ISOLATION AND PURE-CULTURE METHODS 19 



spores, and consequently they may not germinate between the 

 successive sterilizations. On the other hand, if the medium is 

 deep in the vessel, and the exposed surface of the medium' small, 

 much less oxygen gains access, and the spores of anaerobic forms 

 pass more readily into the vegetative condition and are killed by 

 the successive sterilizations. 



Sterilization under pressure. A great time-saving convenience 

 in sterilization is to be found in the use of the autoclave, or steam 



Fig. 5 j5. Autoclave, Horizontal Type, connected with Steam Pipes 

 (All steam apparatus under a hood) 



pressure sterilizer, two types of which are shown in Fig. 5 , a and b. 

 The autoclave is not only more effective than the ordinary steam 

 sterilizer, but by using it the delay of discontinuous sterilization is 

 avoided. In this apparatus the steam is confined, up to any pres- 

 sure desired, instead of being allowed to escape, as in the ordinary 

 steam sterilizer. A good steam pressure gauge on the autoclave 

 is requisite, and a thermometer is not only desirable, but also an 

 additional safeguard. The temperature ordinarily employed is 1 1 5 ,J 

 to 125" C, or about 10 to 20 lbs. pressure. A single incubation 



