68 



BACTERIOLOGY 



when the thermometer registers loo" C, if the sterilizer be pro- 

 vided with a thermometer. With a large Arnold sterilizer a 

 temperature of ioo° C. may not be reached until it has been heated 

 with a rose-burner for twenty to thirty-five minutes. When 

 bulky articles or large amounts of material are to be sterilized, 

 allowance must be made for the time necessary to bring the 

 temperature in the middle of the mass to loo" C. 



Pig. 31. — Steam sterilizer, Massachusetts Board of Health. 



Autoclave Sterilization. — Sterilization in the presence of moist- 

 ure and at temperature above 100" C, requires a pressure greater 

 than that of the atmosphere and the apparatus used for this pur- 

 pose is known as the autoclave. ^ All bacteria and their spores are 

 killed by heating at 110° C, in the presence of water, for fifteen 

 minutes, and in about five minutes at 120° C. The steam pres- 

 sures corresponding to these temperatures are approximately 

 7M pounds and 15 pounds per square inch or }i kilogram and 

 I kilogram per square centimeter, respectively. The autoclave 



