[ 455 ] 



LIV. On Condensation Nuclei produced by tlie Action of 

 Light on Iodine Vapour. By GrWILYM Owen, AL.A., D.&c, 

 and Harold Pealing, M.Sc, University of Liverpool*. 



FROM experiments made by Gr. Owen and A. LI. Hughes 

 (Phil. Mag. Oct. 1907, June 1908) it seems that the 

 gas evolved by a solidified mass of carbon dioxide previously 

 condensed in a dry and dust-free condition contains large 

 numbers of nuclei, the presence of which can be shown by 

 their ability to act as centres for the condensation of super- 

 saturated water- vapour. This fact suggests that the sub- 

 limation of solid C0 2 consists not merely in the escape of 

 separate gaseous molecules, but also in the liberation of large 

 numbers of relatively large molecular aggregations. The 

 question arises as to whether the same is true for other 

 sublimiug substances, and it was during the course of 

 experiments designed to test this point that the effects 

 described in the present paper were observed. 



Experiments were made with camphor, naphthalene T 

 benzoic acid, and iodine — the method being to pass a current 

 of dust-free air through a tube containing one of these 

 volatile substances, and then to test the air for nuclei by 

 suddenly expanding it in a bnlb containing distilled water. 



In fig. 1, A is the glass tube containing the substance to 



w txh&w&itii ap.V' 



be tested. The air swept through this tube was first rendered 

 dust-free by a plug of cotton-wool as shown. Ihe cloud- 

 chamber B was a glass bulb some 6 cms. in diameter and 

 was sealed on to a Wilson expansion apparatus "|\ the 

 piston of which worked in distilled water. The whole 

 apparatus to the right of the tap C was made of glass. Any 

 clouds produced in B were lLiideied visible by focussing on 

 the bulb the light from a Nernst lamp. 11 we take as a 



* Communicated by the Authors, 



t C. T. K. Wilson; Camb. Phil. Soc. Proc. is. p. 323 (1807). 



