BACTERIOLOGICAL TECHNIQUE. 219 



tainly sterilized by putting them in an autoclave (Fig. 

 21) at 110 C. for from fifteen to thirty minutes on 

 two consecutive days. 



Gelatin is sterilized in the same manner except, as 

 already stated, the shorter times are used. Pro- 

 longed heating destroys the congealing properties of the 

 gelatin. 



Blood-serum may be sterilized by fractional sterili- 

 zation and remain fluid, or may be rendered solid by 

 the degree of heat used in sterilizing. 



For the sterilization of fluid serum it is requisite that 

 it be exposed to a temperature of from 62 to 66 C. 

 for one hour on each of six consecutive days. The 

 best apparatus for obtaining and maintaining this tem- 

 perature (about 65 C.) is a small and well-regulated 

 incubator or chamber surrounded by a water space, 

 into which the tubes and flasks containing serum are 

 to be put each day and in which they are to be left for 

 the prescribed time after having been warmed to the 

 desired temperature. 



Serum may be solidified and still remain translucent 

 at a temperature of 76 C., but when heated to a higher 

 degree a more definite coagulation takes place, and the 

 medium becomes opaque. Care must be taken in coagu- 

 lating blood-serum at the higher temperatures to run 

 the temperature up slowly and not to heat above 90 C. 

 until the serum has firmly coagulated; for unless these 

 precautions are taken ebullition is likely to occur, which 

 will lead to the formation of bubbles and an uneven- 

 ness of the surface upon which growth is to be obtained 

 and studied. Serum may be solidified at the tempera- 

 tures mentioned in an incubator, water-oven, or even 

 in an Arnold steam sterilizer, with the top covered by 



