428 Mr Wilson, On Radio-active Rain. 



On Radio-active Rain. By C. T. R. Wilson, M.A., F.R.S., 



Sidney Sussex College. 



[Head 5 May 1902.] 



As the experiments of Elster and Geitel (Physik. Zeitschr. II. 

 p. 590 and in. 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. lxviii. 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. 



