28 



MICRO-ORGANISMS AND DISEASE [chap. 



and white of egg. The now clear fluid is filtered again. 

 The flasks which receive the broth are well plugged with 

 sterilised cotton-wool (see below). In this state the flask is 

 placed over a Bunsen burner on a wire netting, and boiled 

 for half an hour or more ; during the boiling the cotton- 

 wool plug is lifted out for half its length. The flask ought 

 not to contain more broth than about one-half or one-third 

 of its volume, to prevent the broth from rising too much 

 and wetting the plug. When turning off the flame the plug 



Fig. 3.— Two forms of Flasks containing Fluid Nutritive Media. 



is pushed down so as fully to plug the neck and mouth of 

 the flask ; a beaker with sterile cotton-wool cap is placed 

 over the mouth of the flask, and this is allowed to stand for 

 one night. Next day the boiling is repeated for half an 

 hour or more in the same manner as before. If the meat 

 has been fresh and the vessels and cotton-wool have been 

 sterile, twice boiling is found sufficient to destroy every 

 impurity. But to make sure, the broth is placed in the 

 incubator and kept there for twenty-four hours at a tern- 



