STERILIZATION BY HEAT. 49 



medium. Exceptions to this are seen, however, in the 

 ease of certain volatile substances possessing disinfect- 

 ant powers — chloroform and ether, for instance; these 

 bodies, 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 

 materials 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 articles 

 to be treated to a high temperature in a properly con- 

 structed 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 



' An exception to this is the use of chloroform, a volatile disinfectant, that 

 may easily be eliminated after having exercised its germicidal properties. 

 This is, however, not a commonly employed method. 



