Nuclei produced by Action of Lljlil on Iodine Vapour. 475 



expansion, the light had to be kept on for half: a minute or 

 more. This shows that the larger nuclei result from the 

 growth of the smaller nuclei under the influence of the 

 light, 



Flff. 3. 



Sdt 



inc 



^L-y-^au J/ccC 



t~ o4-A/vzta£u 



The clouds obtained at large expansions become less dense 

 as time goes on, showing that the glass-wool gradually loses 

 its efficiency. Further, the decrease in the density of the 

 clouds at large expansions is accompanied by the total dis- 

 appearance of the clouds cf small expansions. This shows 

 that the nuclei only succeed in attaining a considerable size 

 when their number is very large. 



Discussion of the Results. 



These experiments with the glass-wool suggest that the 

 production of the nuclei when the iodine is put directly into 

 the cloud-chamber (fig. 2) is influenced by the condition of 

 the glass walls of the apparatus. 



It now remains to consider the source of this influence. 

 In the first place, it is not probable that the clouds obtained 

 when the iodine is placed in the cloud-chamber are due to 

 the presence of organic impurities on the glass, for they were 

 invariably produced when the apparatus had previously been 

 carefully washed with nitric acid and distilled water. And 

 if this were the case it is difficult to see how the process of 

 rinsing the apparatus with distilled water could bring back 

 the clouds after they had once disappeared. On the other 

 hand, the glass-wool itself is rendered less efficient as an aid 



2 I 2 



