92 LABORATORY GUIDE IN BACTERIOLOGY 



3. Expose a tube of unsteriUzed broth to steam in 

 the arnold, another to steam in the autoclave, at 

 120° C. for 5 minutes each 



4. Set aside in a thermostat, and observe the results. 



EXERCISE 3. PHENOMENA OF STERILIZATION (CON- 

 TINUED) 



Action of Berkefeld and cotton filters. — Berkefeld 

 filters are made of diatomaceous earth, and are porous 

 so as to allow the passage of fluids, while retaining 

 suspended solids, bacteria, etc. Some bacteria, known 

 as "ultramicroscopic bacteria," are so small that they 

 pass through Berkefeld filters. 



1. Arrange a Berkefeld filter so as to connect with a 

 suction pump, and filter a quantity of unsterilized 

 broth (Fig. 3). 



2. Set aside, and observe results. 



The filter (a), after having been connected with 

 the flask, is sterilized in the autoclave. The cotton 

 plug c prevents the air, which is sucked back, from 

 carrying germs into the flask. The flask d is an 

 intercepting or reflux flask, which guards against the 

 broth becoming contaminated from water being sucked 

 back if the pressure suddenly diminishes. 



EXERCISE 4 



Arrange a cotton filter as shown in Fig. 30. Vessel 

 a, provided with a rubber stopper (&) with two holes, is 

 arranged so as to have a glass tube (c) reach to the 

 bottom. This tube is provided with cotton at the' top 

 opening {d) and some nutritive medium (broth, e) is 

 placed inside. Through the other hole a bent glass 

 tube leads out, and this tube is also provided with a 



