BACTERIA OF THE AIR 2QI 



Quantitative Distribution. The number of bacteria in the 

 air varies with different conditions, as has already been indi- 

 cated. They are found to be very sparse in the air over snow- 

 covered mountains, and over large bodies of water. They are 

 comparatively few in the country air and very abundant in 

 the air of cities, or other parts of the country where there is a 

 great deal of dust. Their distribution will depend upon the 

 character of the surface of the soil, the amount and direction 

 of wind, the action of the sunlight, etc. " 



Seasonal Distribution. Season has an influence upon 

 the number of bacteria in the air. Conditions which favor 

 dryness, and hence dust, increase the number of bacteria. 

 Rains not only keep down the dust but they wash the air free 

 from bacteria. The same is true of snow, so that when the 

 air is washed by rain or snow it is much freer from bacteria 

 than at other times. 



The Species of Bacteria in the Air. Bacteria found in the 

 air are mostly saprophytes and are harmless. There are cer- 

 tain parasitic bacteria, however, which may be air-borne. 

 One of the best examples of air-borne, disease-producing bac- 

 teria is Bact. tuberculosis. That this germ can be transported 

 through the air from one individual to another is beyond 

 question, and most people believe that this disease is trans- 

 mitted, largely if not entirely, through the bacteria which get 

 into the air from dried sputum. The work of Cornet and 

 others has shown experimentally that this is possible. Re- 

 cent work by a considerable number of investigators, following 



