1 68 PATHOGENIC BACTERIA. 



a third time, lest a few spores remain alive and capable 

 of spoiling the material. When properly sterilized in 

 this way, culture-media will remain free from contamina- 

 tion until time and evaporation cause them to dry up. 



Fig. 14. — Pasteur-Chamberland filter arranged to filter under pressure. 



If hermetically sealed, a sterile medium will keep indef- 

 initely. 



If it should be necessary to sterilize culture-media at 

 once, not waiting the three days required by the inter- 

 mittent method, it may be done by superheated steam in 



