LESSON xii EFFECT OF FILTRATION 137 



(2) Filter through a sterile Berkefeld filter into a sterile 

 flask. 



(3) Prepare three gelatine plates from the filtrate, and 

 place them in the cool incubator. 



Examine them from day to day. 

 The plates should remain sterile. 



C. EFFECT OF USE ON A BERKEFELD FILTER. 



(1) Take 1000 cc. of sterile water, and add three broth 

 cultures of the Bacillus prodigiosus as above. 



(2) Filter through a sterile Berkefeld filter into a sterile 

 flask. 



(3) Pour the filtrate back into the filtering cylinder and 

 filter again. 



(4) Eepeat this process six times. 



(5) Eventually pour the filtrate back into the cylinder 

 and allow the filter to stand for a day. 



(6) Eepeat the whole process with the same filter on 

 three successive days, without cleaning it out in the mean- 

 time. 



(7) On the last day fix a fresh sterile flask to the filter, 

 and exhaust it with the air-pump. 



(8) From the final filtrate three gelatine plates should 

 be prepared in the usual manner. 



Examine them from day to day. 



The plates will show colonies of the Bacillus pro- 

 digiosus. 



