GAS PRODUCTION 293 



pressure tubing (after removing the cotton-wool plug 

 from the rubber tube), as shown in Fig. 153. 



6. Give the three-way tap half turn to open com- 

 munication between flask and receiver, and seal all 

 joints by coating with a film of melted wax. When 

 the tap is turned, the mercury in the receiver will 

 naturally fall. 



7. Place the entire apparatus in the incubator. 

 (Two hours later, by which time the temperature of 

 the apparatus is that of the incubator, mark the 

 height of the mercury on the receiver.) 



8. Examine the apparatus from day to day and mark 

 the level of the mercury in the receiver at intervals 

 of twenty-four hours. 



9. When the evolution of gas has ceased, remove 

 the apparatus from the incubator ; clear out the wax 

 from the nozzle of the three-way tap (first adjusting 

 the tap so that no escape of gas shall take place) and 

 connect it with the Orsat. 



10. Remove, say, 100 c.c. of gas from the receiver, 

 reverse the tap and force it into the culture flask. 

 Remove 100 c.c. of mixed gases from the culture 

 flask and replace in the receiver. 



Repeat these processes three or four times to ensure 

 thorough admixture of the contents of flask and 

 receiver. 



1 1 . Now withdraw a sample of the mixed gases into 

 the Orsat and analyse. 



In calculating the results be careful to allow for the 

 volume of air contained in the flask at the commence- 

 ment of the experiment. 



For the collection of gases formed under anaerobic 

 conditions a slightly different procedure is adopted: 



1. Fix a culture flask (500 c.c. capacity) with a per- 

 forated rubber stopper carrying an L-shaped piece of 

 manometer tubing, each arm 5 cm. in length. 



2. Prepare a second L-shaped piece of tubing, the 



