28 



many even at a much lower temperature. Thus heating on the 

 first day will kill all non-spored forms. During the twenty-four 

 hours' interval the media is kept at 22 C. in order that spores if 

 present may develop into vegetative forms and be killed during 

 the second heating. In case any of the spores have been slow in 

 developing and thus escaped the second heating the process is 



repeated a third time. Generally 

 speaking this method gives good 

 results ; it is advisable, however, 

 to prolong the heating of media 

 in flasks to half an hour. 



The "Arnold " steam sterilizer 

 (Fig. 7) has almost entirely dis- 

 placed that introduced by Koch 

 for this form of sterilization. It 

 is constructed with a false bottom 

 so that a small quantity of water 

 may be heated to produce steam 

 quickly. A perforated tin dia- 

 phragm permits the steam to 

 stream up and surround the ob- 

 jects placed on it. As the steam 

 rises it passes through an opening 

 in the top of the inner jacket 

 and descends as water of con- 

 densation to again feed the water 

 below. By this method as by 

 that of autoclave sterilization 

 no evaporation takes place from 



the media because it is surrounded with an atmosphere already 

 saturated with moisture. 



An ordinary kitchen steamer over a pot of boiling water may be 

 used if an Arnold sterilizer is not available. 



For the second method, i.e. heating at a lower temperature 

 on six consecutive days, an inspissator (Fig. 8) is generally 

 used. 



FIG. 7. Arnold Sterilizer. 



