i8 



MICRO-ORGANISMS AND DISEASE. [CHAP. 



clear, but if not it is filtered again. If not clear then, it is 

 allowed to stand for several hours. A fine sediment is found 

 at the bottom of the vessel, and from this the clear super- 

 natant fluid is decanted into a sterilised vessel. The broth, 

 if not clear after the first filtering, can be cleared by boiling 

 it with the broken shell 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 



FIG. 2. A BUNSEN BURNER WITH 

 ROSE FOR BOILING FLUIDS IN 

 TEST-TUBES. 



FIG. 3. A FLAXK CONTAINING STERILE 

 STOCK FLUID. 



this state the flask is placed over a Bunsen burner (Fig. 2) 

 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 two-thirds of its volume, to prevent the 

 broth from rising too much and wetting the plug. When 

 turning off the flame the plug 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 



