CULTIVATION OF BACTERIA. 113 



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 

 the culture. The serum thus prepared may be white, or 

 have a reddish-gray color if many corpuscles are pres- 

 ent, and is opaque. It cannot be melted, but once solid 

 remains so. 



Koch devised a very good apparatus (Fig. 18) for coag- 



FIG. 18. Koch's apparatus for coagulating and sterilizing blood-serum. 



ulating blood-serum. The bottom should be covered 

 with cotton, a single layer of tubes placed upon it, and 



