STERILIZATION BY DRY HEAT 



2. Paper covers. For wrapping up vessels and other articles ordinary filter 

 paper may be used, but any common paper of decent texture is equally serviceable 

 and has the merit of being more economical. 



(a) Watch-glasses, Petri "dishes, etc., should be wrapped in several folds of paper. 



(6) Wide-mouthed cylindrical or conical 

 vessels only need to have the opening 

 covered with a double layer of paper, 

 though this should be large enough to allow 

 of it being turned down and twisted [or 

 tied] round the vessel, so that the greater 

 part of the latter is enveloped. In doing 

 this, be careful not to tear the paper, 

 which is apt to split on the edges of the 

 opening. 



[3. Other methods. Petri dishes, pip- 

 ettes, watch-glasses, metal instruments, 

 etc., may be conveniently enclosed in 

 copper boxes of suitable shape, which 

 should have tightly fitting lids with a deep 

 overlap. For ordinary Petri dishes a 

 circular copper cylinder 25 x 12 cm. (fig. 1) 

 containing a moveable tray may be used ; 

 for pipettes a similar but longer and 

 narrower cylindrical metal vessel or rect- 

 angular box is useful. Pipettes may also 

 be enclosed in a piece of large glass tubing, which is then plugged at both ends 

 with wool. The pipettes must of course be themselves plugged at the upper end 

 with wool.] 



Sterilization may be effected in one of several ways, the most generally 

 employed being heat and filtration ; chemical antiseptics are seldom used in 

 bacteriology. The methods of sterilization commonly employed will now be 

 considered in detail. 



FIG. 1. Copper cylinder with deep overlap 

 in which to sterilize Petri dishes. 



SECTION I. STERILIZATION BY DRY HEAT. 



1. Sterilization in a naked flame. 



1. The simplest means of sterilizing a metal instrument is to heat it to 

 redness in a spirit flame or Bunsen burner. This method is always adopted 

 for sterilizing platinum wires and iron and nickel spatulas. 



Knives and similar instruments can also of course be sterilized by heating 

 them in a flame, but on account of the injury done to the instrument the method 

 is very rarely adopted. 



An instrument which has been sterilized by heating to redness must be 

 cooled before it is allowed to touch any material which is to be used for 

 sowing cultures. 



2. An instrument may be sterilized by flaming it, i.e. by passing it rapidly 

 through a hot flame. 



Only pipettes, glass rods, and other instruments with polished surfaces 

 devoid of crevices in which organisms might escape destruction can be 

 sterilized in this way, so that the method is of limited application. 



2. Sterilization by hot air. 



Exposure to hot air is the usual method of sterilizing all glass and porcelain 

 apparatus, instruments with metal handles, etc., but it is not suitable for 

 organic substances, with the exception of wool and paper. 



