8 PRACTICAL BACTERIOLOGY 



one for a thermometer and the other for a thermo-regulator which 

 is usually not necessary and two eyelits for suspending it to the wall, 

 or it can be placed on a stand (see Fig. 1). When suspended against 

 the wall a sheet of asbestos is necessary to protect the wall. It is also 

 important that the copper bottom of the apparatus should be detach- 

 able, thus enabling a new one to be fitted without difficulty. 



In bacteriological work sterilization by dry heat is limited to such 

 articles as glass flasks, plates, small dishes, test-tubes, pipettes, cotton 

 wadding, and such metal instruments as are not seriously injured by 

 the high temperature. The above articles are sterile when heated to 

 150 C. for one hour. The usual method is to remove the Bunsen 

 burner when the thermometer registers 170 C., and allow the 

 apparatus to cool before opening, otherwise glass ware may fly in 

 pieces. 



FIG. 1. Hot- Air Sterilizer. 



FIG. 2. Koch's Steam Sterilizer. 



Fluids, culture media, potatoes, etc., in fact, anything that would 

 be destroyed by the great heat of the hot-air chamber, are sterilized 

 in the ' Koch Steam Sterilizer' 1 (see Fig. 2), or the ' Arnold Steam 

 Sterilizer.'' 



The Arnold apparatus possesses advantages not obtainable with 

 the Koch apparatus. It is smaller but quicker, and is more suitable 



