STERILIZATION 37 



vessels containing the media to be sterilized have been 

 placed within it, screw down the lid. The burners for 

 heating the apparatus should now be lighted. Leave 

 the stopcock open until steam begins to escape freely, 

 then close it, and allow the pressure to rise until the 

 gauge indicates I 5 lb., or a temperature of 1 1 5 to I 2 5C. 

 is reached. Lower the gas- 

 burners, and allow the tempera- 

 ture to remain at the figure just 

 mentioned for half an hour ; 

 then turn off the gas, and cool 

 the autoclave to below 100 C. 

 before opening the stopcock. 



Where the use of the auto- 

 clave is prohibited on account of 

 expense or for other reasons, 

 liquids may be rendered sterile 

 by the process of intermittent 

 sterilization at a temperature of 

 1 00 C. or less. This is effected 

 by heating to 1 00 C. for twenty 

 minutes on three or four succes- 

 sive days. The first heating 

 kills all bacteria which are in FlG - 4.-steam sterilizer. 

 an active growing state, only spores present retaining 

 their vitality. The latter are supposed to germinate 

 in the cool intervals, the organisms arising from them being 

 destroyed by the subsequent heating which they receive ; 

 possibly the alternate heating and cooling on successive 

 days may destroy ungerminated spores. 



For intermittent sterilization of media a steam sterilizer is 

 used (Fig. 1 4). It consists of an iron or copper cylindrical 

 vessel, covered on the outside with felt, asbestos or other 



