256 



Microbes and You 



Devices for filtering known quantities of air for microbiological 

 analysis have been devised by Ruehle and Kulp, by Rettger, and 

 others. These technics are based upon siphoning water from a 

 graduated carboy, and as the water leaves the container, an 



Fig. 42. Electrostatic air sampler showing exposed petri dishes. 

 It employs a high voltage electrostatic field to precipitate organisms from 

 a given volume of air into two specially prepared petri dishes. {Courtesy 

 of the Lamp Division of General Electric Company, Schenectady, New 

 York.) 



equivalent volume of air passes through measured amounts of 

 sterile sand or sterile physiological saline. Plate counts are then 

 made of these trapped organisms using media designed to en- 

 courage the growth of specific organisms. Another device sucks 

 known volumes of air into a special centrifuge which spins the air 



