PREPARATION OF NUTRIENT MEDIA. 65 



thermometer indicates 100 C., for twelve minutes 

 for four or five successive days ; or they may be 

 transferred to the test-tube water-bath and heated 

 for an hour a day for three successive days. 



If the gelatine shows any turbidity after these 

 processes, it must be poured back from the test- 

 tubes into a flask and boiled up for ten minutes, 

 after the addition of the white and shell of an egg- 

 beaten up together. It is once more filtered, and 

 the processes of sterilisation just described must 

 be repeated. 



Sterile Agar Agar Peptone broth, or 

 Nutrient Agar-Agar. Agar-Agar has the ad- 

 vantage of remaining solid up to a temperature of 

 about 45. The preparation of a sterile nutrient 

 jelly is conducted on much the same principles as 

 those already described. Instead, however, of 100 

 grammes of gelatine, only about 20 grammes 

 of agar-agar are employed (1*5 2 per cent.), and 

 to facilitate its solution it must be allowed to 

 soak in salt-water overnight. For the filtration, 

 flannel is substituted for filter paper, or may be 

 used in combination with the latter. The hot- 

 water apparatus is invariably employed, unless, 

 to accelerate the process, the glass funnel and 

 receiver are bodily transferred to the steam 

 steriliser. If the conical cap cannot be replaced, 

 cloths laid over the mouth of the steriliser must be 

 employed instead. It may be necessary to repeat 

 the process of filtration, but it must not be 



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