70 



BIOLOGY AND TECHNIQUE 



laboratory purposes, the original steaming device introduced by Koch 

 has been almost completely displaced by devices constructed on the 

 plan of the so-called " Arnold " sterilizer (Fig. 9) . In such an appara- 

 tus, water is poured into the reservoir 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 sterili- 

 zation at 100 C. is employed. The prin- 

 ciple of this method depends upon 

 repeated exposure of the media for fif- 

 teen 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 tem- 

 perature, 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 ex- 

 posure. 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 vege- 

 tative forms. 



Exceptionally the method may fail even in favorable media when 

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

 served that anaerobic spores, failing to develop under the aerobic con- 

 ditions prevailing during the intervals of fractional steriftzation, have 

 developed after inoculation of the media with other bacteria, when sym- 

 biosis had made their growth possible. Tetanus bacilli have, in this way, 

 occurred in cultures of diphtheria bacilli employed for toxin production. 



FIG. 9. ARNOLD STERILIZER. 



