40 METHODS OF CULTIVATION OF BACTERIA. 



must not be exposed to a temperature above 105 C, and 

 must be sterilised by the intermittent method. Certain 

 precautions are necessary in using the autoclave. In all 

 cases it is necessary to allow the apparatus to cool well 

 below 100 C., before opening it or allowing steam to blow 

 off, otherwise there will be a sudden development of steam 

 when the pressure is removed, and fluid media will be 

 blown out of the flasks. Sometimes the instrument is not 

 fitted with a thermometer. In this case care must be 

 taken to expel all the air initially present, otherwise a 

 mixture of air and steam being present, the pressure read 

 off the gauge cannot be accepted as an indication of the 

 temperature. Further, care must be taken to ensure the 

 presence of a residuum of water when steam is fully up, 

 otherwise the steam is superheated, and the pressure 

 on the gauge again does not indicate the temperature 

 correctly. 



B. (4) Sterilisation at Low Temperatures. Most 

 organisms in a non-spored form 

 are killed by a prolonged expos- 

 ure to a temperature of 57 C. 

 This fact has been taken advan- 

 tage of for the sterilisation 

 of blood serum, which will co- 

 agulate if exposed to a tem- 

 perature above that point. Such 

 a medium is sterilised on Tyn- 

 dalPs principle by exposing it 

 for an hour at 57 C. for eight 

 consecutive days, it being allowed 

 to cool in the interval to the 

 room temperature. The apparatus 

 used (Fig. 5) is a small hot-water 

 jacket heated by a Bunsen placed 

 beneath it, the temperature being 

 controlled by a gas regulator. 

 To ensure that the temperature 

 all round shall be the same, the 



FIG. 5. Steriliser for 

 blood serum. 



