Number and Kinds in the Air 47 



appear on the young plants notwithstanding the most 

 thorough steriHzation of the seed and soil. 



Number of bacteria in the air. — The number of 

 bacteria in the atmosphere, while constantly aug- 

 mented from the various sources enumerated, does not 

 become very large in the open air. This circumstance is 

 readily accounted for by the inroads made upon the 

 bacteria by sunshine and drying. Already dry when 

 raised from the fields and the streets, the bacteria suffer 

 further from the direct light of the sun and succumb to 

 it sooner or later, unless they exist in the spore state. 



Pasteur's method of determination. — Pasteur's early 

 investigations concerning the bacterial content of air 

 demonstrated that the number of organisms in the atmos- 

 phere is not very large and that it is influenced by season, 

 climate, altitude and human activities. Pasteur pre- 

 pared large flasks partly filled with culture solution, 

 drew the neck out to a fine tube, and boiled the contents 

 until all of the air was expelled by the steam. The neck 

 of the flask was then sealed with the aid of a blast lamp, 

 and, on cooling the flask, a partial vacuum was created 

 in it by the condensation of the steam. Such sealed 

 flasks could be taken to any place the atmosphere of 

 which it was desired to examine. On breaking off the 

 neck of the flask, the latter was at once filled with air. 

 The neck was then sealed again. The bacteria, yeasts, 

 and molds present in the air drawn into the flask, soon 

 settled into the liquid and developed there, giving rise 

 to the characteristic turbidity. When the contents of 

 the flask remained clear, the conclusion was inevitable 

 that there were either no microorganisms in that par- 



