620 Dr. G. C. Simpson on the Wilson- Gerdien 



in order to show that in six important particulars the theory 

 is not capable o£ accounting for the observed facts. 



At the end of the paper in which the results o£ the ex- 

 periments already referred to were published, Wilson added 

 the following paragraph*: — 



" In order that the results of these investigations should 

 have a direct bearing on the subject of atmospheric electricity, 

 it is necessary to assume that condensation in the atmosphere 

 frequently takes place from the supersaturated condition. 

 There is very little direct evidence of the existence of super- 

 saturation in the atmosphere ; but there is at least an equal 

 lack of evidence against its existence above the lower cloud 

 layers, even as a normal accompaniment of precipitation. 

 That supersaturation occurs in connexion with thunderstorms 

 is held by V. Bezold and others (Sitzab. Akad. d. Wiss. zu 

 Merlin, 1892)/' 



This idea, that the electrical effects observed with preci- 

 pitation might owe their origin to the greater facility with 

 which water vapour is deposited on negative than on positive 

 ions, was eagerly taken up on the continent, and Elster and 

 Oeitel considered it so promising that they gave to it a 

 prominent place in their lecture before the Brunswick Society 

 of Science in April 1900 1- 



The only serious objection to the suggestion came from 

 Mr. AitkenJ, who was led to doubt, from his classical re- 

 searches on the dust of the atmosphere, that " there is such a 

 thing as dust-free air in our atmosphere." To answer Mr. 

 Aitken's objections Wilson wrote a letter to ' Nature ' § in 

 which he developed his idea into a detailed theory, showing 

 how he considered the dust might be entirely removed from 

 air during thunderstorms, and how the separation of the 

 electricity could be supposed to take place. The following 

 summary of this letter, written as far as possible in the 

 original words, may be taken as giving the essential points 

 of Wilson's theory. 



If an ascending current of air contains drops of w^ater in 

 the form of cloud-particles these drops will lag behind the 

 air-current to a greater or less extent, according to their 

 sizes. Hence, the air which enters the base of a cumulus 

 cloud will pass amongst the water-drops, and ultimately 

 •reach the top. During the passage of the air through the 



* Wilson, Phil. Trans. A. 193. p. 807 (1000). 



t Met. Zeit. xvii. pp. 226-231 (1900). 



t Aitkeu, ' Nature/ xli. pp. 514-515 (1900). 



§ Wilson, 'Nature,' xlii. pp. 149-151 (1900). 



