BACTERIA IN THE AIR. 617 
session of the surface of the culture medium before the others had 
formed visible colonies. Students will do well to employ this simple 
and satisfactory method for the purpose of making themselves familiar 
with the more common atmospheric organisms, and they will find 
the shallow glass dishes with a cover, known as Petri’s dishes, very 
convenient for the purpose. These dishes should be sterilized in the 
hot-air oven and sufficient sterile nutrient gelatin or agar poured 
into them to cover the bottom. After the culture medium has be- 
come solid by cooling, the exposure may be made by simply remov- 
ing the cover and replacing it at the end of the time fixed upon. 
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Fie. 188, 
To determine in a more exact way the number of microérganisms 
contained in a given quantity of air will require other methods. But 
we may say, en passant, that such a determination is usually not of 
great scientific importance. The number is subject to constant fluc- 
tuations in the same locality, depending upon the force and direction 
of the wind. If we have on one side of our laboratory a dusty 
street and on the other a green field, more bacteria will naturally be 
found when the wind blows from the direction of the street than 
when it comes from the opposite direction ; or, if the air is filled with 
dust from recently sweeping the room, we may expect to find very 
