CHAPTER XV. 



EXAMINATION OF AIK, WATEK, AND SOIL. 



Bacteria in the air : Both pathogenic and non-pathogenic 

 bacteria are constantly present in the air in greater or less 

 number, depending on conditions and the quality of the air. 

 The pure air of the mountains contains very few organisms, 

 whereas the dust-laden air of cities and towns contains very 

 many. 



The species also vary according to the temperature and the 

 soil over which the air passes, the number of living creatures 

 moving about in it, and their method of living, hygiene, and 

 sanitation, the presence of decomposing material or the excreta 

 of persons and animals affected with some disease. The air 

 in sick-rooms and hospital wards contains more organisms 

 than the outside air. The air of places frequented by con- 

 sumptives, who expectorate promiscuously on the ground, 

 usually is laden with tubercle bacilli. During those seasons 

 of the year when certain infectious diseases, such as pneu- 

 monia, la grippe, etc., are epidemic, the air is more liable to 

 contain pathogenic organisms than when such epidemics are 

 not prevailing. 



Under ordinary conditions, however, the air usually con- 

 tains only saprophytic bacteria, and these are found always in 

 the lower strata unless disseminated by winds. There are 

 less bacteria found in the air during winter than in the sum- 

 mer, and very few immediately after a rainfall or snowfall. 

 The dust which is shaken from the hides and pelts of animals 

 is very liable to contain anthrax bacilli and their spores. 



In addition to the bacteria, we find also moulds and yeaste. 



The presence of micro-organisms in the air is determined 

 easily by exposing a dish containing sterile gelatin or agar for 

 a few minutes. Special media must be provided for those 



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