56 MANUAL OF BACTERIOLOGY. 



minutes on each of three consecutive days. The object of 

 intermittent sterilization, as explained by Tyndall, who pro- 

 posed it, is this : The culture-medium may be supposed to con- 

 tain fully developed bacteria, and also bacteria in the spore or 

 resting stage. The first sterilization of fifteen minutes will 

 probably be sufficient to destroy all the fully developed bacteria; 

 during the twenty-four hours between the first and second 

 sterilization all of the spores which have survived the first 

 sterilization may be expected to have become fully developed 

 into bacteria which can be destroyed by the second steriliza- 

 tion; the third sterilization is directed against any spore forms 

 which may possibly have survived the second sterilization. 



Although the spore forms which are so extremely resistant 

 are non-pathogenic, as for example spores of the hay bacillus 

 and of the potato bacillus, they nevertheless are capable of 

 ruining the culture-media with which one works. 



It has been shown by Theobald Smith that the discontinuous 

 method cannot be relied upon to sterilize fluids in shallow layers 

 that are freely exposed to the air. For if the spores of anaero- 

 bic bacteria happen to be present in such fluids, they will not 

 develop into the adult form between the applications of heat, 

 under aerobic conditions. 



The sterilization of culture-media is usually effected by 

 seven to ten minutes' sterilization in the autoclave at fifteen 

 pounds' pressure. This is more certain than the fractional 

 method, and may be employed without injury for all media 

 except nutrient gelatin. The gelatinizing property of this is 

 interfered with by the high temperature of the autoclave. It is 

 not advised to sterilize media containing sugars in the auto- 

 clave. 



The form of sterilizer widely used in the United States is that 

 which is known as the Arnold Steam Sterilizer. 



The Arnold sterilizer consists of a cylinder of tin or copper 

 with a cover, which is enclosed in a movable, cylindrical outer 



