CHAPTER XIII. 

 BACTERIA IN AIR, SOIL, WATER AND MILK. 



BACTERIA IN AIR. 



THE bacteria naturally found in air are not pathogenic, 

 but consist usually of spore formers and moulds; in other 

 words, those having some resistance to dryness and sunlight. 

 There are more in the layers of the atmosphere near the 

 earth, circulating in air currents after being raised in dust. 

 When there are no currents, bacteria tend to settle on sur- 

 faces, as they are heavier than air. After rain storms the 

 atmosphere may be nearly free of organisms. More patUo 

 genie forms are found where people live or congregate, so 

 that air currents produced by human activities tend to 

 increase the bacteria floating about. Colon bacilli are 

 sometimes found in the air above streets. Organisms may 

 be carried in currents set up by coughing or blowing. This is 

 well shown by the fact that tubercle bacilli have been found 

 at a distance of twenty feet in front of a coughing consump- 

 tive. No organisms are expelled by quiet respiration. The 

 practical application of this principle is the use of moisture 

 in dusting or sweeping. Surfaces of a sick-room should be 

 wiped with a moist cloth, preferably using a disinfectant 

 that will not hurt the hands. Bacteria have no power to 

 leave a moist surface. They may be freed by the bursting 

 of bubbles of sputum or other infective fluid. They do not 

 travel far by this means if air currents be absent, and there 

 is probably little danger from simple unpleasant odors from 

 drains if protected from these currents. The only possible 

 danger from a faulty drain or sewer is when obstruction has 

 occurred and bubbles form from putrefactions thus liberating, 

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