BACTERIA IN THE AIR AND SOIL 1011 



tions for the occurrence of many bacteria in air are the existence 

 of a prolonged drought followed by a dry wind. Under such condi- 

 tions, even the dark places and unlighted corners of streets and 

 habitations are thoroughly dried out, and bacteria are taken up 

 and carried about together with particles of dust. At such times 

 the dangers from inhalation are much multiplied. By experiments 

 made in balloons, it has been found that bacteria are plentiful below 

 altitudes of about fifteen hundred feet and may be present, though 

 much reduced in numbers, as high up as a mile above the earth's 

 surface. The species of bacteria found in the air are, of course, 

 subject to great variation, depending upon locality. Molds and 

 spore-forming bacteria, being more regularly resistant to the effects 

 of sunlight and drying than bacteria possessing only vegetative 

 forms, are naturally more generally distributed. 



Out of air thus laden with bacteria, they may again settle when 

 the wind subsides and the air becomes quiescent. The process of 

 settling, however, is extremely slow, since the weight of a bacterium 

 is probably less than a billionth of a gram, and it may be held in 

 suspension in air for considerable periods. Rains, snow, or even 

 the condensation of moisture from a humid atmosphere, hastens 

 this process considerably and large quantities of bacteria may settle 

 out from air, in a comparatively short time, in ice chests, in operating 

 rooms, or in other places in which much condensation of water vapor 

 takes place. 



The importance of the air as a means of conveying disease is 

 still a problem upon which much elucidation is needed. The im- 

 portance of fhis manner of conveyance in smallpox, in measles, in 

 scarlet fever, and in other exanthemata, can not be denied. As 

 regards the diseases of known bacterial origin, conveyance by air 

 is of importance in the case of tuberculosis, where infection by 

 inhalation may take place, and in the case of anthrax, where inhaled 

 anthrax spores may give rise to the pulmonary form of the disease. 

 The importance of air conveyance for any great distance in pneu- 

 monia, in influenza, in diphtheria, and in meningitis is by no means 

 clear and requires much further study. The expulsion of bacteria 

 from the lungs and naso-pharynx does not take place during simple 

 expiration, since an air-current passing over a moist surface is not 

 sufficient to dislodge microorganisms. Expulsion of bacteria in 

 these conditions must take place together with small particles of 

 moisture carried out in sneezing, coughing, or any forced expiration. 



