124 MANUAL OF BACTERIOLOGY. 



Bacteria of the Air. — ^The bacteria of the air will be found 

 for the most part clinging to solid particles in suspension 

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

 cannot be blown from moist surfaces, they are not removed 

 by currents of air. Conditions of dryness and wind tend 

 to increase the number of microorganisms in the air. They 

 are fewer after a fall of rain or snow, and the number 

 is smaller in winter than in summer. The air of cities 

 contains more bacteria than that of the country. The atmos- 

 phere over the sea and at the tops of high mountains is nearly 

 or wholly free from bacteria. The bacteria which do occur in 

 the air are seldom pathogenic. Their character depends upon 

 the character of the dust. It is obvious that dust which consists 

 in part of the dried, pulverized expectoration of cases of pul- 

 monary 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 disease), 

 and is due to the inhalation of anthrax spores attached to the 

 wool. The atmosphere in the immediate vicinity of cases of 

 the exanthematous fevers is liable to 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 or thin slices of boiled potato for a few minutes, replacing 

 it, and allowing the organisms to develop. In most cases a large 

 proportion of the growth that appears 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 number of 

 organisms that are present depends chiefly upon whether the 

 air is quiet or has recently been disturbed by draughts, gusts 

 of wind, or sweeping. These facts are of fundamental impor- 

 tance in laboratory work, where plate-cultures are being 



