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XLIY. On Condensation Nuclei produced by the Action of 

 Light on Iodine Vapour. By Harold Pealing, M.Sc'., 

 Lecturer in Physics at South African College, Cape Town *. 



I^HE object of this research was to continue the investi- 

 gation I carried out in collaboration with Professor 

 Gwilym Owen, D.Sc. (an account of which we published in 

 the Phil. Mag. for April 1911) ; and to test and examine the 

 objections which Ramsauer made to the explanation we gave 

 of our experiments. 



From experiments we made with a Wilson's expansion 

 apparatus it was shown : — 



(1) That when light fell on a mixture of moist air or 

 oxygen and iodine vapour contained in a freshly cleaned 

 glass vessel, nuclei are produced possessing the following- 

 properties : 



The nuclei are very unstable, disappearing in a few 

 seconds in the dark, and carry no electricity and need oxygen 

 and moisture for their production. The light required for 

 their production need not be very intense, nor of a high 

 degree of ref rangibility. They usually reach their maximum 

 size in less than one second, and as a rule they were not large 

 enough to be caught by an expansion of less than 18*5 cm. 



(2) No nuclei are produced after the iodine has been in the 

 apparatus for some days, but if the apparatus was washed 

 with nitric acid and finally with distilled water, nuclei 

 reappeared. 



(3) Glass-wool possesses the peculiar property of facili- 

 tating the formation of nuclei, the number produced when 

 iodine-laden air is admitted into the apparatus through a 

 plug of glass-wool being much greater than the number 

 obtained on placing iodine directly in the cloud-chamber. 

 This property becomes less and less marked as the wool 

 becomes more and more saturated with iodine. We were 

 unable to decide whether this action of the glass-wool was 

 due to some impurity on its surface. In the case of the 

 nuclei produced without the aid of the glass-wool, we were 

 of the opinion that they were not produced by an impurity 

 on the surface of the glass, but were produced by some 

 chemical action between the iodine,, water-vapour, and 

 oxygen. To explain the disappearance of the effect we 

 supposed that the chemical action was reversible, and that as 

 soon as chemical equilibrium was established no more nuclei 

 were produced. 



The most conclusive evidence we put forward for this 

 * Communicated by the Author. 



