262 A MANUAL OF BACTERIOLOGY 



Aitken concludes (1) that the earth's atmosphere is 

 greatly polluted with dust produced by human 

 agency ; (2) that this dust is carried to considerable 

 elevations by the hot- air rising over cities, by the 

 hot and moist air rising from sun-heated areas of the 

 earth's surface, and by winds driving the dusty air 

 up the slopes of hills ; (3) that none of the tests 

 made of the Mediterranean sea air show it to be very 

 free from dust ; and (4) that the amount of dust in 

 the atmosphere of pure country districts varies with 

 the velocity and the direction of the wind : fall of 

 wind being accompanied by an increase in dust. 

 Winds blowing from populous districts generally 

 bring dusty air. 



It is stated that a man in the town inhales about 

 37,500 germs every twenty-four hours, and no 

 fewer than 2,250,000 inorganic particles every 

 minute. 1 ' Most of these are merely annoying, 

 though a few are real messengers of disease and 

 death. If the lungs are warm and moist, they can 

 repel the particles ; but with cold and dry lungs the 

 suffering from the clogging must soon begin/ 



Besides the inorganic or dead particles, the air 

 is more or less laden with living particles. The 

 majority of these are of the non-pathogenic or harm- 

 less kind, but there is plenty of evidence to show 

 that pathogenic microbes lurk about in the atmo- 

 sphere, and that many infectious diseases are propa- 

 gated by means of air-carried microbes. Hence the 

 reason that the study of aerial microbes is peculiarly 



1 A cigarette smoker sends no fewer than 4,000,000,000 of 

 particles (more or less) into the air with every puff he makes. 



