traversing 
the aluminium or gold leaf, to increase the rate of leak to four or 
five times its normal value. 
In several of these experiments the rain-water was filtered ; 
the radio-activity is not due to the visible solid particles in the 
rain-water. 
The showers on which tests for radio-activity were made fell 
on the dates given below : — 
March 29. (Peebles) about 10 A.M. Fine but copious rain. 
April 3. (Peebles) 9.0 to 9.25 A.M. Rain and wet snow. 
April 22. (Cambridge) about 12.30 P.M. Rain. 
May 2. (Cambridge) 11.50 A.M. to 12.5. Rain. 
May 3. (Cambridge) noon. Heavy shower beginning as hail. 
In the Peebles observations the rain was collected in a large 
porcelain vessel (developing tray) placed in an open space in 
a garden ; in the Cambridge experiments, in a large glass funnel 
placed on the roof of the Cavendish Laboratory. 
As already stated, in all these cases considerable radio-activity 
was detected, and the variations in the nature of the precipita- 
tions did not seem to influence much the magnitude of the effect 
obtained. 
The radio-activity obtained by the evaporation of rain dis- 
appears in the course of a few hours, falling to less thao half its 
initial value in one hour. 
From the evaporation of distilled water or of tap-water or of 
rain-water which had stood for some hours no radio-activity was 
obtained. Nor did such water when exposed for some hours in 
the open air and then boiled down to dryness yield any radio- 
activity. 
