ASSIMILATION OF CARBON 



57 



from the gas flame below passing between the two walls and thus rendering the 

 heating uniform. 



Objects that cannot endure dry heat are sterilized in a steam sterilizer, 

 such as Koch's apparatus. This is a cylinder of tinned sheet iron or copper 

 with a cover above. The lower part is filled with water and the objects to be 

 sterilized are placed upon a perforated rack in the upper part. A burner below 

 the cylinder heats the water to boiling and the contained objects are sterilized 

 by water vapor at ioo°C. The apparatus is covered with felt or asbestos, to 

 retard the escape of heat. d 



Fig. 34. — Arnold steam sterilizer. 



Instead of a steam sterilizer the autoclave is frequently used for steriliza- 

 tion (Fig. 35). This is nothing more than a Papin's digester, operating with 

 superheated steam, under pressure up to two atmospheres or more and at 

 temperatures of from ioo° to i34°C. or higher. At a temperature of i2o°C. 

 sterilization need last only fifteen minutes. At a temperature of 130 all 

 germs are instantly killed, so that repeated treatment, necessary in the case 

 of steam sterilization, is here superfluous. 



c For most satisfactory work the oven should have an automatic temperature-regulator, 

 various forms of which are available for gas. Electrically heated, automatically regulated 

 ovens are also obtainable, some of which are so well insulated that but little heat escapes to the 

 exterior. — Ed. 



d One of the various forms of the Arnold type of steam sterilizer is most convenient and 

 efficient in operation. (Fig. 34.) This keeps but a small amount of water boiling at any one 

 time and a large portion of the water that is boiled away is condensed and returned to the 

 reservoir. — Ed. 



