WHAT WE BREATHE 37 



top of the mountain. As was to be anticipated, 

 frequently no bacteria at all were found, and it 

 was only when such comparatively large volumes 

 of air as one thousand litres (about 200 gallons) 

 were explored that microbes in numbers varying 

 from four to eleven were discovered. The air of 

 the country is far freer from microbial life than 

 that of cities ; whilst open spaces, such as those 

 afforded by the London parks, are paradises of 

 purity compared with the streets with their 

 attendant bacterial slums. 



That it is no exaggeration to describe streets 

 from the bacterial point of view as slums is to be 

 gathered from the fact that much less than a 

 thimbleful of that dust which is associated with 

 the blustering days of March and the scorching 

 pavements of summer may contain from nine 

 hundred to one hundred and sixty millions of 

 bacteria. But investigators have not been content 

 to merely quantitatively examine street dust ; in 

 addition to estimating the numerical strength of 

 these bacterial dust-battalions, the individual char- 

 acteristics of their units have been exhaustively 

 studied, and the capacity for work, beneficent or 

 otherwise, possessed by them has been carefully 

 recorded. The qualitative discrimination of the 

 bacteria present in dust has resulted in the dis- 

 covery of, amongst other disease germs, the 



