PART II 

 BACTEEIA IN AIR AND WATER 



CHAPTER VI 

 BACTERIA IN THE ATMOSPHERE 



THE air that surrounds our earth is never at a stand- 

 still. It is kept in motion by its waves and storms, and 

 is affected by season, climate, and proximity to human 

 habitation. Mingled with the gases and vapor of which 

 it is composed, there is a quantity, at times very great, 

 of dust particles, derived from the soil, the water, and 

 the streets of cities. These dust particles of varying 

 degree of fineness are borne hither and thither by the 

 changing winds, and, as they pass from place to place, 

 carry with them their minute bacterial passengers. 



The richness of the air in dust particles is easily per- 

 ceived as we watch the rays of the sun pass into a dark 

 room through a hole in the window curtain. The light 

 reflected from the countless number of these particles 

 shows them to be suspended and restless. The wind, as 

 it sweeps over the bare fields and roadsides, raises clouds 

 of dust, particles of soil and of vegetable and animal 

 materials, and carries them away on a more or less dis- 

 tant journey. The fine spray from the ocean, as it dashes 

 against the shore, and the spray from river and lake, 



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