Microbes in the Static Condition. 15 



tant media : atmospheric air, waters, soil, foods, the 

 living organism, dwellings, vehicles, clothing, etc. 



Air. — The germs which are found suspended in the 

 atmosphere can not multiply there, and hence come 

 from other media from which they are carried off by 

 atmospheric currents along with pulverulent matters. 

 They are deposited in the calm (tranquil air, hollow 

 places). Their number and nature vary with climatic 

 and other conditions ; their number increases during 

 desiccation of the soil (summer), and diminishes after 

 rains. They are most 'numerous in inhabited places 

 and in the vicinity of marshes, whilst the air of moun- 

 tains and of the surface of seas is almost completely 

 free from them. 



The germs of the air do not long resist the com- 

 bined action of oxygen and light ; nevertheless, through 

 the agency of the winds their effects may be mani- 

 fested at great distances. 



The study of atmospheric germs is made by simple 

 enumeration with the microscope or by various cul- 

 ture processes. The latter method is much to be pre- 

 ferred since it allows of the separation of dead germs, 

 the number of which is very large. 



Watei^s. — Subterranean waters, having filtered 

 through thick layers of earth, are free from all germs, 

 but quickly become infected on contact with the surface 

 soil and the air. The waters of wells are always in- 

 fected; their pollution is, moreover, easy to under- 

 stand : the masonry having no support at the bottom 

 eventually sinks down and fissures are produced 

 through which infiltrate the waters of the neighboring 

 surface soils often impregnated with germs. For this 



