STERILISATION BY MOIST HEAT. 



31 



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 

 1 1 5° C. or 120° C. To boil at 115° C, water re- 

 quires a pressure of about 23 lb. to the square 

 inch {i.e. 8 lb. plus the 15 lb. of ordinary atmos- 

 pheric pressure). To boil at i20°C., a pressure 

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

 In such an apparatus the desired temperature is maintained by 



Fig. 5. — Autoclave, 



a, safety valve. 



b, blow-oif pipe. 



c, gauge. 



