~Dry Heat.~--The _naked flame_ of the Bunsen burner is invariably used for sterilising the platinum needles (which are heated to redness) and may be employed for sterilising the points of forceps, or other small instruments, cover-glasses, pipettes, etc., a very short exposure to this heat being sufficient. _Ether Flame._--In an emergency small instruments, needles, etc., may be sterilised by dipping them in ether and after removal lighting the adherent fluid and allowing it to burn off the surface of the instruments. Repeat the process twice. It may then be safely assumed that the apparatus so treated is sterile. [Illustration: FIG. 25.--Muffle furnace.] _Muffle Furnace_ (Fig. 25).--Although this form of heat is chiefly used for the destruction of the dead bodies of small infected animals, morbid tissues, etc., it is also employed for the sterilisation of porcelain filter candles (_vide_ p. 42). Filter candles are disinfected immediately after use by boiling in a beaker of water for some fifteen or twenty minutes. This treatment, however, leaves the dead bodies of the bacteria upon the surface and blocking the interstices of the filter. To destroy the organic matter and prepare the filter candle for further use proceed as follows: 1. Roll each bougie up in a piece of asbestos cloth, secure the ends of the cloth with a few turns of copper wire, and place inside the muffle (a small muffle 76×88×163 mm. will hold perhaps four small filter candles). 2. Light the gas and raise the contents of the muffle to a white heat; maintain this temperature for five minutes. 3. Extinguish the gas, and when the muffle has become quite cold remove the filter candles, and store them (without removing the asbestos wrappings) in sterile metal boxes. NOTE.--The too rapid cooling of the candles, such as takes place if they are removed from the muffle before it has cooled down to the room temperature, may give rise to microscopic cracks and flaws which will effectually destroy their efficiency. _Hot Air._--Hot air at 150° C. destroys all bacteria, spores, etc:, in about thirty minutes; a momentary exposure to a temperature of 175° to 180° C. will effect the same result and offers the more convenient method of sterilisation. This method is only applicable to glass and metallic substances, and the small bulk of cotton-wool comprised in the test-tube plugs, etc. Large masses of fabric are not effectually sterilised by dry heat--short of charring--as its power of penetration is not great.