EXAMINATION OF CULTURES 117 



determine what change has taken place. The most usual 

 change is the coagulation of the casein, giving a firm, solid 

 coagulum. Some organisms precipitate the casein. The 

 amount of clear serum over the coagulum should be noted. 

 Some organisms, like B. suipestifer, saponify the milk, 

 giving it a brownish, translucent appearance in old cultures. 

 The examination of bouillon cultures containing sugars in 

 fermentation tubes includes the determination of the extent 

 of the growth in the open and closed parts of the tube, the 

 presence or absence of gas, the rapidity with which it is 

 produced, and the reaction of the liquid. After the gas pro- 

 duction is completed, it is desirable to determine the quan- 

 tity and its approximate composition. 



Determination of the quantity of gas. It is desirable to 

 determine the quantity of gas collected in the closed branch 

 in terms of the capacity of the tube. To do this, measure the 

 length of the closed branch and the length of that portion 

 of the tube filled with gas. Thus, if the length of the tube is 

 10 cm. and the length of the portion filled with gas is 3 cm., 

 the gas fills three-tenths of the branch. This cannot be deter- 

 mined until the gas formation has ceased, which sometimes 

 requires several (4 to 6) days. The closed branch of the fer- 

 mentation tube should be straight and the connecting part of 

 the tube should be narrow. If the tube stands too long before 

 the quantity of gas is determined, some of it is liable to be 

 absorbed. To determine the ratio of C0 2 to H in the gas 

 produced remove the plug from the fermentation tube and 

 fill the open bulb with a 2% solution of caustic soda. Place 

 the thumb tightly over the open end of the tube and tip it up 

 so that the gas will pass through the liquid and come into 

 the open bulb. It is then returned. This should be repeated 

 several times. Remove the thumb when the open bulb is 

 full, and the liquid will rush up into the closed branch to fill 

 the space occupied by the C0 2 which has been absorbed by 

 the caustic soda. Measure the portion of the tube first occu- 

 pied with gas and now filled with the liquid. This will indi- 

 cate the quantity of C0 2 . The remainder of the gas is H. 



