﻿524 



Prof. J. A. Pollock on the 

 Table II. 



per cent. 



l/«. 



sjs 2 . 



= 1. 





= 25. 



s v 



4-0 



1250 



11-4 



3-3 Xl0- 7 



94 



2-7x10-7 



43 2 



2000 



12-5 



3-6 „ 



10-4 



3-0 ,. 



83-5 



3000 



13-6 



3-9 „ 



11-3 



33 „ 



100-0 



i . 



3440 



14-0 



4-1 „ 



11-6 



3-4 „ 



Diameters of the large ion for various values of the mobility. 



For equilibrium, under usual atmospheric conditions, the 

 value o£ the free surface energy must be influenced by the 

 attraction of the core, so the thickness of the surrounding 

 film of water cannot, from this point of view, be great com- 

 pared with the range of molecular force. The preceding- 

 calculation makes the diameter of the whole ion about a 

 tenth of the value, 5x 10" 6 centimetre, usually accepted for 

 this range. The nuclei of these large ions may, then, be very 

 minute, and although their actual size is unknown it is pos- 

 sible that they may be fairly uniform in diameter, as there is 

 evidence that the mobilities of the fully developed ions under 

 given atmospheric conditions lie within somewhat narrow 

 limits. 



It is not quite clear how the electrical energy of the ions 

 is related to their diameter. The charge is, however, not 

 necessary for equilibrium, and it is not unlikely that the 

 conclusions as to the nature of the ions, only rendered pos- 

 sible by the happy chance of their electrification, may apply 

 with perhaps little modification to many of the far more 

 numerous class of unelectrified nuclei which exist in ordinary 

 air. 



A detailed discussion of the large ion was published by 

 Sutherland in the Philosophical Magazine for September 

 1909. In his own words his view is as follows*: — "To 

 account for the very small mobility of the large ion of 

 Langevin I have imagined the structure already described, 

 namely, a nucleus of (H 2 0) 2 or (H 2 0) 3 or both in a state 

 very similar to that of a liquid surrounded by an envelope of 

 HoO vapour which is kept highly concentrated close to the 

 nucleus. This envelope is similar to the surface film of 

 vapour of H 2 deposited on the grains of fine powders. 

 The number of H 2 molecules per cm. 3 close to the nucleus 

 * Sutherland, Phil. Mag. xviii. p. 366 (1909). 



