BACTERIOLOGIC EXAMINATION OF AIR. 229 



fluenza, and sometimes of pneumonia, causes the specific 

 bacteria of these diseases to be far from uncommon in 

 street-dust. 



Gunther points out that the majority of the bacteria 

 which occur in the air are cocci, sarcina being very 

 abundant. Most of them are chromogenic and do not 

 liquefy gelatin. It is unusual to find a considerable 

 variety of bacteria at a time ; generally not more than 

 two or three species are found. 



It is an easy matter to determine whether bacteria are 

 present in the air or not, all that is necessary being to 

 expose sterile plates or Petri dishes of gelatin to the air 

 for a while, close them, and observe whether or not bac- 

 teria grow upon them. 



To make a quantitative estimation is, however, much 



Fig. 44. — Hesse's apparatus for collecting bacteria from the air. 



more difficult. Several methods have been suggested, of 

 which the most important may be considered. 



The method suggested by Hesse is simple and good. 

 It consists in making a measured quantity of the air to 



