CH. viii.] OF AN ELECTRON 79 



This brilliant research has actually been carried out 

 by Professor J, J. Thomson, by means of a method 

 partly due to Mr. C. T. K. Wilson, supplementing a 

 fact discovered by Mr. Aitken, and interpreted in the 

 light of hydrodynamic principles laid down long ago 

 by Sir George Stokes. 



I must be excused for waxing somewhat enthusiastic 

 over this matter : it seems to me one of the most 

 brilliant things that has recently been done in experi- 

 mental physics. Indeed I should not need much 

 urging to cancel the " recently " from this sentence ; 

 save that it is never safe for a contemporary to usurp 

 the function of a future historian of science, who can 

 regard matters from a proper perspective. 



The matter is rather long to explain from the 

 beginning, and I must take it in sections. 



Aitken and Cloud Nuclei. 



First of all, Mr. John Aitken,* of Edinburgh, dis- 

 covered in 1880 that cloud or mist globules could 

 not form without solid nuclei, so that in perfectly 

 clear and dust-free air aqueous vapour did not 

 condense, and mist did not form. (See, for instance, 

 my lecture to the British Association at Montreal, in 

 1884, on "Dust" Nature, vol. 31, p. 268.) 



Without solid surfaces, in clear space, vapours 

 could become supersaturated ; but the introduction 

 of a nucleus would immediately start condensation, 

 and according to the number of nuclei, or conden- 

 sation centres, so will be the number of cloud 

 globules formed. 



Every cloud or mist globule is essentially a 

 minute falling raindrop, not floating in the least, but 

 falling through a resisting medium falling slowly 



* Trans. R.S. Edin., vol. 30, pp. 337-368 (1883). 



