64 ESSENTIALS OF BACTERIOLOGY 



while moist heat at 100 C. killed the spores of the anthrax 

 bacillus in one hour, it required three hours of dry heat at 

 140 C. to produce death. 



For obtaining dry heat that is, a temperature of 150 C. 

 (about 300 F.) a sheet-iron oven (Fig. 15) is used which 

 can be heated by a gas-burner. If it have double walls (air 

 circulating between), the desired temperature is much more 

 quickly obtained. A small opening in the top to admit a 

 thermometer is necessary. These chests are usually about 

 i foot high, \Y feet wide, and ^ foot deep. In them glass- 

 ware, cotton, and paper can be sterilized. When the cotton 

 is turned slightly brown, it usually denotes sufficient steriliza- 

 tion. All instruments, where practicable, should be drawn 

 through the flame of an alcohol lamp or Bunsen burner. One 

 hour in the oven at 170 C. usually sterilizes glassware, while 

 the ordinary germs in liquids may be killed by boiling for 

 five minutes if no spores are present. The boiling of any 

 fluid at 1 00 C. for one and one-half hours nearly always 

 insures sterilization. 



Moist Heat. Steam at 100 C. in circulation has been 

 shown to be a very effective application of heat. 



The steam chest devised by Koch consisted of a long 

 double boiler divided by a perforated shelf on which the 

 material could rest while subjected to streaming steam. 



Arnold's steam sterilizer will answer every purpose of the 

 Koch steam-chest. It is cheaper, also requiring less fuel to 

 keep it going. The steam does not escape, but is condensed 

 in the outer chamber. 



The autoclave (Fig. 16), which produces steam under 

 pressure and allows a temperature of 120 C. to be obtained, 

 is a most effective method of sterilization, but the higher 

 temperatures are not suitable for gelatin or sugar solution. 

 Gelatin loses its power of solidifying if the boiling is pro- 

 longed. 



Instead of sterilizing for a long time at once, successive 

 sterilization is practised with nutrient media, so that the 

 albumin will not be too strongly coagulated. Fifteen minutes 



