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 
