192 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



Filtration through Cotton. A large glass funnel is lined with a double 

 layer of absorbent cotton; the layers are placed at right angles, 

 thus laying the fibers of cotton at right angles. The cotton is moistened 

 with a small amount of hot water if agar or gelatin is to be filtered. 

 The medium is then carefully poured into the funnel, care being taken 

 that the cotton is not displaced by the force of the inflowing fluid. The 

 first portion of filtrate may not be clear and it is somewhat diluted 



FIG. 14. Hot-air sterilizer. Lautenschlager form. (Park.) 



with the water originally used to wet the cotton hence it should be 

 returned and refiltered. Agar and gelatin filter slowly, which may 

 lead to congelation, therefore the top of the funnel should be covered 

 to prevent undue loss of heat. Funnels surrounded by a hot water 

 jacket are sometimes used in the filtration of these media. Media 

 that are fluid when cold may be often advantageously clarified by 

 filtration through a good grade of heavy filter paper, with or without 

 a preliminary clarification with eggs, as occasion demands. 



