Ultra-Violet Light on Moist Air. 311 



ordinary dust cloud produced by expansion has not had time to 

 completely settle. 



These experiments have left many points of interest un- 

 determined. Of these the relation of the very slow moving 

 positive ions observed by Lenard to the cloud drops is one of 

 the chief. 



Conclusion. As far as these experiments go they seem to 

 point to the effect of ultra-violet light on moist air being of a 

 two- fold character. 



It manufactures some unidentified substance which is less 

 volatile than water and a solution of which in water forms the 

 droplets. It ionises the air and the ions get on the cloud drops 

 just as ions produced by any other process would be caught by 

 any finely divided substance floating in the air. 



I wish to thank Prof. J. J. Thomson, F.R.S., and Mr C. T. R. 

 Wilson, F.R.S., who have kindly given me their help and advice 

 during the course of these experiments. 



5yvv*a 



