52 Bacteria in Relation to Country Life 



INFLUENCE OF SEASON, CLIMATE, AND ALTITUDE 



The investigations of Miquel, extending over a period 

 of years, show conclusively that seasonal influences are 

 an important factor in the increase or decrease of bac- 

 teria in the air. There is an evident increase from winter 

 to spring and from spring to summer, and a decrease 

 from summer to autumn. Yet, while the temperature 

 and other seasonal influences can be easily traced in 

 these averages, there are temporary disturbances that 

 threaten to obscure the general results. For instance, 

 the number of bacteria diminishes after a rain, and in- 

 creases rapidly as the soil begins to dry at the surface. 

 When, however, the drying period continues for ten or 

 fifteen days, the number of bacteria decreases again. 



The causes for these variations are not far to seek. 

 The number of bacteria in the air decreases in periods 

 of rainfall because of their being carried down by the 

 rain. Similarly, in periods of dry weather, the wind 

 carries away some of the finer particles of the surface soil; 

 among them, the bacteria that have multiplied rapidly 

 under favorable moisture conditions. Prolonged drying, 

 on the contrary, and the accompanying germicidal 

 action of direct sunlight, involves the partial destruction 

 of the bacteria floating in the air, as well as of those in 

 the dust mulch at the very surface of the soil; and for 

 these reasons the dust-laden atmosphere of droughts 

 may not be rich in germs. 



The seasonable influences are also noticeable in the 

 atmosphere of densely populated districts, although the 

 number of organisms found here is much greater than 



