78 BACTERIOLOGY 



volatile substances possessing disinfectant powers chloro- 

 form and ether, for instance; these bodies, after exhibiting 

 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, ordinarily, 

 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 apparatus, etc., or where 

 destructive changes in the composition of the substances 

 to be treated, as in-the case of old cultures, infected tissues, 

 pathological exudates, feces, etc., are a matter of no conse- 

 quence. On the other hand, for the sterilization of all 

 materials to be used as culture media heat only is employed. 1 



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 constructed 

 oven this is known as dry sterilization; by subjecting 

 them to the action of streaming or live steam at the tem- 

 perature of 100 C.; and by subjecting them to the action 

 of steam under pressure, under which circumstance the 

 temperature to which they are exposed becomes more and 

 more elevated as the pressure increases. 



Experience has taught us that the process of sterilization 

 by dry heat is of limited application because of its many 



1 An occasional exception to this is the use of chloroform, mentioned above. 



