
356 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 
electroscope four or five times, after the rays had traversed a thin 
layer of aluminium or gold leaf. The activity disappeared in the 
course of a few hours, falling to half value in about 30 minutes. 
Rain water, which had stood for some hours, showed no trace of 
activity. Tap water, when evaporated, left no active residue. 
The amounts of activity obtained from a given quantity of rain 
water were all of the same order of magnitude, whether the rain 
was precipitated in fine or in large drops, by night or by day, or 
whether the rain was tested at the beginning or at the end of a 
heavy rainfall lasting several hours. 
The activity obtained from rain is not destroyed by heating 
the platinum vessel to a red heat. In this and other respects it 
resembles the excited activity obtained on negatively charged 
wires exposed in the open air. 
C. T. R. Wilson? obtained a radio-active precipitate from rain 
water by adding a little barium chloride and precipitating the 
barium with sulphuric acid. An active precipitate was also 
obtained if alum was added to the water, and the aluminium 
precipitated by ammonia. The precipitates obtained in this way 
showed a large activity. The filtrate when boiled down was quite 
inactive, showing that the active matter had been completely 
removed by precipitation. The production of active precipitates 
from rain water is quite analogous to the production of active 
precipitates from a solution containing the emanation X of thorium 
(see section 178). 
The radio-activity of freshly fallen snow was independently ob- _ 
served by C. T. R. Wilson? in England, and Allan* and McLennan‘ 
in Canada. In order to obtain a large amount of activity, the 
surface layer of snow was removed, and evaporated to dryness 
in a metal vessel. An active residue was obtained with radio- 
active properties similar to those observed for freshly fallen rain. 
Both Wilson and Allan found that the activity of rain and snow 
decayed at about the same rate, the activity falling to half value 
in about 30 minutes. McLennan states that he found a smaller 
amount of radio-activity im the air after a prolonged fall of snow. 
1 Proc. Roy. Suc. Vol. 12, 1902. 
2 Proc. Camb. Phil. Soc. 12, p. 85, 1903. 
3 Phys. Rev. 16, p. 106, 1903. 4 Phys. Rev. 16, p. 184, 1903. 
