138 BACTERIOLOGICAL ANALYSIS LESSON xn 



(9) Now clean the "candle" of the filter thoroughly, 

 scrubbing it with a brush, and sterilise the whole apparatus 

 in the autoclave. 



(10) When it has cooled down, filter through it 1000 cc. 

 of sterile water, to which three broth cultures of the Bacillus 

 prodigiosus have been added. 



(11) From the filtrate prepare three gelatine plates, 

 which should be kept in the cool incubator. 



Examine them from day to day. 

 The plates should remain sterile. 



The experiment proves that filters at best can do only 

 a limited amount of work, and must be cleansed 

 or sterilised from time to time. 



Other filters may be tested in the same manner, 



