GAS PRODUCTION BY BACTERIA 6 1 



day and the quantity of gas indicated. Note the bubbles of 

 gas rising through the -liquid to the top. When the gas pro-: 

 duction 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 reaction of the culture should be ' de- 

 termined and noted at this and the next exercise. Explain- 

 the chemical ; formulse for the production of the gas, and if 

 the reaction changes, give the explanation. 

 . Liquefy, two large tubes of gelatin and two of agar, and 

 pour the hquefied media from each tube into a Petri dish. 

 After it sets, place the dishes on the table in. the laboratory, 

 and remove the covers. Allow one each of the dishes contain- 

 ing gelatin and agar to be exposed for 5 minutes and the 

 others for 10 minutes. Label, and place the agar plates in 

 the incubator and the gelatin ones in the cabinet for gelatin 

 plates. Examine at the next exercise. 



94. Determine 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. 



95. To determine the Ratio of COj 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 



