258 Mr Townsend, On Electricity in Gases etc. [Feb. 8, 



negatively electrified oxygen forms round it a cloud of larger 

 drops than the positive for the same density of electrification. 

 The negative cloud does on this account fall more rapidly 

 than the positive, so that eventually the upper layers of the 

 atmosphere would have a light positive charge. This is exactly 

 what does exist in the atmosphere, namely, that its normal state is 

 positive, as Lord Kelvin points out in a pamphlet on " Electrifica- 

 tion of Air." {Philosophical Society of Glasgow, 1894-5.) He 

 also refers to experiments of Elster and Geitel's, which show that 

 when rain or snow falls into a basin it generally shows a large 

 electrical effect whether positive or negative, even in the space of 

 a quarter of a minute, with however on the whole a preponderance 

 of negative electrification. When we come to seek a theoretical 

 explanation of the formation of these clouds the way is perfectly 

 clear up to a certain point. It can be shown (J. J. Thomson, 

 Applications of Dynamics to Physics and Chemistry, Chap. XI.) 

 that if a drop having an electrification E and radius a be in a 



E 2 



saturated atmosphere near a flat surface, then when a 3 = — — ™, T 



being the surface tension of water, a particular size of drop is 

 obtained such that if a is less than this value, moisture will be 

 condensed on the drop, and when a is greater than this value, the 

 drop will evaporate and become smaller. So that on these con- 

 siderations only we get a stable nucleus formed, which in the case 



10~ 7 

 of the oxygen would have a radius of -^- . It would be prema- 

 ture to go more deeply into this subject in this paper, as experi- 

 ments are in the process of being carried out which may throw 

 more light upon it. In conclusion I desire to state that my best 

 thanks are due to Professor Thomson for the advantages that 

 have been derived from his very valuable suggestions. 



Monday, 22 February, 1897. 

 Mr F. Darwin, President, in the Chair. 



The following were elected Fellows of the Society : — 



A. Liversidge, M.A., Christ's College. 



H. C. Robson, M.A., Fellow of Sidney Sussex College. 



The following, having been nominated by the Council, were 

 elected Associates : 



S. W. Richardson, Trinity College. 

 H. F. Hyndman, Trinity College. 



