STERILISA TION B Y HIGH-PRESSURE STEAM. 39 



steriliser, especially when a large bulk of medium is to be 

 sterilised, it is best to put the media in while the apparatus 

 is cold, in order to make certain that the whole of the food 

 mass reaches the temperature of 100 C. The period of 

 exposure is reckoned from the time boiling commences in 

 the water in the steriliser. At any rate allowance must 

 always be made for the time required to raise the tempera- 

 ture of the medium to that of the steam surrounding it. 



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. 



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 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 

 perforated diaphragm. The source of 

 heat is a large Bunsen beneath. The 

 temperature employed is usually 1 15 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 tern- 

 perature 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 sufficient 

 to kill all organisms or spores. Here, again, care 

 must be taken when gelatine is to be sterilised. It 



o o o o ooo 



FIG. 4. Autoclave. 



