50 LABORATORY BACTERIOLOGY 
absence of gas in the closed branch. Indicate the quantity 
and note its rate of formation from time to time. Test the 
reaction of the liquid with litmus paper. 
The fermentation tubes are also used to enable one to 
determine the quantity and kinds of gases produced and the 
aérobic or anaérobic tendencies of the organism. 
In studying the cultures in the fermentation tubes they 
should be examined each day and the quantity of gas indi- 
cated. Note the bubbles of gas rising through the liquid to 
the top. When the gas production has ceased, the liquid 
begins to clear near the surface in the closed branch. The 
final record should not be made until this occurs. The reac- 
tion of the culture should be determined and noted at this 
and the next exercise. Explain the chemical formule for the 
production of the gas, and if the reaction changes, give the 
explanation. 
77. 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. 
78. To determine the ratio of CO, to H in the gas produced. 
This can be approximately determined by the use of caustic 
soda. 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 
