DRY HEAT 29 



1. Roll each bougie up in a piece of asbestos cloth, 

 secure the ends of the cloth with a few turns of cop- 

 per wire, and place inside the muffle (a small muffle 

 76X88X163 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 appli- 

 cable 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 steril- 

 ised by dry heat short of charring as its power of 

 penetration is not great. 



Sterilisation by hot air is effected in the hot-air oven 

 (Fig. 1 8). This is a rectangular, double- walled metal 

 box, mounted on a stand and heated from below by a 

 large Bunsen burner. The interior of the oven is 

 provided with loose shelves upon which the articles 

 to be sterilised are arranged, either singly or packed 

 in square wire baskets or crates, kept specially for this 

 purpose. One of the sides is hinged to form a door. 

 The central portion of the metal bottom, on which 

 the Bunsen flame would play, is cut away, and replaced 

 by firebrick plates, which slide in metal grooves and 



