STERILIZATION OF CULTURE MEDIA. 57 
It is always advisable to test the sterilization of culture material 
before making use of it. This is done by placing it fora few days 
in an incubating oven at 30° to 35° C. If a considerable quantity of 
material in test tubes has been prepared at one time, it will be suffi- 
cient to put a few tubes in the incubating oven to test sterilization. 
Failure to make this test often leads to serious complications in 
experimental investigations. A laboratory sometimes becdmes in- 
fected with resistant spores, which are not all destroyed by the usual 
methods of sterilization, and these may not develop until some time 
has elapsed after the supposed sterilization. 
Sterilization of Blood Serum.—Blood serum which has been 
collected in test tubes or small flasks, as heretofore directed, is 
Fia. 28. 
sterilized in a water bath at 60° C. (140° F.) by the method of dis- 
continuous heating. It is usually left in the hot-water bath for 
about an hour, and thisis repeated, at intervals of twenty-four hours, 
for five to seven days. This rather tedious process may be avoided 
by collecting the serum in the first instance with proper precautions 
to prevent it from becoming contaminated with atmospheric organ- 
isms. <A special apparatus was devised by Koch for sterilizing blood 
serum, but an improvised hot-water bath which is regulated toa 
temperature of 60° C. by an automatic thermo-regulator will answer 
the purpose. After being sterilized the serum is solidified by careful 
exposure to a temperature of about 68° C., which causes it to co- 
agulate, forming a transparent, jelly-like mass. When coagulated 
at a higher temperature it becomes opaque. The time required for 
this operation varies from half an hour to an hour, and it is best to 
remove the tubes from the receptacle in which they are exposed ta 
