STERILIZATION BY HEAT. 49 



to this are seen, however, in the case of certain volatile 

 substances possessing disinfectant powers, chloroform and 

 ether, for instance ; these substances, after performing 

 their germicidal activities, may be driven off by gentle 

 heat, leaving the media quite suitable for purposes of 

 cultivation. They are not, however, in general use in 

 this capacity. 



The circumstances under which chemical sterilization 

 or disinfection is practised in the laboratory are ordi- 

 narily, either those in which it is desirable to render ma- 

 terials free from danger that are not affected by the 

 chemical action of the agents used, such as glass appa- 

 ratus, etc., or where destructive changes in the compo- 

 sition of the substances to be treated, as in the case of 

 old cultures, infected tissues, etc., are a matter of no 

 consequence. On the other hand, for the sterilization 

 of all materials to be used as culture media, heat only is 

 employed. 



The two processes will be explained in this chapter, 

 beginning with 



STERILIZATION BY HEAT. 



Sterilization by means of high temperature is accom- 

 plished in several ways, viz., by subjecting the sub- 

 stance, to be treated to a high temperature in a properly 

 constructed oven, this is known as dry sterilization ; by 

 subjecting them to the action of streaming or live steam 

 at the temperature of 100 C. ; and by subjecting them 

 to the action of steam under pressure, under which 

 circumstances the temperature to which they are ex- 

 posed becomes more and more elevated as the pressure 

 increases. 



