2. Close the oven door, and open the ventilating slide, in order that any moisture left in the tubes, etc., may escape; light the gas below; set the electric alarm to ring at 100°C. 3. When the temperature of the oven has reached 100°C., close the ventilating slide; reset the alarm to ring at 175°C. 4. Run the temperature up to 175°C. 5. Extinguish the gas at once, and allow the apparatus to cool. 6. When the temperature of the interior, as recorded by the thermometer, has fallen to 60°C.--_but not before_--the door may be opened and the sterile articles removed and stored away. NOTE.--Neglect of this precautionary cooling of the oven to 60° C. will result in numerous cracked and broken tubes. 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 described 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.~--_Water 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 sterilisation of such albuminous "fluid" media as would coagulate 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 albuminous 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, extinguish 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. 5. Destroy these new vegetative forms by a similar exposure to 56° C. on the second day, whilst others, of slower germination, may be caught on the third day, and so on. 6. In order to ensure thorough sterilisation, repeat the process on each of six successive days. This method of exposing liquids to a temperature of 56° C. in a water-bath for half an hour on each of six successive days is termed _fractional sterilisation_.