BACTERIOLOGICAL TECHNIQUE. 215 
Fermentation broth is made usually by adding 1 per 
cent. of glucose to the above. For accurate work the 
meat sugars are first extracted by allowing the colon 
bacillus to grow in the broth over night. The bouillon 
is then sterilized and the peptone and salt added, and 
the process already given gone through with. 
Fermentation bouillon is usually placed in a tube of 
special construction, known as a fermentation tube (see 
Fig. 14, p. 82). This is essentially a tube 1.5 em. in 
diameter, bent at an acute angle, closed at one end, and 
provided with a bulb at the other end, which latter 
should be large enough to receive all the fluid in the 
closed branch should gas in any considerable quantity 
collect there. The tube also serves a most important end 
in giving information as to the aérobic and anaérobic 
growth of the species under consideration, for the con- 
necting tube being constricted serves to prevent, to a 
great degree, the entrance of oxygen of the air into the 
closed branch, and the free oxygen in the medium is 
driven out by the heat during sterilization; from which 
it may be seen that growth in the bulb is aérobie and 
growth in the closed branch is anaérobic. For the study 
of fermentation alone small tubes may be inverted into 
larger ones or tubes may be bent on themselves. 
Nutrient Gelatin. To the bouillon already prepared 
as described add 10 per cent. of sheet gelatin and neu- 
tralize. Add the whites of two eggs for each litre and 
boil for a few minutes. Filter, place in tubes or flasks, 
and sterilize. Instead of adding gelatin to bouillon 
already prepared, it may be added to the meat infusion 
at the same time the peptone and salt were added in 
preparing nutrient bouillon as just described. 
Nutrient Agar. This is prepared by adding to stock 
