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132                PATHOGENIC BACTERIA.

them. If these have any tendency to cling to the glass,
each one should be given a few violent twists, so as to
break away the fibrinous attachments. After this the
jars are carried to the laboratory and stood upon ice for
forty-eight hours, by which time the clots will have re-
tracted considerably, and a moderate amount of clear
serum can be removed by sterile pipettes and placed in

FIG. 22.—Koch's apparatus for coagulating and sterilizing blood-serum.

sterile tubes. If the serum obtained is red and clouded
from the presence of corpuscles, it may be pipetted into
sterile cylinders and allowed to sediment for twelve hours,
then repipetted into tubes. It is evident that such com-
plicated maneuvring will offer many possible chances of
infection; hence the sterilization of the serum is of the
greatest importance.

If it is desirable to use the serum as a liquid medium, it
is exposed to a temperature of 60° to £5° C. for one hour
upon each of five consecutive days. If it is thought best
to coagulate the serum and make a solid culture-medium,
it may be exposed twice, for an hour each time—or three
times if there is distinct reason to think it contam-
inated—to a temperature just short of the boiling-point.
During the process of coagulation the tubes should be
inclined, so as to offer a large surface for the growth of