30 METHODS OF CULTIVATION OF BACTERIA 



p o o o o o o 



are supported on a perforated diaphragm. The 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 atmospheric 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 main- 

 tained 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 tem- 

 peratures above 105 C., and is best sterilised 

 by the intermittent method. Certain pre- 

 cautions 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 develop- 

 ment 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 accurate 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 tempera- 

 ture 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. 



FIG. 4. Autoclave. 



a. Safety-valve. 



b. Blow-off pipe. 



c. Gauge. 



