118 BACTERIOLOGY 



the ingredients are added at once and boiled for a long time 

 is to weigh out the necessary amount of agar-agar, 10 or 15 

 grams, and place this in 1300 or 1400 c.c. of water and boil 

 down over a free flame to 1000 c.c. The peptone, salt, and 

 beef-extract are then added and the boiling continued until 

 they are dissolved. The clarification with egg-albumen may 

 then be done, and usually the mass filters quite clear and 

 does not show the presence of precipitates upon cooling. 

 If the mixture is positively alkaline, it is not only cloudy, 

 but it filters with difficulty; if it is acid, it is usually quite 

 clear, and filters more quickly, but, as Schultz has pointed 

 out, it loses at the same time some of its gelatinizing 

 properties. 



Another method by which agar-agar can be easily and 

 quickly melted is by steam under pressure. If the flask 

 containing the mixture of bouillon and agar-agar be kept 

 in the digester or autoclave for ten minutes with the steam 

 under a pressure of about one atmosphere, as shown by the 

 gauge, the agar-agar will be found at the end of this time 

 completely melted, and filtration may then be accomplished 

 with but little difficulty. 



If glycerin is to be added to the agar-agar, it is done after 

 filtration and before sterilization. The nutritive properties 

 of the media for certain organisms, particularly the tubercle 

 bacillus, are increased by the addition of glycerin in the 

 proportion of 5 to 7 per cent. 



If after filtration a fine flocculent precipitate is seen, look 

 to the reaction of the medium. If it is quite alkaline, boil, 

 neutralize, and filter again. If the reaction is neutral or 

 only very slightly acid, dissolve and again clarify with egg- 

 albumen by the method given. 



The most important feature of all the media, aside from 



