FERMENT A TION. 



219 



FIG. 38. 



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 modification 

 of the ordinary fermentation-tube, which 

 is constructed in the following man- 

 ner : test-tubes of about 10 or 12 c.c. 

 capacity are placed in an inverted posi- 

 tion within a larger test-tube, and the 

 latter plugged with cotton in the usual 

 way and sterilized. (See Fig. 38.) 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 air-bubble is 

 completely expelled, and the small tube 

 contains nothing but the liquid. 



The medium that Durham employs 

 for the fermentation-test is a 1 per cent, 

 solution of Witte's peptone in distilled 

 water, to which have been added known amounts of 

 some such fermentable sugar as glucose, saccharose, 

 lactose, mannite, etc., as the case may demand. He 



