242 STERILIZATION, ANTISEPSIS AND DISINFECTION 



necessary to dry the material by holding the loop near the flame before 

 incineration to prevent "spattering." The "bacteria incinerator" 

 made by de Khotinsky is particularly to be recommended for this 

 purpose. 1 



(6) Dry Heat. Test tubes, flasks, Petri dishes, pipettes and other 

 laboratory glassware are sterilized in the hot-air sterilizer an oven 

 heated with a gas flame. An exposure of one and a half hours at 160 

 C. or one hour at 180 C. will effectually kill all spores. The heat 

 should be applied gradually and reduced gradually to diminish the 

 danger of cracking. Dry heat has but little power of penetration. 

 Glassware is conveniently wrapped in paper before sterilization to 

 protect it from dust prior to its use. The cotton plugs of flasks and 

 beakers are also covered with paper before sterilization, for the same 

 reason. 



(c) Moist Heat. 1. The most satisfactory agent for the steriliza- 

 tion of articles uninjured by moisture is steam under pressure. Many 

 kinds of media and laboratory apparatus, and fomites as well are 

 quickly and completely sterilized by steam. The autoclave is com- 

 monly used for laboratory purposes. It consists essentially of a 

 double-walled chamber with close-fitting cover, into which steam 

 may be introduced. There are many patterns, but the essential 

 features are the steam should enter the chamber from the top, and 

 the bottom of the chamber should be provided with a stop-cock, 

 through which the residual air and condensation can escape. 



Operation. A single layer of apparatus should be sterilized at one 

 time. If several layers are introduced, condensation water from the 

 upper layer may collect on the lower layers, permitting of subsequent 

 contamination. Steam is admitted to the chamber to displace the 

 air, and the air-cock should remain open until live steam flows freely 

 from it, because hot air is far less efficient than steam for sterilization. 

 Also, the condensed steam escapes through the same orifice. When 

 all the air is replaced by dry steam the pressure is gradually in- 

 creased until fifteen pounds are recorded on the pressure gauge. 

 This pressure is maintained from ten to twenty minutes, depending 

 upon the nature of the material to be sterilized. In general, media 



1 It consists essentially of a tube about 12 cm. in length and 1 cm. in diameter, of 

 fire clay surrounded by a resistance coil of sufficiently fine wire and numerous layers 

 to heat the interior of the tube to a white heat. The charged platinum wire is placed 

 in the tube, and within a few seconds it becomes white-hot. There is absolutely no danger 

 from "spattering," because the extruded organisms fall upon the hot walls of the tube 

 (see Fig. 23, page 205). 



