142 



BACTERIOLOGY. 



The emptying continues by siphon action, and au- is drawn in along 

 the cylinder to replace the water. When the upper flask is empty, 

 the position of the two is reversed, and the flow again started. 

 When a sufiicient volume has been drawn through the cylinder, the 

 outer cap and the cotton-wool plug are replaced, and it is set aside 

 for the colonies to develop. As an example, twenty-five litres of air 

 from an open square in Berlin gave rise to three colonies of bacteria 

 and sixteen moulds ; on the other hand, two litres from a school- 



L-^ 



Fig. 71. — Hesse's Appakatus. 



room just vacated by the scholars gave thirty-seven colonies of 

 bacteria and thirty-three moulds. 



Porous substances, such as sand, powdered glass, or sugar, may 

 be used for the filtration of samples of air ; and an apparatus is 

 employed in a convenient form to be conveyed to the laboratory for 

 the subsequent examination. 



Petri's Apparatus consists of a glass-tube 9 cm. long, containing 

 two sand-filters separated from each other. A known volume of air 

 is aspirated through the tube. The bacteria are arrested and can 



