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BACTERIOLOGY. 



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

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

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

 When a sufficient 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- 



FiG. 71. — Hesse's Apparatus. 



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 Appa/ratus 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 



