

Condensation Nuclei produced in Gases by Eontgen Rays, &c. 127 



"On the Condensation Nuclei produced in Gases by the Action of 

 Eontgen Kays, Uranium Kays, Ultra-violet Light, and other 

 Agents." By C. T. R WILSON, M.A., Clerk-Maxwell Student 

 in the University of Cambridge. Communicated by Pro- 

 fessor J. J. THOMSON, F.K.S. Received October 29 Kead 

 November 24, 1898. 



(Abstract.) 



The experiments here described consist mainly in determinations of 

 the least degree of supersaturation necessary to cause water vapour to 

 condense on nuclei from various sources. 



As in former experiments* the supersaturation is brought about by 

 very sudden expansion of air or other gas originally saturated with 

 water vapour, the expansion required to produce a fog or shower of 

 drops being measured. The expansion is expressed in terms of vjv lf 

 the ratio of the final to the initial volume. 



The following classes of nuclei have been studied in this way : 



1. Nuclei produced by Kontgen rays. 



2. ,, ,, uranium rays. 



3. ,, ,, ultra-violet light. 



4. sunlight. 



5. ,, ,, metals in contact with the gas. 



6. the action of ultra-violet light on a negatively 



electrified zinc plate. 



7. ,, the discharge of electricity from a pointed 



platinum wire. 



In addition the behaviour of the nuclei in an electric field has been 

 studied, with the object of distinguishing between " ions " and nuclei 

 which carry no charge of electricity. 



1. The action of strong X-rays on the gas differs from that of weak 

 raysf merely in the number of nuclei produced, the supersaturation 

 required to cause water to condense on the nuclei remaining unaltered. 

 The value of v.Ji\ corresponding to this degree of supersaturation 

 (approximately fourfold) is equal to 1*25. 



2. Uranium compounds, whether inside the expansion apparatus in 

 immediate contact with the gas or contained in a glass bulb outside the 

 apparatus!, produce nuclei requiring the same degree of supersatura- 

 tion as those produced by X-rays. 



* ' Phil. Trans.,' A, vol. 189 (1897), p. 265 ; ' Camb. Phil. Soc. Proc.,' vol. 9- 

 (1*97), p. 333. 



t ' Phil. Trans.,' loc. cit. 



% 'Camb. Phil. .Soc. Proc.,' loc. cit. 



