236 



The Bacteriology of the Air 



melted gelatin it can be rolled out into a film like that of an Esmarch tube. 

 This cylindric expansion is divided into squares which make the counting of the 

 colonies very easy. 



Roughly, the number of germs in the atmosphere may be estimated at from 

 100 to 1000 per cubic meter. 



The bacteriologic examination of air is of very little importance 

 because of the numerous errors that must be met. Thus, when the 

 air of a room is quiescent it may contain very few bacteria; let some 



Fig. 81. Petri's sand filter for air- 

 examination. 



Fig. 82. Sedgwick and Tucker's ex- 

 panded tube for air-examination. 



one walk across the floor so that dust rises, and the number of bac- 

 teria becomes considerably increased; if the room be swept, the in- 

 crease is enormous. From these and similar contingencies it be- 

 comes very difficult to know just when and how the air is to be ex- 

 amined, and the value of the results is correspondingly lessened. 



The most sensible studies of the air aim rather at the discovery 

 of some definite organism or organisms than at the determination 

 of the total number per cubic meter. 



