Nuclei produced by Action of Light on Iodine Vapour. 469 



admitted the light, and the other permitted an inspection of 

 the cloud obtained. The slits were provided with flaps, 

 which could, be closed when it was desired to keep the 

 apparatus in complete darkness. 



Table II. shows the effect of light. The results given in 

 the last column were obtained by producing the expansion 

 in the dark and then admitting the light immediately 

 afterwards. 



Table IT. 



No Iodine in the Iodine in the 

 Apparatus. Light on ' Apparatus. Light on 

 Bulb continuously. Bulb continuously. 



Iodine in Apparatus. 

 Expansion made in the 



dark. Light put on 

 immediately afterwards. 



Pressure- 

 fall. 



Observation. 



Pressure- 

 fall. 



Observation. 



Pressure- 

 fall. 



Observation. 



15-o 

 17-5 



18-5 



Few drops. 

 Thin shower. 

 Good shower. 



145 

 15-5 

 17-5 



18-5 



1 



Few drops. 

 Fair shower. 

 Dense shower. 

 Coloured cloud. 



14-2 



i 



j 17T> 

 18-5 







Thin shower. 

 Good shower. 



The above table shows clearly : — 



(a) That the addition of the iodine causes an immense 



number of fre^h nuclei to appear. 



(b) That the majority of the nuclei are small and con- 



sequently require a large expansion corresponding to a 

 pressure-drop of 18' 5 cms. to catch them. 



(c) That all these nuclei are wholly produced by the action 



of light, for when the expansions are made in the 

 dark the effects observed on admitting the light imme- 

 diately afterwards are the same as in the absence of 

 iodine. 



These effects Mere obtained with a Nernst lamp as the 

 source of illumination. Other modes of illumination — arc 

 lamp, fishtail burner, Nernst light screened by red glass, 

 and diffused daylight — were tried with the same results. In 

 the last case the clouds were observed by illuminating the 

 bulb with a Nernst lamp after the expansion had been 

 made. 



