NUTRIENT CULTURE-MEDIA 



73 



having been placed in the thermostat, are kept at a temper- 

 ature of 65 to 68 C. until coagulation occurs; then removed 

 and sterilized by fractional sterilization. 



Sterilization of Blood-serum. The tubes are placed 

 three to four days in incubator at 58 C., and those tubes 

 which show any evidences of organic growth are discarded. 



If, now, at the end of a week, the serum remains sterile at 



Fig. 21. Thermostat or inspissator for blood-serum. 



the ordinary temperature of the room, it can be used for 

 experimental purposes. 



Perfectly prepared blood-serum is transparent, of a gelatin- 

 like consistence, and straw color. It will not liquefy by heat, 

 though bacteria can digest it. Water of condensation 

 always forms, which prevents the drying of the serum. 



Short Method. Blood-serum may be prepared in a shorter 



