BACTERIA IN THE AIR. 625 



are borne upon projecting pedicles by which they are removed from 

 the moist material upon which and in which the mycelium develops 

 (Fig. 188), and, being dry, are easily carried away by currents of air. 



Bacteriologists have given much attention to the study of the mi- 

 croorganisms suspended in the atmosphere, with especial reference to 

 hygienic questions. The methods and results of these investigations 

 will be considered in the present section. 



Pasteur (1860) demonstrated the presence of living bacteria in the 

 atmosphere by aspirating a considerable quantity of air through a 

 filter of gun-cotton or of asbestos contained in a glass tube. By dis- 

 solving the gun-cotton in alcohol and ether he was able to demon- 

 strate the presence of various microorganisms by a microscopical ex- 

 amination of the sediment, and by placing the asbestos filters in 

 sterilized culture media he proved that living germs had been filtered 

 out of the air passed through them. 



FIG. 189. 



A method employed by several of the earlier investigators con- 

 sisted in the collection of atmospheric moisture precipitated as dew 

 upon a surface cooled by a freezing mixture. This was found to con- 

 tain living bacteria of various forms. The examination of rain water, 

 which in falling washes the suspended particles from the atmosphere, 

 gave similar results. 



The first systematic attempts to study the microorganisms of the 

 air were made by Maddox (1870) and by Cunningham (1873), who 

 used an aeroscope which was a modification of one previously de- 

 scribed by Pouchet. In the earlier researches of Miquel a similar 

 aeroscope was used. This is shown in Fig. 189. The opening to the 

 cylindrical tube A is kept facing the wind by means of a wind vane, 

 and when the wind is blowing a current passes through a small aper- 

 ture in a funnel-shaped partition which is properly placed in the 

 cylindrical tube. A glass slide, upon the lower surface of which a 



