Agar-agar 193 



2 grams of beef-extract in solution, 10 grams of peptone, 5 grams of 

 salt, and 100 grams of gelatin (" Gold label" is the best commercial 

 article) are added, and heated until the ingredients are dissolved. 

 The solution reacts strongly acid and must be corrected by titra- 

 tion, as already described. It must then be returned to the fire 

 and boiled for about an hour. As gelatin is apt to burn when boiled 

 over the direct flame, double boilers have been suggested, but unless 

 the outer kettle is filled with brine or saturated calcium chlorid solu- 

 tion, they are very slow, and when proper care is exercised there is 

 really no great danger of the gelatin burning. It must be stirred 

 occasionally, and the flame should be so distributed by wire gauze 

 or by placing a sheet of asbestos between it and the kettle as not to 

 act upon a single point. At the end of the hour the albumins 

 of the meat-infusion will be coagulated and the gelatin thoroughly 

 dissolved. Giinther has shown that the gelatin congeals better 

 if allowed to dissolve slowly in warm water before boiling. As 

 much water as has been lost by vaporization during the process of 

 boiling should be replaced. It is well to cool the liquid to about 

 6o°C., add the water mixed with the white of an egg to clear the 

 liquid, boil again for half an hour, and filter. 



If the filter paper be of good quality, properly folded (pharma- 

 ceutic filter), wet with boiling water, and if the gelatin be properly 

 dissolved, the whole quantity should pass through before cooling 

 too much. Should only half go through before cooling, the re- 

 mainder must be returned to the pot, heated to boiling once more, 

 and then passed through a new filter paper. As a matter of fact, 

 gelatin usually filters readily. A wise precaution is to catch the 

 first few centimeters in a test-tube and boil them, so that if cloudi- 

 ness show the presence of uncoagulated albumin, the whole mass 

 can be boiled again. The finished gelatin, which is perfectly trans- 

 parent and of an amber color, is at once distributed into sterilized 

 tubes and sterilized like the bouillon by the intermittent method. 

 The sterilization can also be satisfactorily performed by the use of 

 the autoclave at iio°-ii5°C. for fifteen minutes, but this method is 

 probably less well adapted to the sterilization of gelatin than of the 

 other media, as the high degree of heat injures its subsequent solidi- 

 fying power. 



Gelatin becomes liquid at 37°C. It cannot, therefore, be used 

 with advantage for cultures that must be kept at body temperatures. 



AGAR-AGAR 



Agar-agar is the commercial name of a preparation made from a 

 Ceylonese sea-weed. It reaches the market in the form of long 

 shreds of semi-transparent, isinglass-like material, less commonly in 

 long bars of compressed flakes, rarely in the form of powder. It 

 dissolves slowly in boiling water with a resulting thick jelly when 



