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 65 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 



