BACTERIA IN THE AIR. 



627 



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. 



FIG. 190. 



To determine in a more exact way the number of microorganisms 

 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 



