2 METHODS OF STERILISATION 



On removal from the oven, the cotton-wool plugs 

 will probably be slightly brown in colour. 



Metal instruments, such as knives, scissors, and 

 forceps, may be sterilised in the hot-air oven as de- 

 scribed above, but exposure to 175 C. is likely to 

 seriously affect the temper of the steel and certainly 

 blunts the cutting edges. If, however, it is desired 

 to sterilise surgical instruments by hot air, they should 

 be packed in a metal box, or boxes, and heated to 

 130 C. and retained at that temperature for about 

 thirty minutes. 



Moist Heat Wafer at 56 C. This temperature, if 

 maintained for thirty minutes, is sufficient to destroy 

 the vegetative forms of bacteria, but has practically 

 no effect on spores. Its use is limited to the sterilisa- 

 tion of such albuminous " fluid" media as would coagu- 

 late at a higher temperature. 



METHOD. 



1. Fit up a water-bath, heated by a Bunsen flame 

 which is controlled by a thermo-regulator, so that the 

 temperature of the water remains at 56 C. 



2. Immerse the tubes or flasks containing the albu- 

 minous fluid in the water-bath so that the upper level 

 of such fluid is at least 2 cm. below the level of the 

 water. (The temperature of the bath will now fall 

 somewhat, but after a few minutes will again rise to 

 56 C). 



3. After thirty minutes' exposure to 56 C., ex- 

 tinguish the gas, remove the tubes or flasks from the 

 bath, and subject them to the action of running water 

 so that their contents are rapidly cooled. 



4. The vegetative forms of bacteria present in the 

 liquid being killed, stand it for twenty-four hours in 

 a cool, dark place; at the end of that time some at least 

 of such spores as may be present will have germinated 

 and assumed the vegetative form. 



