Mr Wilson, On Radio-activity from Snow. 85 



On Radio-activity from Snow. By C. T. R. Wilson, M.A., 

 F.R.S., Sidney Sussex College. 



[Read 16 February 1903.] 



An experiment of the same nature as those already made with 

 freshly fallen rain and described before this Society (Proceedings, 

 vol. xi. p. 428 and xn. p. 17, 1902) was made with freshly fallen 

 snow at Peebles on Jan. 10th. The snow was melted and 50 c.c. 

 of the water were evaporated to dryness in a porcelain basin. 

 This was then inverted over the thin aluminium roof of the 

 ionisation apparatus used as a detector of radio-activity (described 

 in the first of the above-mentioned papers). 



The rate of leak, measured 10 minutes after the drying was 

 complete, was increased from 0'19 division per minute, its normal 

 value, to 3*6 divisions per minute ; the ionisation inside the vessel 

 was thus increased by the radiation to about 18 times its normal 

 value. An exactly similar experiment, made with 50 c.c. of rain 

 on August 17, 1902, gave ten minutes after drying a leakage of 

 37 divisions per minute, the leakage due to " spontaneous " 

 ionisation being 01 7 per minute. In both cases the collecting 

 vessel was exposed for about three-quarters of an hour in the 

 open air to catch the rain or snow, and the evaporation was com- 

 pleted 20 minutes after the vessel was brought in. 



There is thus no indication of any difference in the intensity 

 of the radio-activity obtained from equal weights of snow and 

 rain. 



