DISTRIBUTION OF BACTERIA. 135 



Bacteria of the Air. The bacteria of the air will be 

 found for the most part clinging to solid particles in suspen- 

 sion in the shape of dust. As has already been stated, 

 bacteria will not rise from moist surfaces unless forcibly 

 removed, as by agitation or currents of air. Conditions of 

 dryness and wind tend to increase the number of micro- 

 organisms in the air. They are fewer after a fall of rain 

 or snow, and the number is smaller in winter than in sum- 

 mer. The air of cities contains more germs than that of 

 the country. The atmosphere over the sea and at the tops 

 of high mountains is nearly or wholly free from germs. 

 The bacteria which do occur in the air will seldom be 

 pathogenic. Their character will depend upon the character 

 of the dust. It is obvious that dust which consists in part 

 of the dried, pulverized expectoration of cases of pulmonary 

 tuberculosis may contain tubercle bacilli. Anthrax of the 

 lungs sometimes arises in men who handle the wool of 

 sheep that were infected with anthrax (Wool-sorter's dis- 

 ease) , and is due to the inhalation of anthrax spores attached 

 to the wool. It is likely that the atmosphere in the imme- 

 diate vicinity of cases of the exanthematous fevers may 

 contain the organisms, whatever they may be, that cause 

 these diseases. 



In a rough way one may obtain some knowledge of the 

 character of the organisms in the air of a given locality by 

 removing the cover of a Petri dish containing sterilized 

 gelatin or agar for a few minutes, replacing it, and allow- 

 ing the organisms to develop. In most cases a large pro- 

 portion of the growths that appear will be moulds. Yeasts 

 are also common, and among the bacteria the micrococci 

 are abundant. Chromogenic varieties are likely to be 

 present. 



A few studies of this character will show that the num- 

 ber of organisms that are present depends chiefly upon 



