684 BACTERIA IN AIR, SOIL, WATER, AND MILK 



tions, even the dark places and imlighted corners of streets and habita- 

 tions 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 com- 

 paratively 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 importance of 

 this manner of conveyance in smallpox, in measles, in scarlet fever, 

 and in other exanthemata, can not be denied. As regards the dis- 

 eases 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 pneumonia, in influenza, in diph- 

 theria, and in meningitis is by no means clear and requires much fur- 

 ther study. The expulsion of bacteria from the lungs and naso-pharynx 

 does not take place during simple expiration, since an air-current pass- 

 ing 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. The bacteria thus discharged are then subject to the 

 process of drying and often are exposed to direct sunlight for a con- 

 siderable period before they are again taken up in the air. 



The methods of estimating the bacterial contents of the air are not 



