20 MICRO-ORGANISMS AND FERMENTATION. 



a plug of asbestos. The contents of the flask can remain 

 for years without undergoing any change. During cooling 

 the air drawn into the flask is partially filtered through 

 the asbestos-plug ; the tube may, however, be open 

 while the cooling process is going on ; the germs are 

 deposited in the lowest part of the bend, or, at the 

 most, they do not pass the enlargement of the narrow tube, 

 and therefore do not come into contact with the liquid. 

 Hence, it is evident that the lower part of the bent tube 

 must be heated whenever the flask is to be agitated or 

 emptied through the straight tube, without exposing it to 

 contamination. If the flask is to be opened and placed in 

 connection with another flask, this must be effected either in 

 some small germ-free space, or the opening and connecting 

 must be done in a flame. A Bunsen burner is placed 

 directly in front of the operator, the flask to be emptied to 

 the left, and the one that is to receive the liquid or culture 

 to the right, close to the burner. Then the tube of the 

 left-hand flask is opened in the flame by quickly removing 

 the india-rubber tube with its glass stopper : while the open 

 tube is in the flame, the glass stopper of the flask to the 

 right is quickly withdrawn, and the hot tube of the first 

 flask is introduced into the india-rubber tube of the second 

 flask. The liquid is now poured into the 

 latter flask, the bent tube of the former flask 

 being at the same time heated. Then the 

 side tube of the left flask is again introduced 

 into the flame, while the stopper of the 

 right flask is heated and put back into its 

 place ; finally, the left flask is closed in the 

 flame with its tube and stopper. When the 

 operation is quickly performed, there is seldom 

 any danger of contamination. 



Pasteur flasks will be found indispensable 

 in certain operations ; as, for instance, in 

 PIG. 4. CHAMBERLAND physiological researches where one has to deal 



Flask. * J 



with large quantities of liquids. 



In recent years various other flasks and vessels have been 

 brought into use, notably the Chamberland flask (Fig. 4), the 



