STERILISATION BY HIGH-PRESSURE STEAM 29 



The temperature 



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B. (3) Sterilisation by Steam at High Pressure. This is 

 the most rapid and effective means of sterilisation. It is effected 

 in an autoclave (Fig. 4). This is a gun-metal cylinder supported 

 in a cylindrical sheet-iron case ; its top is fastened down with 

 screws and nuts and is furnished with a safety valve, pressure- 

 gauge, and a hole for thermometer. As in the Koch's steriliser, 

 the contents are supported on a perforated diaphragm. The 

 source of heat is a large Bunsen beneath, 

 employed is usually 115 C. or 120 C. 

 To boil at 115 C., water requires a pres- 

 sure 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 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 a 

 temperature above 105 C., and is best 

 sterilised by the intermittent method. Cer- 

 tain precautions are necessary in using the 

 autoclave. In all cases it is necessary to 

 allow the apparatus to cool well below 1 00 

 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 



FIG. 4. Autoclave. 



a. Safety-valve. 

 &. Blow-off pipe, 

 c. Gauge. 



