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XXIV. 



UNCHARGED NUCLEI PEODUCED IN MOIST AIE BY 

 ULTRA-VIOLET LIGHT AND OTHER SOURCES. 



By the LATE PROFESSOR J. A. McCLELLAND, F.R.S, 



AND 



J. J. M'HENRY, M.Sc, 

 University College, Dublin. 



[Read May 24. Published August 19, 1921.] 



I. — Nuclei peoduced by Ultka-violet Light. 

 Ultka-violet light acting on moist air or oxygen produces in the gas small 

 particles or nuclei which are not electrically charged. C. T. R. Wilson, in a 

 paper on condensation nuclei (Proc. Roy. Soc, London, 64, p. 127), describes 

 some experiments with the^ condensation nuclei, which he detected and 

 studied by hieaus of his expansion apparatus. When the radiation was 

 weak, he found that the nuclei produced required as great a degree of super- 

 saturation to cause water to condense upon them as the ions produced by 

 X-rays, uranium, &c. With stronger radiation, however, the nuclei grew in 

 size, requiring a smaller degree of supersaturation to produce condensation. 

 Under favourable conditions they even became visible. 



In our experiments the nuclei were detected electrically. Wilson's 

 method of supersaturation gave but a rough idea of the size of the uncharged 

 nuclei, and it could not be applied to dry air. In the following paper an 

 account is given of an attempt to examine the nuclei more closely by an 

 electrical method. Some experiments were also performed on other types of 

 unchai'ged nuclei. 



When uncharged nuclei pass through air ionized by uranium. X-rays, &c., 

 they are electrified by the small ions, and thus large ions are formed, of 

 smaller mobility, under an electric field. These large ions recombine less 

 rapidly than the small ions ; and hence, if time is allowed for the ions to 

 recombine, the presence of nuclei originally uncharged causes an increase in 

 the current measured. 



P. Lenard and C. Ramsauer have also studied in great detail the nuclei 

 produced by ultra-violet light. Using light of such short wave-length that 

 it was able at once to ionize the air and to produce the nuclei, they obtained 

 large ions, which they attribvited to the combination of nuclei and ions pro- 



