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 

 aerobic or anaerobic 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 formulae 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 C0 2 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 



