CHAPTER XIII. 

 BACTERIA IX AIR, SOIL, WATER, AND MILK. 



BACTERIA IN AIR. 



THE bacteria naturally found in air are not patho- 

 genic, 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 surfaces, as they are heavier 

 than air. After rain storms the atmosphere may be 

 nearly free of organisms. More pathogenic 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 

 has been found at a distance of twenty feet in front of 

 a coughing consumptive. 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. 



