STERILIZATION. 



57 



steam upon the removal of the pressure which may blow the 

 media out of their tubes and flasks. The apparati:is must, 

 therefore, be kept closed until the gauge shows that the atmos- 

 pheric pressure is as great as the pressure within, or, what is 

 equivalent, until the temperature has fallen to ioo° C. Gelatin, 

 especially, may be damaged by sterilization with the autoclave, 

 if it be heated too long or to too high a temperature. Media 

 containing sugar should not be sterilized 

 in the autoclave (see page 62). 



Sterilization by Filtration.— Ordi- 

 nary filters are useless for this purpose, 

 but the tubes or bougies of unglazed por- 

 celain devised by Pasteur and Chamber- 

 land are effective when properly employed. 

 They are made in several different 

 grades of porosity. In the Berkefeld filter 

 bougies made of infusorial earth are used, 

 and the pores in this are larger than those 

 of the Pasteur filter. The coarser of 

 these filters permit the passage of very 

 small bacteria. Bacteria of average size, 

 like bacillus coH communis, may grow 

 through the pores in the walls of both the 

 Berkefeld and Pasteur filters if suffi- 

 cient nutrient material is present to 

 permit of their multiphcation.* 



Filters of these kinds are widely used for water, and will 

 be spoken of in connection with the chapter on water. Simi- 

 lar tubes are employed for the filtration of certain organic 

 nutrient media whose ingredients would be damaged .by steril- 

 ization with heat, chiefly extracts of organs, such as the thymus 

 gland. The soluble "toxins" of bacteria may be obtained 

 by filtration of fluid-cultures through such tubes, which remove 

 * Wherry. Journal oj Medical Research. Vol. VIII. 1902. 



Fig. 16. — KiTASATo 

 Filter. 



