12 PRACTICAL BACTERIOLOGY 



The process of fractional sterilization at low temperatures is con- 

 ducted in a somewhat similar manner to TyndalPs method, but re- 

 quires a greater number of exposures to a temperature not exceeding 

 68 to 70 C., and is employed to sterilize easily decomposable 

 materials which would be rendered unfit for culture purposes by 

 steam such as blood serum. Sterilization by steam can also be 

 practised by what is known as the ' direct method, 1 by a single ex- 

 posure in the Koch Steam Sterilizer, but a prolonged exposure is 

 necessary at times several hours to obtain complete sterilization. 



When an autoclave is used, a single exposure of fifteen minutes is 

 sufficient to practically destroy all bacilli and their spores, provided 

 the thermometer registers 122 C. The autoclave when used must 

 either remain closed until cool, or until the gauge indicates that 

 pressure no longer exists, for if opened when the steam within is still 

 under pressure, the steam will escape so rapidly that all fluids within 

 the chamber thus suddenly relieved of pressure, will boil violently, and 

 as a rule boil quite out, of the tubes, blowing out the plugs. 



Open vessels before sterilization are closed with cotton-wool plugs 

 or stoppers. Glass plates and pipettes are sterilized in copper or 

 sheet-iron boxes specially manufactured for the purpose, and removed 

 when required. It is convenient to wrap each pipette in paper. 



Petri dishes are most conveniently sterilized in a special cylindrical 

 copper box with a capped lid, there being a round hole in the box 

 and in the lid. These holes are placed opposite each other during 

 sterilization, thus allowing a current of hot air to circulate in the 

 interior of the cylinder. When the process of sterilization is com- 

 pleted, the lid is slightly turned, closing the holes. The interior of 

 the cylinder is fitted with a special apparatus, enabling one or any 

 number of dishes to be withdrawn when required. 



STERILIZATION OF POST-MORTEM INSTRUMENTS. 



1. Wipe the dirty knife with a piece of cotton wool and dip in a 

 strong solution of caustic soda. 



2. Wash in alcohol, heat in a flame, boil in strong soda solution, or 

 place in the steam sterilizer. 



Instruments may be quite conveniently sterilized in the laboratory 

 by dipping them in benzine and inflaming the latter. The steriliza- 

 tion is thus completed without exposing the instruments to as high a 

 temperature as when they are drawn through the flame. 



