PHYSICAL AGENTS 



123 



nature of the material. Apparatus for this purpose may now 

 be obtained from sizes as small as one or two gallons up to 

 huge structures which will take one or two truckloads of 

 material (Figs. 73-85) . The latter type is in common use 

 in canning factories, dairy plants, hospitals, public institu- 

 tions and municipal and governmental disinfecting stations. 

 Very frequently there is an apparatus attached for pro- 



FiG. 76. — A "process kettle" (steam-pressure sterilizer) used in canning 

 Diameter, 40 inches; height, 72 inches. 



ducing a vacuum both to exhaust the air before sterilizing, 

 so that the steam penetrates much more quickly and thor- 

 oughly, and for removing the vapor after sterilizing, thus 

 hastening the drying out of the material disinfected. 



The smaller types of pressure sterilizers are called ''auto- 

 claves" and have become indispensable in laboratory work. 

 Fifteen pounds' pressure maintained for fifteen minutes is 

 commonly sufficient for a few small objects. For larger 

 masses much longer time is needed. The author found that 



