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IX. OH the Comparative Efficiency an Condensation Nuclei of Positively and Negatively 



Charged Ion*. 



By 0. T. R. WILSON, M.A. 



Communu-itti'tl />// the. Meteorological Count-it. 

 



Received May 11, Read June 15, 1899. 



THE work, of which the results are given in this communication, forms part of an 

 investigation on atmospheric electricity, on which I am engaged on behalf of the 

 Meteorological Council. 



The relation tetween rain and atmospheric electricity was one of the problems it 

 was suggested I should study experimentally. The importance in connection with 

 that question of the subject dealt with in this paper has already been noticed by 

 Professor J. J. THOMSON, who points out* that "if the negative ions, say, were 

 to differ in their power of condensing water around them from the positive, then we 

 might get a cloud formed round one set of ions and not round the other. The ions 

 in the cloud woidd fall under gravity, and thus we might have separation of jx>sitive 

 .UK! negative ions and the production of an electric field, the work required for 

 tin- production of the field being done by gravity." 



To make this process worthy of consideration as a source of atmospheric electricity, 

 ii would be necessary to show reason for believing (l) that atmospheric air in the 

 regions in which rain is formed is likely to contain free ions, (2) that the positively 

 and negatively charged ions differ in their efficiency as condensation nuclei. 



With respect to the first of these questions, former experiments furnish considerable 

 evidence in favour of an affirmative answer. When moist dust-free air is allowed to 

 '\pand suddenly, a slight rain-like condensation always takes place if the maximum 

 MH>ersaturation attained exceeds a certain limit. This limit is identical with that 

 which is necessary for the formation of fogs in air, in which a supply of ions has been 

 produced by the action of Hontgrn rays or other ionising agent. The nuclei, on 

 which tin- drops are formed in air exposed to the rays, were shown experimentally to 

 In- identical with the ions to which the conductivity of the gas when exposed to the 

 rays is due. The equality of the expansion required to give the comparatively few 

 drops in the absence of the rays, with that required to cause water to condense on 

 the ions, is so exact as to furnish what is at first sight almost convincing evidence 



' I'l.il. Mag., 1 December, 1898, p. 533 

 CXCIII. A. 2P 18.12.99 



