ASSIMILATION OF CARBON 



55 



Objects that cannot endure dry heat are sterihzed 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 tilled 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.'' 



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 100° to i34°C. or higher. At a temperature of i2o°C. 

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



Fig. 34. — Arnold steam sterilizer. 



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

 of steam sterilization, is here superfluous. 



Liquids may also be sterilized by filtration. The most convenient arrange- 

 ment for this purpose is the Chamberland filter, a hollow cylinder of porous 

 porcelain, closed at one end. The liquid to be sterihzed is passed, under pres- 

 sure, through the porous walls of the previously sterilized filter. 



Various disinfecting materials are also used for the chemical destruction of 

 microorganisms. The most effective of these is corrosive sublimate, or mercuric 

 chloride (HgCl2). A solution of i g. of mercuric chloride in a hter of distilled 

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

 efScient 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. 



