﻿Nature of the Large Ions in the Air. 525 



will have a value N 2s like a saturation value, and the number 

 will diminish with increasing distance from the nucleus till 

 it becomes N 2 where its influence has ceased. " 



For the velocity u of the ion in an electric field he gives 

 the expression 



e^- = rV^T^ENs + G + HN 2 >, 

 ax 



where r is the distance between the centres of a molecule of 

 H 2 and one of air when in collision, q the number of ions, 

 and N 3 the number of air molecules, per cubic centimetre, 

 T the absolute temperature, and E, Gr, and H, on the right- 

 hand side of the equation, constants. 



The form of the expression within the brackets was sug- 

 gested by a preliminary result of my own measurements that 



1/m=1200 + 107-5 h 3 



where h is the humidity in grammes per cubic metre. Such 

 a form of relation only holds approximately as may be seen 

 from an inspection of fig. 1, and the numerical values in the 

 equation need modifying in the light of the later measures, 

 and may require considerable alteration if it turns out that 

 the observations at low humidities require correction. 



Stress is laid by Sutherland on the point that according 

 to his view of the movement of gaseous ions the mobility 

 should depend on the density of ionization. In my obser- 

 vations the number of ions per cubic centimetre varied from 

 650 to 32900, but no dependence of mobility on this factor 

 is apparent. 



Sutherland's discussion has the great merit that it involves 

 no unreasonable assumptions. It was developed, however, 

 before the evidence in favour of a rigid core in connexion 

 with the large ions was fully appreciated, and from what I 

 have said in this paper I believe his investigation requires 

 modification. 



A correspondence with this gifted author during the 

 progress of the experimental work was a source of great 

 encouragement in a most tedious investigation. 



"&' 



Summary, 



1. The large ions in the air, which were discovered by 

 Langevin in 1905, have a mobility which at constant atmo- 

 spheric pressure is a function of the relative humidity only. 

 At standard pressure the mobility varies from 1 1250 when 

 the humidity is 1 per cent., to 1/3110 when the pressure is 

 that of saturation. 



