292 Bcientific Proceedings^ Royal Dublin Society. 



water surface at O'^C. before being exposed to the spark. A small definite 

 quantity of water vapour was thus contained in the air, and a small number 



Q 



of nuclei were formed, j^ being found to be 1'25. On now including a 

 second water surface at room temp. ll^C. after the quartz tube, it was thought 



Q 



that ~ would increase, as if, for example, HjO. were formed in the partly 



moist air more nuclei would be formed in passing over the second water 



C 



surface. -^ was found, however, to have the value 1'24 in this case, so that 



no additional nuclei were formed. 



Many physicists have tried to demonstrate the presence of H2O2 in the 

 nuclei formed by ultra-violet light in moist air. Vincent (Camb. Phil. Soc. 

 Proc, 1904, 12) used a sensitive photographic test, but failed to detect any 

 H2O2. Miss Saltmarsh {he. cit.) showed that prepared HjOo when introduced 

 into the expansion apparatus caused the production of nuclei. The presence 

 of H2O2 in these nuclei could be demonstrated' as, when about 19 expansions 

 were made, the showers falling into a small vessel containing titanic acid, 

 the acid became yellow. On the other hand, 150 expansions on the nuclei 

 produced by ultra-violet light caused no colouration of the acid. Miss 

 Saltmarsh concluded that the part played by H2O2 was at most a very small 

 one in the formation of nuclei. 



On the other hand, Barkow (Ann. der Physik, 23, 1907) found that 

 H2O2 does not assist cloud-formation unless when decomposed by strong 

 sunlight. When the HoOj is exposed to strong sunlight, numerous nuclei 

 are produced, wliich persist for long pei-iods. Ultra-violet light is not necessary 

 for this decomposition of H2O2, but it was not brought about by light from a 

 strong arc lamp. Barkow tested for hydrogen peroxide in the nuclei pro- 

 duced by ultra-violet light in moist air by drawing them slowly through a 

 solution containing titanic dioxide in concentrated sulphuric acid. This 

 solution turns brown with H2O2, but Barkow obtained no colouration even 

 after 150 hours, using in all 134 litres of moist air. 



Barkow investigated the effect of nitrogen peroxide on the condensation 

 of water vapour. He passed nitrogen dioxide into the cloud-chamber. 

 Spontaneous condensation immediately took place, and many expansions were 

 required to remove the nuclei. He also ozonized oxygen, and passed it into the 

 expansion apparatus. The characteristic blue cloud was obtained without 

 expansion. This spontaneous condensation with ozonized oxygen, however, 

 Barkow attributed to the presence of small quantities of nitrogen in the 

 oxygen used, oxides of nitrogen being formed. 



