476 Dr. G. Owen and Mr. H. Pealing on Condensation 



c 



to the formation of the nuclei by soaking it in strong nitric 

 acid and then washing it with distilled water. A small ping 

 of glass-wool so treated did not cause a cloud when placed 

 (in a dry condition) in the cloud-chamber. This same treat- 

 ment, however, did not prevent the process of sweeping 

 the iodine vapour through a long plug of it fas in fig. 3) 

 from greatly increasing the density of the cloud at 18'5 

 pressure-fall, although it entirely stopped the formation of 

 the large nuclei caught at small expansions. 



If the nuclei originate as a surface action at the glass 

 surface, this effect of the nitric acid on the glass-wool may 

 be regarded in two ways : — (a) That the acid alters the 

 catalytic properties of the surface of the glass-wool, or 

 (b) that it partly removes from it some substance the presence 

 of which aids the formation of the nuclei. One would not 

 expect glass-wool to contain an " impurity " in the ordinary 

 sense of the word. It is well known, however, that there is 

 an action between glass and pure water resulting in the forma- 

 tion of minute traces of NaOH at the surface. Supposing 

 for a moment that the production of the nuclei depends upon 

 this action, it is not unreasonable to suppose that artificially 

 increasing the alkalinity at the glass surface would increase 

 ihe number of nuclei produced. To test this point the 

 apparatus was rinsed out with a weak solution (5 per cent.) 

 of NaOH and then worked with this same solution in the 

 bulb and expansion cylinder instead of distilled water, as in 

 the previous experiments. Under these circumstances we 

 found the iodine to be entirely without effect, for the nuclea- 

 tion was identical with the Wilson effects which we studied 

 before introducing the iodine. Thus the nuclei are not due 

 to the minute traces of NaOH usually present on moist glass; 

 or, at all events, the degree of extra alkalinity that we 

 happened to try, instead of facilitating the production of 

 nuclei, prevented it completely. We also found that treating 

 the apparatus in the same way with a dilute solution of 

 H 2 S0 4 prevented the production of the nuclei. 



We thus find it difficult to say definitely what the nuclei 

 are. They are evidently minute particles of some unstable 

 compound requiring the co-presence of iodine-vapour, oxygen, 

 and water-vapour for its production. 



The effect is clearly photochemical in character, and in all 

 probability belongs to that class of phenomena studied by 

 Tyndall *, Aitken f, and 0. T. P. Wilson J. Tyndall found 



* Tyndall, Phil. Trans, vol. clx. p. 333 (1870). 



t Aitken, Proc. Roy. Soc. Edin. vol. xxxix. i. p. 15 (1897). 



\ Wilson, Phil. Trans, vol. cxcii. p. 403 (1899). 



