202 



BACTERIOLOGY 



Fig. 37 



hydroxide solution gives the proportion of CO2 absorbed. 

 The explosive character of the residue is determined as 

 follows: the thumb is placed again 

 over the mouth of the bulb and the 

 gas from the closed branch is allowed 

 to flow into the bulb and mix with 

 the air there present. The plug is 

 then removed and a lighted match 

 inserted into the mouth of the bulb. 

 The intensity of the explosion varies 

 with the amount of air present in the 

 bulb." 



Durham's Fermentation-tube. — Dur- 

 ham employs a convenient modifica- 

 tion of the ordinary fermentation-tube, 

 which is constructed in the following 

 manner: test-tubes of about 10 or- 12 

 c.c. capacity are placed in an inverted 

 position within a larger test-tube, and 

 the latter plugged with cotton in the 

 usual way and sterilized. (See Fig. 

 37.) The small tube should fit loosely 

 within the larger one. The medium 

 to be used is run into the larger tube 

 until there is present about 50 per 

 cent, more than the volume of the 

 smaller tube. The whole is then 

 sterilized in streaming steam by the 

 fractional method. After the first 

 sterilization the small tube will be 

 found almost filled with fluid, over which a small air- 

 bubble lies. After the second or third sterilization this 



Durham's fermentation- 

 tube. 



