336 Mr Wilson, On the Action of Uranium rays [Oct. 25, 



II. vt/vj = 1-260. 



Result. A dense shower was always seen when the expansion 

 was made with the air in A exposed to the rays ; only a few 

 drops appearing when the Uranium salt was removed. 



III. v 2 /v x = 1-273. 



Result. Slight shower without the rays, a very dense shower 

 under the action of the rays. 



IV. v,/v,= 1-286. 



Result. The same as in III. The showers were not appre- 

 ciably denser than in III. 



More exact measurements were now made of the minimum 

 expansion at which the drops first begin to appear under the 

 action of the rays. The experimental numbers are given in the 

 following table. 



Pressures are all given in millimetres of mercury. 

 Barometer = 769, 

 f C. = 15-5°, 

 it = vapour-pressure = 13, 

 P 1 = pressure of the air before expansion 

 = £-7r=769-13 = 756. 



Thus the minimum value of v. 2 fv 1 required to bring about 

 condensation under the influence of Uranium rays lies between 

 1*245 and 1'249. This agrees very closely with the value 1252, 

 found with very different apparatus for air under ordinary con- 

 ditions (Phil. Trans, loc. cit.). 



