Dust and Smoke on the Ionization of Air, 665 



The maximum values on very clear days were : — 



+ to centre. — to centre. Ratio n-\-/n—. 



Oct. 30 21-1 19-1 1-11 



Sept. 25 20-0 20'0 ? 1*00? 



and the minimum values, occurring on smoky or misty 

 days, were : — 



Oct. 9 12-1 11-2 1-08 



Oct. 21 13-3 11-8 1-12 



Oct. 26 14-4 11-5 1-24 



Hence, the number of ions detected in very clear weather 

 was nearly twice as great as on a dull, smoky, misty day. 

 Large values were always obtained with " clear shining after 

 rain/' when the air had been largely freed from impurities. 



Indeed, apparatus of the type used might prove efficient 

 for the purposes of testing the degree of purity of the air, at 

 any time or place, as regards the amount of smoke, dust, or 

 mist present. 



It should be explained that the position of the Physics 

 Building at McGill University is such that a slow moving 

 wind from the city brings with it much smoke from the soft 

 coal used in the fires of the ever increasing factories, whilst a 

 wind from the west or north-west is of exceptional purity. 

 The maxima and minima recorded above indicate this 

 difference well. 



If the window was shut, an artificial fog of tobacco-smoke, 

 steam from a boiling kettle, and vapours from the flame of a 

 two-ringed Bunsen burner were found to lower the ionization 

 current still further to about one-third of the maximum. 



By blowing tobacco-smoke directly into the cylinder V, 

 the ionization current at B may be reduced to a very small 

 value. This is merely a repetition of the experiment of 

 Owens*, who first showed the marked diminution of the 

 ionization current between two plates when tobacco-smoke 

 was introduced between them. 



What then becomes of the ions ? 



The explanation usually given is not altogether satisfactory, 

 namely, that " in dusty air, the rate of recombination is much 

 more rapid than in dust-free air as the ions diffuse rapidly to 

 the comparatively large dust particles distributed throughout 

 the gas" (Rutherford's 'Radioactivity/ 2nd edition, p. 42). 

 And again, "The effect produced by dust is easily explained, 



* I'liil. Mag. Oct. 1899. 



