STERILISATION BY MOIST HEAT. 



that all bacteria in a non-spored form are killed by the tempera- 

 ture of boiling water, while if in a spored form they may not 

 be thus killed. Thus by the sterilisation on the first day all the 

 non-spored forms are destroyed the spores remaining alive. 

 During the twenty-four hours which intervene before the next 

 heating, these spores, being in a favourable medium, are likely 

 to assume the non-spored form. The next heating kills these. 

 In case any may still not have changed their spored form, the 

 process is repeated on a third day. Experience shows that 

 usually the medium can now be kept indefinitely in a sterile con- 

 dition. Steam at 100 C. is therefore under most conditions 

 available for the sterilisation of all ordinary media. In using 

 the Koch's steriliser, especially when a large bulk of medium is 

 to be sterilised, it is best to put the media in while the appa- 

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

 mass reaches the temperature of 100 C., and it is preferable 

 to prolong the period of exposure to half an hour, always reck- 

 oning from the time boiling commences in the water in the 

 steriliser. 



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

 dium, 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 Pres- 

 sure. This is the most rapid and effective 

 means of sterilisation. It is effected in an 

 autoclave (Fig. 5). This is a gun-metal cylin- 

 der 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 steriliser, the 

 contents are supported on a perforated dia- 

 phragm. 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 re- 

 quires a pressure of about 23 Ib. to the square 

 inch (i.e. 8 Ib. plus the 1 5 Ib. of ordinary atmos- 

 pheric pressure). To boil at I2OC, a pressure 

 of about 30 Ib. (i.e. 15 Ib. plus the usual pressure) is necessary. 

 In such an apparatus the desired temperature is maintained by 



OF THE 

 UNIVERSITY 



OF 



00 0000 



FIG. 5. Autoclave. 



a, safety valve. 



b, blow-off pipe. 



c , gauge. 



