THE BACTERIA OF SOIL, AIR, WATER, ETC. 137 



Bacteria of the Aij-.— The bacteria of the air will be found 

 for the most part clinging to solid particles in suspension in the 

 shape of dust also, as shown by Fliigge, to particles of sputum 

 thrown out by efforts in coughing. The minute air bubbles 

 thrown off in this way remain suspended for some time. Still, 

 as has already been stated, bacteria cannot be blown from 

 moist surfaces, they are not removed by currents of air. Con- 

 ditions 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 smaher in winter than in summer. 

 The air of cities contains more bacteria than that of the 

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

 mountains is nearly or wholly free from bacteria. The bac- 

 teria which do occur in the air are seldom pathogenic. Their 

 character depends upon the character of the dust. It is ob- 

 vious 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 arise 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 im- 

 mediate 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 oranisms in the air of a given locality by re- 

 moving the cover of a Petri dish containing sterilized gelatin 

 or agar or thin slices of boiled potato for a few minutes, re- 

 placing 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 



