CHAPTER 'V. 
STERILIZATION. 
STERILIZATION is the process of killing micro- 
organisms. A body is said to be sterile when all 
the bacteria in or upon it have been killed or re- 
moved. Sterilization is effected by the application 
of heat, by treating with certain chemicals, or, in 
the case of fluids, by filtration. 
When spores are not present, bacteria are killed 
by exposure for twenty minutes to boiling water or 
steam. ‘To destroy spores, however, a much longer 
exposure is necessary, in some cases several hours. 
Steam under a pressure of thirty pounds will give 
a temperature of 120° C.; this will kill all organ- 
isms and spores in fifteen minutes. 
Dry heat is not so fatal to either bacteria or their 
spores as is moist heat or steam ; therefore to steril- 
ize by dry heat a higher temperature, sustained for 
a longer time, is necessary ; 150° C. continued for 
one hour will kill all ordinary bacteria and their 
spores. Liquids and objects liable to be injured by 
dry heat can be sterilized only by steam. 
The method of dzscontinuous sterilization is em- 
ployed in sterilizing objects which would be injured 
by long exposure to a high temperature. The ob- 
jects to be sterilized are first subjected to a temper- 
ature sufficiently high to destroy all bacteria not 
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