Efficiency of Ions as Condensation Nuclei. 



289 



" On the Comparative Efficiency as Condensation Nuclei of posi- 

 tively and negatively charged Ions." By C. T. E. Wilson, 

 M.A. Communicated by the Meteorological Council. Ee- 

 ceived May 11,— Eead June 15, 1899. 



(Abstract.) 



The experiments described in this paper were undertaken with the 

 object of throwing some light upon what appeared to be fundamental 

 questions in connection with the electrical effects of precipitation. It 

 was hoped in this way to make some advance towards an understand- 

 ing of the relation between rain and atmospheric electricity. 



It was pointed out by Professor J. J. Thomson* that if positive and 

 negative ions differed in their power of condensing water around them, 

 drops might be formed upon one set of ions only, and separation of 

 positive and negative electricity would then take place by the falling 

 of the drops, the work required for the production of the electric field 

 being due to gravity. 



To make this process worthy of consideration as a possible source of 

 atmospheric electricity, it would be necessary to show reason for 

 believing (1) that atmospheric air in the regions in which rain is 

 formed is likely to contain free ions, (2) that positively and negatively 

 charged ions differ in their efficiency as condensation nuclei. 



It is mainly with the second of these questions that this paper deals. 

 The result of this part of the investigation was to prove that water 

 condenses much more readily on negative than on positive ions. The 

 experiments consisted in measurements of the expansion required to 

 cause condensation in the form of drops in air initially saturated and 

 containing ions, alternately nearly all positive and nearly all negative. 

 The ratio of the final to the initial volume being indicated by v 2 /vi, 

 then, to cause water to condense on negatively charged ions, the super- 

 saturation must reach the limit corresponding to the expansion v 2 /vi = 

 T25 (approximately a fourfold super saturation). To make water con- 

 dense on positively charged ions, the supersaturation must reach the 

 much higher limit, corresponding to the expansion v 2 /vi = 1*31 (the 

 supersaturation being then nearly sixfold). 



We see, then, that if ions ever act as condensation nuclei in the 

 atmosphere, it must be mainly or solely the negative ones which do so, 

 and thus a preponderance of negative electricity will be carried down 

 by precipitation to the earth's surface. 



Incidentally it was proved that the difference between the effects as 

 condensation nuclei of the positively and negatively charged ions is 

 not to be explained by supposing that the charge carried by the nega- 



* ' Phil. Mag.,' December, 1898. 



