THE EXAMINATION OF AIR 



331 



examination of air. Some observers simply expose plates 

 covered with nutrient medium to the air for a given time, 

 and then count and examine the various organisms as soon 

 as they have grown sufficiently well. Other investigators 

 have used various filtering materials, sugar, sand, etc. A 

 certam volume of the air to be examined is aspirated 

 through a tube containing one of these materials, which 



Fig. 31. — Hesse's Appaeatds. 



is afterwards treated with sterile water ; this is then examined 

 by the methods as given under ' The Examination of 

 Water.' The most satisfactory method is a modification of 

 the Esmarch roll culture, devised by Hesse. 



Hesse's Method. — The apparatus consists essentially of a 

 glass cylinder about 70 centimetres long and 3"5 centimetres 

 in diameter ; this tube is covered at one end by two rubber 



