180 BACTERIA IN AIR, SOIL, WATER AND MILK 



forms parasitic to animals are found. Tetanus bacilli are, 

 perhaps, the most widespread of pathogenic bacteria in the 

 soil. Their resistant spores remain alive an unlimited time. 

 Persons going barefoot and subject to wounds or bruises may 

 contract the disease. Tubercle bacilli, in dead persons, live 

 only a few months, but when contained in sputum spat upon 

 the earth survive for a long period. The important condi- 

 tions in which microorganisms in the soil play a part are 

 gas bacillus infection, tetanus, anthrax and uncinariasis, 

 hookworm disease. 



BACTERIA IN WATER. 



in water there are many hundreds of species, but it may 

 be said in general that all the disease-producing kinds are 

 in water because discharges from human disease have been 

 put into it. Of course this may not be direct, but through 

 the agency of soil as mentioned above. Some bacteria may 

 be carried into streams by rain which brings down the dust. 

 Rain itself is free of germs. Bacteria may be present in water 

 up to the billions without altering greatly its clearness or 

 giving it an odor, while, on the other hand, a cloudy water 

 does not necessarily indicate bacterial pollution, for the 

 turbidity may be due to harmless inorganic chemical matter. 

 Of the two water sources recognized by hygienists, ground 

 water (deep wells) and surface water (ponds, lakes, and rivers) 

 the second is by far the more important and the more easily 

 polluted. Large bodies of water, either still or moving, tend 

 to rid themselves of bacteria. In still or slowly moving 

 bodies, such as reservoirs, germs settle with other organic 

 and inorganic matters. For water-courses of any character 

 purification is aided by changes in temperature during the 

 day and night and the very efficient disinfecting properties 

 of direct sunlight. Oxygen absorbed from the air also assists 

 in destroying bacteria. There are certain saprophytes in 

 water and sewage capable of breaking up organic matter 

 and freeing oxygen, which, either free or in going into com- 



