Mr Wilson, Experiments on radio-activity from rain. 17 



Further experiments on radio-activity from rain. By C. T. R. 

 Wilson, M.A., F.R.S., Sidney Sussex College. 



[Read 27 October 1902.] 



In a paper read before this Society on May 5th, 1902, experi- 

 ments were described which showed that a vessel, in which freshly 

 fallen rain has been evaporated to dryness, shows radio-active pro- 

 perties lasting for a few hours only. 



Many samples of freshly fallen rain have since that date been 

 tested both here and at Peebles, and all have shown this effect. 

 The magnitude of the effect obtained from a given quantity of 

 rain has nearly always been of the same order, whether the rain 

 has consisted of large or small drops, and whether it has been 

 collected by day or by night, at the beginning of a shower or after 

 some hours of continuous heavy rainfall. Once however during a 

 thunderstorm an abnormally large effect was obtained. 



The radio-activity is obtained equally well, whether the rain is 

 boiled down in platinum or porcelain vessels. It is not destroyed 

 by heating the vessel to dull redness ; in this as in other points 

 it resembles the induced radio-activity obtained on negatively 

 charged wires. 



A radio-active precipitate may be obtained from freshly-fallen 

 rain by adding a little barium chloride and precipitating the 

 barium with sulphuric acid ; the barium sulphate is temporarily 

 radio-active. A more convenient method, suggested to me by 

 Dr A. Scott, is to add a small quantity of alum to the rain water 

 and then precipitate the aluminium as hydrate by means of 

 ammonia. The hydrate thus precipitated from a given volume of 

 rain gives even larger effects than would have been obtained by 

 evaporation of the rain, while the filtrate when boiled down to 

 dryness now shows no trace of radio-activity. From 190 c.c. of 

 rain a precipitate was in this way obtained sufficiently radio-active 

 to increase the ionisation within the testing vessel to about 

 100 times its normal value ; to enter the vessel the rays had to 

 penetrate aluminium about "00032 cm. in thickness. The intensity 

 of the radio-activity falls to about \ of its initial value in an hour, 

 like that obtained by evaporation. Similar precipitates formed in 

 tap-water, or in rain-water that has stood for 24 hours, are quite 

 inactive. 



VOL. XII. PT. I. 



