84 BIOLOGY AND TECHNIQUE 



A and flows from there into the shallow receptacle B, formed by 

 the double bottom. The flame underneath rapidly vaporizes the 

 thin layer of water contained in B, and the steam rises rapidly, 

 coursing through the main chamber C. Steam which escapes through 

 the joints of the lid of this chamber is condensed under the hood 

 and drops back into the reservoir. Exposure to steam in such an 

 apparatus for fifteen to thirty minutes insures the death of the 

 vegetative forms of bacteria. 



In the sterilization of media by such a device, the method of 

 fractional sterilization at 100 C. is employed. The principle of this 

 method depends upon repeated exposure of the media for fifteen 

 minutes to one-half hour on three succeeding days. By the first 

 exposure all vegetative forms are destroyed. The media may then 

 be left at room temperature, or at incubator temperature (37.5 C.) 

 until the following day, when any spores which may be present will 

 have developed into the vegetative stage. These are then killed 

 by the second exposure. A repetition of this procedure on a third 

 day insures sterility. It must always be remembered, however, that 

 this method is applicable only in cases in which the substance to 

 be sterilized is a favorable medium for bacterial growth in which 

 it is likely that spores will develop into vegetative forms. 



Exceptionally the method may fail even in favorable media when 

 anaerobic spore-forming bacteria are present. Thus, it has been 

 observed that anaerobic spores, failing to develop under the aerobic 

 conditions prevailing during the intervals of fractional sterilization, 

 have developed after inoculation of the media with other bacteria, 

 when symbiosis had made their growth possible. Tetanus bacilli 

 have, in this way, occurred in cultures of diphtheria bacilli employed 

 for toxin production. 



In noting the time of an exposure in an Arnold sterilizer, it is 

 important to time the process from the time when the temperature 

 has reached 100 C. and not from the time of lighting the flame. 



The principle of fractional sterilization at low temperatures is 

 applied also to the sterilization of substances which can not be sub- 

 jected to temperatures as high as 100 C. This is especially the 

 case in the sterilization of media containing albuminous materials, 

 when coagulation is to be avoided, or when both coagulation of the 

 medium and sterilization are desired. 



In such cases fractional sterilization may be practiced in simply 

 constructed sterilizers, such as a Koch inspissator or, in the case 



