Agar-agar 195 



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 through gauze and 

 cotton until clear. 



The finished gelatin, which is perfectly transparent and of an amber 

 color, is at once distributed into sterilized tubes and sterilized Kke 

 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 lessens its subsequent soUdifying power. To 

 overcome this evil it is recommended to plunge the freshly sterihzed 

 media into ice water as soon as it has slightly cooled. 



Gelatin becomes Uquid 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, and recently in the form of powder. 



The "Bacto-agar" made by the Digestive Ferments Co. of Detroit, 

 Michigan, is a very satisfactory preparation. It dissolves slowly in 

 boiling water with a resulting thick jelly when cold. The jelly, 

 which solidifies between 40° and 5o°C., cannot again be melted 

 except by the elevation of its temperature to the boiling-point. 

 The culture-medium made from agar-agar is nearly transparent. 

 In addition to its ability to liquefy and solidify, it has the advantage 

 of remaining soh'd at comparatively high temperatures so as to 

 permit keeping the cultures grown upon it at the incubation tem- 

 perature— j.e., 37°C., — at which temperature gelatin is always 

 liquid. 



It is prepared as follows: To 1000 cc. of bouillon made as described 

 above, preferably of meat instead of beef-extract, 10 to 15 grams of 

 agar-agar are added. The mixture is boiled vigorously for an hour 

 in an open pot over the direct gas flame or in the double boiler with 

 saturated calcium chlorid solution in the outside pot. After being 

 cooled to about 6o°C., and after the correction of the reaction by • 

 titration, an egg beaten up in water is added, and the hquid again 

 boiled until the egg-albumin is entirely coagulated, when it is filtered 

 through gauze and cotton. 



Ravenel* prepares agar-agar by making two solutions, one con- 

 sisting of the meat-infusion, but twice the usual strength, the other 

 the agar-agar dissolved in one-half the usual quantity of water. 

 The agar-agar is dissolved by exposure to superheated steam in the 



* "Journal of Applied Microscopy," June, 1898, vol. i, No. 6, p. 106. 



