Dust and Smoke on the Ionization of Air. 659 



the intermediate ions with the small ions. Pollock finds that 

 the number of intermediate ions appears to have in the main 

 a linear relation with the amount of water vapour present in 

 the atmosphere. It is also important to note that Pollock 

 finds more negative than positive large ions, the mean values 

 of their numbers being n + , 1914; n— ,2228; n — /n + , 1-16. 

 This, of course, suggests that more negative than positive 

 small ions transform into large ions, a result which might be 

 expected from the known physical properties of the two kinds 

 of ions. 



The ionization of the atmosphere is difficult to investigate 

 on account of the number of variable and unknown conditions, 

 mainly meteorological. Laboratory experiments have been 

 made with much frequency and accuracy, but in these care 

 has been taken to avoid as far as possible all the disturbing 

 elements, such as dust, smoke, water vapour and mist. It 

 seemed^ then desirable to make some experiments on the 

 ionization of air as it occurs, and without removal of 

 impurities, using a strong and constant ionizing source. 



A large paper or metal cone (fig. 1) was, therefore, placed 



Fkr. 1. 



r- 



~Z 1 















■-.- 



\ ; S 



r\l 











i— , r- 











/ 



L 





f / 



\ \ \— 



r 





I / 



\ J 









0Ra 



above the Ebert instrument, and the <y rays from 14 mo-, of 

 radium bromide, about | to 2 m. distant, were made to 

 traverse the upper part of the cone, so as to ionize the air on 

 its passage to the testing vessel, which was, of course, 

 screened by thick lead from the rays from the radium. The 

 results obtained by Bates and the writer were published in 

 a letter to 'Nature/ 11th March, 1909. The observations 

 were made in a large class-room in the Arts Buildinc at 

 McGill University, under the very dry conditions of a 

 Canadian winter. The temperature was about 18° C, and 

 Smoke and dust were avoided as far as possible, apart from 



2U2 



