[ 403 ] 
IX. On the Condensation Nuclei jyroduced in Gases hy the Action of Rontgen Rays, 
Uranium Rctys, Ultra-violet Light, amd other Agents. 
By C. T. R. Wilson, M.A., Clerh-Maxivell Student in the University of Cambridge. 
Communicated hy lUofessor J. J. Thomson, F.R.S. 
Received October 29,—Read November 24, 1898. 
Contents. 
Page 
1. Introduction. 403 
2. Expansion apparatus. 405 
3. Nuclei produced by Rontgen ray.s. 408 
4. ,, ,, ,, uranium ,, 410 
5. ,, ,, ,, ultra-violet light. 412 
6. ,, „ „ sunlight . 428 
7. ,, ,, ,, metals in contact with the gas. 431 
8. ,, ,, ,, the action of ultra-violet light on a negatively electrified zinc plate 433 
9. ,, ,, ,, discharge of electricity from a pointed platinum wi- c. 439 
10. Behavioui- of the various kinds of nuclei in an electric field. 444 
11. On ions and condensation. 451 
In a previous communication (‘Phil. Trans.,’A, vol. 189, p. 265, 1897) I described 
experiments proving that when dust-free air, initially saturated with water vapour, 
is allowed to expand adiabatically, condensation takes place, if the maximum degree 
of supersaturation resulting from the expansion exceeds a certain limit. Using Vo/Vi 
the ratio of the final to the initial volume,as a measure of the expansion, we may 
describe the phenomena briefly as follows :— 
Condensation only takes place throughout tlie gas if V 2 /V 1 exceeds 1‘25 ; the drops 
are comparatively few, provided a second limit {voli\ — P38) is not exceeded. Beyond 
this second limit the rate of increase in the number of drops with increasing expansion 
is extremely rapid, very dense fogs resulting from expansions even slightly exceeding 
this limit. 
The view was taken that when the degree of supersaturation (approximately eight¬ 
fold) corresponding to the second limit is reached, the vapour condenses independently 
of any nuclei other than its own molecules or those of the gas with which it is mixed. 
The rain-like condensation which takes place in air when lies between 1’25 
3 F 2 12.5.99 
