302 Scientific Proceedings, Royal Dublin Society. 



SUMMARY. 



I. 



The nuclei produced by ultra-violet light in moist air are detected by an 

 electrical method, and their size indicated by their mobility, when charged 

 in an electrical field. The number and size of the nuclei depend upon the 

 time of exposure to the light, the intensity of the light, and the amount of 

 moisture in the air. 



The nuclei are probably small water-drops which owe their formation 

 to the production in the moist air by ultra-violet light of some hygroscopic 

 substance, such as hydrogen peroxide or oxides of nitrogen. An account of 

 work by other experimenters is given, and the evidence, though not conclusive, 

 points to hydrogen peroxide as the parent substance of the nucleus. 



Some experiments were performed in air containing, instead of water, 

 the vapours of methyl alcohol, ethyl alcohol, and toluene respectively. No 

 nuclei are found by our method, probably because the nuclei grow so big, 

 and decrease so much in number, that they fail to affect the currents in our 

 apparatus. The presence of very slight traces of either of these vapours in 

 water-vapour prevents the formation of nuclei such as can be detected by 

 our apparatus, and probably for the same reason. 



II. 



The nuclei which are emitted by a glass tube when heated not too 

 strongly can be detected and measured in a manner similar to that described 

 for the nuclei produced by ultra-violet light. The emission of nuclei when 

 the heating is not too strong lasts only a few minutes, but the tube "recovers" 

 under certain conditions. 



The nuclei can be charged by ultra-violet light, probably by the photo- 

 electric discharge of electrons from the rather large particles. The positive 

 ions are found to be much more numerous and of much greater size than the 

 negative. In all probability only particles greater than a certain size are 

 acted on by the light. The electrons then become attaclied to the smaller 

 particles or the air molecules. The negative ions thus formed are numerically 

 more decreased by diffusion to the walls of the apparatus than the larger 

 positive ions. 



The experiments on the recovery of glass tubes fatigued by heat seem to 

 indicate a difference between those nuclei which can be charged by ultra- 

 violet light and those which can be charged by the small ions produced by 



