CUL 77 VA TION OF BA CTERIA . 127 



For the preparation of bouillon, as well as gelatin, 

 agar-agar, and glycerin agar still to be described, beef- 

 extract (Liebig's) may be employed, but for the most 

 delicate work this is rather undesirable, because of its 

 unstable composition and because of the precipitation of 

 meat-salts, which can scarcely be filtered out of the agar- 

 agar, owing to the fact that they only crystallize when 

 the solution cools. When it is desirable to prepare the 

 bouillon from beef-extract, the method is very simple. 

 To 1000 c.cm. of clean water 10 grams of Whitte's dried 

 beef-peptone, 5 grams of sodium chlorid, and about 2 

 grams of beef-extract are added. The solution is boiled 

 until the constituents are dissolved, neutralized, if neces- 

 sary, and filtered when cold. If it is filtered while hot, 

 there is always a subsequent precipitation of meat-salts, 

 which clouds it. 



Bouillon and other liquid culture-media are best dis- 

 pensed and kept in small receptacles test-tubes or flasks 

 in order that a single contaminating organism, should 

 it enter, may not spoil the entire bulk. A very con- 

 venient simple apparatus used by bacteriologists for fill- 

 ing tubes with liquid media is shown in Figure 21. It 

 need not be sterilized before using, as the culture-medium 

 will be sterilized by the intermittent method after the 

 tubes are filled. The test-tubes and flasks into which 

 the culture-medium is filled must, however, be previously 

 sterilized by dry heat. The dry-heat sterilization is done, 

 of course, after the cotton plugs are in place. 



Bouillon is the basis of most of the culture-media. 

 The addition of 10 per cent, of gelatin makes it "gela- 

 tin ;" that of i per cent, of agar-agar makes it "agar- 

 agar. ' ' The preparation of these media, however, varies 

 somewhat from that of the plain bouillon. 



Gelatin. The culture-medium known as gelatin has de- 

 cided advantages over the bouillon, not only because it is 

 an excellent food for bacteria, and, like the bouillon, trans- 

 parent, but because it is also solid. Nor is this all : it is 

 a transparent solid which can be made liquid or solid at 



