16 MICRO-ORGANISMS IX WATER 



sulphur normally present in a litre of the various 

 culture media employed, thus : 



1 As used for purposes of cultivation. 



The composition of the culture materials is of 

 course by no means absolutely constant, for the extrac- 

 tion of the juices from the beef must vary as well as 

 the proportion of pure muscular material present in the 

 latter. 



Agar-agar. For the preparation of agar-agar jelly 

 20 grams of agar-agar is employed in the place of 

 the 100 grams of gelatine used in the ordinary gela- 

 tine-peptone medium described above. The agar-agar 

 must be cut up into as small pieces as possible, as this 

 materially facilitates its solution. 



The composition of the nutrient agar medium is 

 1 litre of beef extract, 20 grams of agar-agar, 10 grams 

 peptone, 5 grams common salt. This mixture is heated 

 for a number of hours in the steam steriliser until the 

 agar-agar has completely dissolved. The liquid is then 

 rendered faintly alkaline with carbonate of soda, clari- 

 fied with white of egg, strained through a piece of 

 linen, and finally filtered, all exactly in the same way 

 as already described for the gelatine peptone, excepting 

 that the filtration of the agar-agar may be carried on 

 at a higher temperature in the steam-steriliser, as the 

 melting-point of the agar-agar is not thereby reduced. 

 When in a fluid condition the agar looks transparent 

 and of a rather darker yellow-brown colour than gela- 

 tine, but when it solidifies it loses its complete trans- 



