EXAMINATION OF AIR. 451 



The particles of dust and the free organisms suspended 

 in the atmosphere are specifically heavier than the air and 

 hence tend to settle. This sedimentation is very marked 

 even in a few hours, in a room where currents of air are 

 absent. To a certain extent this takes place in the open 

 air, but the purification of the out-door atmosphere is chiefly 

 accomplished by the precipitation of rain and snow, and by 

 the washing- of the air as it passes over the surface of lakes 

 and seas. For the same reason expired air will contain but 

 a very small fraction of the organisms present in that which 

 was inspired. Thus, when the former contained 20,000 

 germs per cubic meter, the latter contained in a like volume 

 only 40. It is evident, therefore, that air which contains 

 the fewest organisms is to be met with at high altitudes, as 

 on the tops of mountains and glaciers, and in mid-ocean. 



The actual number of organisms present in the air is 

 greatly over-estimated. This is indicated in the marked 

 freedom from contamination by air germs, of the nutrient 

 media, in ordinary routine bacteriological work. Naturally, 

 the air of a recently swept room will contain more germs 

 than that outside of the house. Likewise, the air of cities 

 will be relatively rich as compared with that of the open 

 country. Moreover, the number of organisms in the air 

 during the winter season will be less, nearly one-half, than 

 in the spring or summer. 



The following table will be of interest showing the 

 variation in the number of bacteria and moulds in different 

 seasons and in country and city air. The figures represent 

 the average numbers obtained by Miquel, who made monthly 

 examinations extending over a period of 10 years. The re- 

 sults in the first half of the table were obtained at the 

 Mont-souris observatory, situated in a park at the southern 

 edge of Paris; whereas, the results in the other half of the. 

 table were obtained in the square before the City Hall, only 

 about 2 miles distant from the former place. The figures 



