STERILIZATION BY HEAT. 59 



while steam under pressure may answer very well for 

 routine sterilization, still it presents insurmounteble 

 obstacles to its use in more delicate experiments where 

 time-exposure to definite temperature is of importance. 



For sterilization bv live steam the apparatus com- 

 monly employed has, until recently, been the cylin- 

 drical boiler recommended by Koch. (See Fig. 8.) 



Its construction is very simple, essentially that of the 

 ordinary potato-steamer used in the kitchen. It con- 

 sists of a copper cylinder, the lower fifth of which is 

 somewhat larger in circumference than the remaining 

 four-fifths, and acts as a reservoir for the water from 

 which the steam is to be generated. Covering this sec- 

 tion of the cylinder is a wire rack or grating through 

 which the steam passes, and which serves to support 

 the articles to be sterilized. Above this, comprising 

 the remaining four-fifths 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 ma- 

 nometer 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 

 serves to diminish the loss of heat and deflection of the 

 flame through the action of draughts. The apparatus 

 is simple in construction, and the only point which 

 is to be observed while using it is the level of the water 

 in the reservoir. On the reservoir is a water-gauge 

 which indicates at all times the amount of water in the 

 apparatus. The amount of water should never be too 



