GENERAL BACTERIOLOGY 
101 
distilled water, 2 c.c. of medium are measured, and to 
this 8 c.c. of redistilled water are added; 10 drops of 
indicator are also added to the tube, thoroughly mixed, 
and the color is compared against the scale, and, if 
found to be too acid, NaOH is added drop by drop, 
until the proper color is reached; the amount of NaOH 
necessary to bring the entire medium to the desired P H 
concentration is calculated, added, and the medium 
titration is completed. 
Tubing the Media. —After the media have been pre¬ 
pared they are poured into test tubes (about 10 c. c. 
into each) or into flasks (about 30 to 50 c.c. into each) ; 
this is usually done by pouring the medium into a fun¬ 
nel to the tip of which a rubber tubing had been fitted, 
with a pinchcock, and the tubes and flasks are so filled 
from this; a special apparatus has been devised (see 
Fig. 19) which consists of a large glass bulb with a 
glass outlet, a stopcock, and a gauge which permits to 
pour into the tubes or flasks equal amounts of the medium. 
Sterilization of Media. —The ordinary media which 
do not contain sugar, glycerin, gelatin' or animal serum, 
are sterilized in the steam pressure sterilizer (autoclave) 
at fifteen pounds pressure for fifteen to thirty minutes; 
the media containing substances which are apt to be in¬ 
jured by the high temperature must be sterilized by the 
fractional method (see the section on Destruction of Bac¬ 
teria) ; that is, by exposure for twenty minutes in Ar¬ 
nold’s sterilizer, on three successive days. 
The media containing animal serum must be sterilized 
in water-bath or hot-air sterilizer at the temperature of 
60° to 70° C. for half an hour on three successive days. 
Some media are best sterilized by passing through the 
