﻿Nature of the Large* Ions in the Air. 515 



The main facts are as follows : — 



1. Ions with a mobility of 1/3000 under usual atmo- 

 spheric conditions form a well denned class; this was 

 Langevin's original discovery. McClelland and Kennedy * 

 in their investigation found no evidence of any other 

 ions. In my own measurements there is no indication of 

 another type of ions with a mobility approximating to that 

 of the Langevin ion, though there is a class of ions with a 

 mobility intermediate between those of the large and small 

 ions. 



2. Langevin and Moulin f mention that the simultaneous 

 variations in the numbers of the large and small ions in the 

 air are opposed in direction, a fact which is also shown in 

 the measures made on the few occasions when I have taken 

 continuous observations of both classes of ions at Sydney. 

 Langevin and Moulin further state that the number of the 

 large ions is the greater the more numerous the (dust) par- 

 ticles in the air, and they consider the large ions as created 

 by the attachment of: small ions to these neutral particles. 



From the results of G. T. R. Wilson's % investigations on 

 the formation of clouds in closed vessels, it may be inferred 

 that these large ions do not exist in air recently freed from 

 dust, and that they are not developed in intervals of time 

 extending to days in dust-free air has been shown by workers 

 in this laboratory. 



3. The mobility depends on the humidity §. The first 

 suggestion of the probability of a connexion between these 

 ions and the moisture in the air was made by Sir Ernest 

 Rutherford in his book on Radioactive Transformations. 



These facts lead one to picture the Langevin ion as a col- 

 lection of water molecules surrounding a dust particle, the 

 whole being electrified by the attachment of a small ion. 

 Judging from the mobility measurements, the size of the ion 

 at constant temperature depends on the vapour-pressure. If 

 any change of vapour-pressure occurs, the radius changes 

 until equilibrium is again established, and there is still 

 equilibrium when the vapour is saturated ||, for cloud con- 

 densation experiments with unfiltered air show that the ion 

 does not grow to a visible drop until there is some slight 

 super saturation. 



* McClelland and Kennedy, Proc. R. I. Acad. xxx. A. p. 71 (1912). 



f Langevin and Moulin, Le Radium, iv. p. 218 (1907). 



X C. T. R. Wilson, Phil. Trans. A. clxxxix. p. 265 (1897). 



§ Pollock, Jonrn. and Proc. Roy. Soc.N.S. Wales, xliii. p. 198(1909). 



j| The term saturation will be used throughout to denote the condition 

 of the vapour when in equilibrium with a plane water surface having a 

 thickness great compared with the range of molecular force. 



2 L 2 



