EXAMINATION OF AIR, SOIL, AND WATER 



233 



This form of analysis, however, has not yielded any very 

 practical results, and is not much resorted to. 



Hesse's Method. Hesse's method requires an apparatus 

 called an aeroscope, which, by means of siphoning bottles 

 (aspirator), sucks air through 

 a cylinder lined with gelatin, 

 and by regulating the rate of 

 flow an approximate idea of 

 the number of bacteria per 

 liter of air can be obtained. 

 A less complicated method is 

 known as Petri's method. 



Fig. 115. Petri's sand-filter for 

 air-examination (McFarland). 



Fig. 116. Sedgwick's expanded 

 tube for air - examination (Mc- 

 Farland). 



Sand is sterilized by heating to redness, and while still 

 warm placed in test-tubes, which are then plugged. 



