58 ESSENTIALS OF BACTERIOLOGY 



Koch has invented a safety burner by which the gas supply is 

 shut off should the flame accidentally go out. 



Coagulation of Blood-serum. The tubes of blood-serum 

 having been placed in the oven, are kept at a temperature 

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

 sterilized. 



Fig. 25. Thermostat for blood -serum. 



Sterilization of Blood-serum. The tubes are placed three 

 to four days in incubation 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 the 

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

 mental 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. Blood-serum 



