84 



VETERINARY BACTERIOLOGY 



used, however, in the sterilization of liquids or of any organic mater- 

 ial which might be decomposed at such a temperature. 



Sterilization by Streaming Steam. It is found in practice that 

 live steam is the most efficient sterilizing agent for many of the 

 media used in the laboratory. Steam under atmospheric pressure 

 at sea-level has a temperature of about 100. Some type of 

 apparatus is used such that the live steam comes in direct con- 

 tact with the material to be sterilized. One type of the apparatus 

 is called the Arnold steam sterilizer (Fig. 49). It consists essen- 

 tially of a pan with a double bottom opening into the sterilizing 



Fig. 48. Oven for sterilization by hot air (Jordan). 



chamber above. The water between the bottoms is quickly 

 heated to boiling temperature and is automatically replaced 

 from the supply on the exterior through small holes as rapidly 

 as it boils away. A single exposure to live steam for fifteen 

 minutes is sufficient to kill all vegetative bacteria, but spores 

 are not thus destroyed. It is customary, therefore, to heat for 

 fifteen minutes on one day, keep the medium for twenty-four 

 hours at a temperature suitable for the germination and devel- 

 opment of any spores present, then heat again for fifteen minutes 

 in the same manner. Those spores which have germinated will 



