473 Nuclei produced by Action of Liglit on Iodine Vapour. 



will present a considerable area of fresh glass surface at 

 which the action may proceed. On the other hand, the part 

 played by the glass-wool in the experiments where the iodine 

 vapour is swept through it before admission into the cloud- 

 chamber is not quite so clear. It either means that the glass 

 fibres by some preparatory catalytic action on the iodine- 

 laden air facilitate the subsequent formation of the nuclei, or 

 it means that there is some unexpected u impurity " in the 

 glass-wool, which, reacting with the iodine, produces the 

 results described. Since clean glass is known to affect 

 certain chemical reactions, we are inclined to adopt the 

 former view. 



Summary of the Main Results. 



In contradistinction to the results previously obtained with 

 solid C0 2 , we find that 



(1) Camphor, napthalene, benzoic acid, and iodine do not 

 sublime in the form of particles sufficiently large to act as 

 condensation nuclei for water-vapour. But 



(2) When moist air (or oxygen) containing iodine vapour 

 is illuminated, nuclei are produced, possessing the following 

 properties : — 



The nuclei are very unstable, disappearing in a few seconds 

 in the dark. They do not carry an electric charge. They 

 are not obtained except in the presence of oxygen and water- 

 vapour. They grow under the action of the light, but, 

 generally, do not attain a size greater than that requiring 

 a pressure-fall of 18'5 cms. in order to catch them. The 

 light required for their production need not be very intense 

 nor of a high degree of refrangibility. 



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

 the apparatus for some days. The cessation of the action is 

 probably due to a state of chemical equilibrium having been 

 attained. The equilibrium is destroyed by rinsing the glass 

 walls of the apparatus with water. 



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

 tating the formation of the 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 gets 

 more and more saturated with the iodine. The action of the 

 glass-wool is regarded either as being of a catalytic nature 

 or as evidence of the wool being an unexpected source of 



