40 METHODS OF CULTIVATION OF BACTERIA. 



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. 



(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 inter- 

 val 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 lid also 

 is hollow and filled with water, 

 and there is a special gas burner 

 FIG. 5. steriliser for at tne side to heat it. 



blood serum. 



THE PREPARATION OF CULTURE MEDIA. 



The general principle to be observed in the artificial 

 culture of bacteria is that the medium used should approxi- 



