CULTIVATION OF BACTERIA. 



195 



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 special apparatus (Fig. 22) for coag- 

 ulating- blood-serum. The bottom should be covered 



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



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

 the temperature elevated until coagulation occurs. The 

 repeated sterilizations may be conducted in this appa- 

 ratus, or may be done equally well in the steam appa- 

 ratus, the cover of which is not completely closed, for if 

 the temperature of the serum is raised too rapidly it is 

 certain to bubble. 



Like other culture-media, blood-serum and its combi- 

 nations may be sterilized in the autoclave, thus saving 

 much time. The serum should be coagulated first, else 

 bubbling is apt to occur. The autoclave temperature is 

 apt to make the preparation very firm and hard, consider- 

 able fluid being pressed out of it. 



