294 Scientific Proceedings, Royal Dublin Society. 



All three tests answered for hydrogen peroxide, and the experiment is 

 apparently conclusive. It may be objected, however, that the hydrogen 

 peroxide detected was not present in tlie nuclei, but simply formed in the 

 air by the ultra-violet light, and not affecting the condensation. 



We, accordingly, tried a test in which the nuclei after being charged by 

 uranium were driven by the field on to an electrode covered with moist 

 paper. The paper had been dipped into a mixture of starch solution, potass, 

 iodide, and ferrous sulphate. No blue colouration was observed even after 

 twelve hours. The quantity of HjOi may, however, have been too small to 

 be detected by this method. 



The following experiments were performed by Lenard and Eamsauer on 

 the origin of the nuclei which were detected by their effect on the steam-jet. 

 Air was freed Trom all vaporous impurities by cooling to - 76° C. No nuclei 

 were formed in this air by ultra-violet light. Adding slight traces of water- 

 vapour to the air caused a slight but perceptible effect on the steam-jet. 

 Saturating the pure air with pure water-vapour by passing it over a large 

 'water surface produced a marked effect on the steam-jet, but not nearly so 

 strong as that of ordinary moist laboratory air. It is evident, then, that 

 water-vapour is only partially responsible for the production of the nuclei 

 in ordinary air, and that cooling to - 76° C. removes, besides water-vapour, 

 some other effective agent. Slight traces of water-vapour along with 

 vapours from india-rubber gave a strong effect on the steam-jet, while the 

 same amount of water- vapour alone, or of vapour from india-rubber alone, 

 gave only a slight effect. We shall see later in Lenard's experiments upon 

 other vapours, besides water- vapour in air, that ammonia gave many nuclei 

 on exposure to ultra-violet light. It is possible that ammonia is the other 

 effective agent besides water-vapour in ordinary air. Lenard attributes the 

 nucleation in the case of ammonia to the formation of ammonium nitrite, 

 and ammonium nitrate, and, where water-vapour is alone present, to the 

 formation of hydrogen peroxide. 



Experiments on Air Sattiratcd with other Vapours. 



Experiments were performed on air containing vapours of ethyl alcohol, 

 methyl alcohol, and toluol. The room air, after being carefully dried, was 

 passed over a surface of the liquid before being exposed to the ultra-violet 

 light. Ci and Cz were then measured, as in the case of water-vapour, but in 

 all three cases examined Cg was equal to Ci, so that nuclei similar to those 

 obtained with water-vapour in the air were not formed. 



It was noticed in these experiments that if minute traces of either of the 

 three vapours mentioned above were present they prevented the formation 



