STERILIZATION APPARATUS. 55 



equivalent to a temperature of 122° C. to which the 

 organisms are exposed. 



The objection to both of these methods of direct 

 sterilization by steam is that many substances that it 

 is desirable to retain in as near their normal condition 

 as possible are materially altered by this energetic form 

 of treatment. Gelatin is not only rendered cloudy, but 

 often loses the power of gelatinizing. Many of the other 

 media contain always a fine precipitate after this method ; 

 in fact, for most of the media which are employed the 

 discontinued method at the temperature of streaming 

 steam gives the most satisfactory results. 



For sterilization by steam the apparatus commonly 

 employed has, until recently, been the cylindrical boiler 

 recommended by Koch. (See Fig. 8.) 



Its construction is very simple. It consists of a 

 copper cylinder, the lower fourth of which is somewhat 

 larger in diameter than the remaining three-fourths, and 

 acts as a reservoir for the water from which the steam 

 is to be generated. Covering this section of the cylinder 

 is a wire rack or grating through which the steam 

 passes, and which serves as a bottom upon which the 

 objects to be sterilized rest. Above this, comprising 

 the remaining three- fourths of the cylinder, is the cham- 

 ber for the reception of the materials over and through 

 which the steam is to pass. The cylinder is closed by 

 a snugly-fitting cover through which are usually two 

 perforations into which a thermometer and a manometer 

 may be inserted. The whole of the outer surface of 

 the apparatus is encased in a non-conducting mantle of 

 asbestos or felt. 



The water is heated by a gas-flame placed in an en- 

 closed chamber, upon which the apparatus rests, which 



