54 Bacteria in Relation to Country Life 



marked extent. On the other hand, the frequent and 

 copious rainfall tends to remove the bacteria from the 

 air, while the direct tropical sunshine largely contributes 

 to their destruction. 



There are, thus, two distinct and opposite tendencies, 

 one favoring the accumulation of bacteria in the atmos- 

 phere, the other hastening their destruction. In dry, 

 tropical countries, the number of bacteria in the air 

 must, of necessity, be limited, a fact also true of dry, 

 cold countries. In countries with temperate or cold 

 climates, the number of bacteria present in the air in 

 the winter is small, because of the retarded or suspended 

 development of the organisms. This is true, also, because 

 of the covering of ice and snow which prevents the re- 

 moval of the bacteria from the soil and the surface of 

 lakes and streams into the atmosphere. 



The influence of altitude. — The. air over mountain 

 peaks contains scarcely any bacteria at all. Their num- 

 ber diminishes as the distance from the level of the sea 

 is increased. Evidently, the lower temperature prevail- 

 ing in the mountains retards bacterial development, 

 even when conditions are otherwise favorable. More- 

 over, bare rock is an unsatisfactory place for bacterial 

 development. Again, the distance from the fioor of 

 the valleys is frequently too great to allow the transpor- 

 tation of any large number of their bacteria to the air 

 of the higher peaks. 



Bacteria and respiration. — The number of bacteria 

 in the air over the ocean, over high mountains, in polar 

 regions, or in countries of scant rainfall, is relatively 

 very small.. Conditions there prevent both the addition 



