Mr Wilson, On Radio-active Rain. 



429 



Glass windows in the front and back of the apparatus enabled 

 the position of the gold leaf to be read by a microscope of which 

 the eye-piece was provided with a micrometer scale. The time 



taken by the gold leaf to travel over an exact number of scale 

 divisions was observed, under normal conditions and when the 

 vessel in which the rain had been evaporated was placed inverted 

 over the ionisation apparatus as shown in the figure. With this 

 apparatus the movement of the gold leaf due to the spontaneous 

 ionisation was at the rate of about 11 divisions per hour. 



In the experiments made at Cambridge the apparatus used for 

 detecting radio-activity differed from that just described, mainly 

 in being inverted, the bottom being closed by a sheet of gold leaf. 

 The rain was evaporated in a platinum bowl, which was cooled 

 by floating on water and then placed below the ionisation appa- 

 ratus. The bowl was a shallow one, wider than the ionisation 

 apparatus, so that the surface on which the rain had been dried 

 could be brought close up to the gold leaf bottom of the detecting 

 apparatus. 



The general result of these experiments has been the same 

 at both places. In all cases in which a convenient quantity of 



