STERIL1SA TION B Y HIGH-PRESSURE STEAM. 39 



o o o 



is reckoned from the time the ebullition commences in the 

 water in the steriliser. At any rate allowance must always 

 be made for the time required to raise the medium to the 

 temperature in which it is placed. 



If we wish to use such a substance as blood serum as a 

 medium, the albumin would be coagulated by a temperature 

 of 100 C. Therefore other means have to be adopted in 

 this case. 



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

 the most rapid and effective means of 

 sterilisation. It is effected in an auto- 

 clave (Fig. 4). This is a copper cylinder 

 on legs, the top of which 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 steri- 

 liser, the contents are supported on a per- 

 forated 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 1 5 Ibs. of ordinary 

 atmospheric pressure). To boil at 120 

 C., a pressure of about 30 Ibs. (i.e. 1 5 Ibs. 

 plus the usual pressure) is necessary. When 

 in such an apparatus the desired tempera- FIG. 4. Autoclave, 

 ture has been reached, the latter is a. Safety valve, b. 

 maintained by adjusting the safety valve * 

 so as to blow off. One exposure of media to such 

 temperatures for a quarter of an hour is sufficient to 

 kill all organisms or spores. Here, again, care must be 

 taken when gelatine is to be sterilised. It must not 

 be exposed to a temperature above 105, 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 



