30 METHODS OF CULTIVATION OF BACTERIA 



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source of heat is a large Bunsen beneath. The temperature 

 employed is usually 115 C. or 120 C. To boil at 115 C., 

 water requires a pressure of about 23 Ibs. to the square inch 

 (i.e. 8 Ibs. plus the 15 Ibs. of ordinary atmo- 

 spheric pressure). To boil at 120 C., a 

 pressure of about 30 Ibs. (i.e. 15 Ibs. plus 

 the usual pressure) is necessary. In such 

 an apparatus the desired temperature is 

 maintained by adjusting the safety-valve so 

 as to blow off at the corresponding pressure. 

 One exposure of media to such temperatures 

 for a quarter of an hour is amply sufficient 

 to kill all organisms or spores. Here, again, 

 care must be taken when gelatin is to be 

 sterilised. It must not be exposed to a 

 temperature above 105 C., and is best 

 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 super- 

 heated, 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 exposure to a 

 temperature of 57 C. This fact has been taken advantage of 

 for the sterilisation of blood serum, which will coagulate if 

 exposed to a temperature above that point. Such a medium is 

 sterilised on Tyndall's 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 shown in 

 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 around shall be the same, 



FIG. 4. Autoclave. 



a. Safety-valve. 

 6. Blow-off pipe. 

 c. Gauge. 



