12 LABORATORY BACTERIOLOGY 



16. The Preparation of Nutrient Gelatin. — Take a flask of 

 bouillon containing 300 c.c. and pour it into a small agate iron 

 dish and add 30 grams of sheet gelatin which has been cut 

 into small pieces, heat the bouillon, with frequent stirring, in a 

 water bath until the gelatin is dissolved. Allow it to cool to 

 a temperature between 45° and 50° C. and then add the white 

 of one egg and mix it thoroughly by stirring, or better by pour- 

 ing the gelatin many times from one flask or beaker to another. 

 After the egg albumen is completely diffused, return the liquid 

 gelatin to the large covered water bath and boil until the egg 

 albumen is coagulated. This takes about 20 minutes. It is 

 now ready for filtering, which must be done while the gelatin is 

 hot. Filter through properly folded but ordinary filter paper, 

 first moistened with boiling water. (For illustrations and direc- 

 tions for folding filter paper, see Abbott's " Prin. of Bact.," p. 9 7. 

 Filter paper already folded may be procured.) Distribute the 

 filtrate as directed. In pouring the gelatin into the tube use 

 a small beaker or graduate and see that the gelatin does not 

 touch the sides of the upper part of the tube. Stand the tubes 

 in a wire basket and sterilize them by boiling in a closed water 

 bath or by steaming in the Arnold's steam sterilizer for 30 

 minutes. The small flasks can be sterilized in the same man- 

 ner. Place tubes and small flasks in the incubator and allow 

 them to remain there for two days. If the gelatin in any of 

 the tubes becomes cloudy, the medium in those tubes must be 

 rejected. Carefully wipe all of the other tubes with a moist 

 cloth, label, and place them in the locker where they can be 

 kept until used. 



17. The Preparation of Nutrient Agar. — Weigh out 3 grams 

 of agar and cut it into small pieces with a pair of scissors. Put 

 the finely cut agar into an agate iron dish and add 50 c.c. of 

 distilled water and boil it over a gas flame with constant stir- 

 ring, to prevent scorching, until the agar is dissolved, giving a 

 thick homogeneous pasty substance. Pour 300 c.c. of bouillon 

 (11) from a flask into the cup containing the dissolved agar. 



