STERILIZATION. 



6 9 



FIG. i 6. 



movable top, which is fastened down tightly during sterili- 

 zation. It is furnished with a thermometer, a pressure- 

 gauge, and a safety-valve which allows the steam to escape 

 if too high a pressure is attained. Heat is furnished by a 

 gas-burner underneath. The lower part of the cylinder 

 contains water. The objects to be sterilized are supported 

 above this water on a perforated bottom or shelf. 



It is necessary to follow certain precautions in the use of 

 the autoclave, especially during cooling. The apparatus 

 must not be opened while the steam contained within it is 

 still under pressure, as there may be a 

 sudden evolution of steam upon the re- 

 moval of the pressure which may blow 

 the media out of their tubes and flasks. 

 The apparatus must, therefore, be kept 

 closed until the gauge shows that the 

 atmospheric pressure is as great as the 

 pressure within, or, what is equivalent, 

 until the temperature has fallen to 100 

 C. Gelatin, especially, may be dam- 

 aged by sterilization with the autoclave, 

 if it be heated too long or to too high 

 a temperature. 



Sterilization by Filtration. Ordi- 

 nary filters are useless for this purpose, 

 but the tubes or bougies of unglazed 

 porcelain devised by Pasteur and Cham- 

 berland are effective when properly em- 

 ployed. The Berkenfeld filter employs 

 bougies made of infusorial earth, and its pores are larger than 

 those of the Pasteur filter. Both of these are made in several 

 grades according to the coarseness or fineness of the pores. 

 The coarser of these filters permit the passage of very small 

 bacteria. Bacteria of average size, like bacillus coli com- 



Kitasato Filter. 



