12 LABORATORY BACTERIOLOGY 
dish, add 30 grams of sheet gelatin, and heat, 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 pouring 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 firmly 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 which has been mois- 
tened with boiling water. 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 steam- 
ing in the Arnold steam sterilizer for 30 minutes. The small 
flasks can be sterilized in the same manner. Place tubes and 
small flasks in the incubator and allow them to remain there 
for 2 days. Ifthe gelatin in any of the tubes becomes cloudy, 
the medium in those tubes must be rejected. Carefully wipe 
all the other tubes with a moist cloth, label, and place them 
in a locker, where they may be kept until used. 
18. The preparation of nutrient agar. Weigh out 4.5 grams 
of agar and cut it into small pieces with a pair of scissors. 
Put the finely cut agar into an agate-iron dish, add roo cc. of 
distilled water, and boil over a gas flame, with constant stir- 
ring to prevent scorching, until the agar is dissolved, giving a 
thick, homogeneous, pasty substance. Pour 300 cc. of bouillon 
(§ 11) from a flask into the cup containing the dissolved agar. 
Place the dish containing the mixed agar and bouillon in a 
closed water bath and boil for 20 minutes, then cool it to a 
1 For illustrations and directions for folding filter paper, see Abbott’s 
Principles of Bacteriology, 6th edition, p. 101. Filter paper already 
folded may be procured. 
