CHAPTER XII. 

 THE BACTERIOLOGIC EXAMINATION OF THE AIR. 



It has been repeatedly emphasized — and indeed at the 

 present time almost every one knows — that micro-organ- 

 isms float almost everywhere in the air, and that their 

 presence there is a constant source of danger, not only 

 of contamination in our bacteriologic researches, but 

 also a menace to our health. 



Such micro-organisms are neither ubiquitous nor equally 

 disseminated, but are much more numerous where the air 

 is dusty than where it is pure — much more so where men 

 and animals are accustomed to live, than upon the ocean 

 or upon high mountain-tops. The purity of the atmo- 

 sphere bears a distinct relation to the purity of the soil 

 over which its currents blow. 



The micro-organisms that occur in the air are for the 

 most part harmless saprophytes which have been sepa- 

 rated from their nutrient birthplace and carried about by 

 the wind. They are almost always taken up from dried 

 materials, experiment having shown that they arise from 

 the surfaces of liquids in which they grow with much dif- 

 ficulty. They are by no means all bacteria, and a plate 

 of sterile gelatin exposed for a brief time to the air will 

 generally grow moulds and yeasts as well as bacteria. 



The bacteria present are occasionally pathogenic, espe- 

 cially in localities where the discharges of diseased animals 

 have been allowed to collect and dry. For this reason the 

 atmosphere of the wards of hospitals and of rooms in 

 which infectious cases are being treated is much more 

 apt to contain them than the air of the street. However, 

 the dried expectoration of cases of tuberculosis, of in- 



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