l6 LABORATORY GUIDE IN BACTERIOLOGY 



At the end of this period the gas is shut off and the 

 pressure allowed to decrease gradually. The valve 

 should not be opened, nor the lid removed, until 

 atmospheric pressure has been restored, otherwise the 

 sudden release of pressure would cause the media to 



boil suddenly and push 

 the plugs out of place. 



When large autoclaves 

 are used (Fig. 10) pro- 

 vision must be made for 

 proper circulation of air, 

 or "air cushions" form 

 and media will not be 

 sterilized. If an aperture 

 remains in the outlet the 

 discharge of air is facili- 

 tated. Attachment of a 

 vacuum pump is often 

 desirable to remove all air. 

 Blood serum or egg 

 media are the most diffi- 

 cult to sterilize. The tem- 

 perature of coagulation of 

 these media is relatively 

 low, and sudden heating 

 causes the mass to break 

 up, form bubbles, and be- 

 come useless for cultural purposes. The Koch inspis- 

 sator may be used, or, with certain precautions, the 

 autoclave. The Koch inspissator (Fig. n) allows the 

 tubes to rest in an inclined position and to be heated 

 gradually to 75 C. This temperature is maintained 



