6 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



3. A steam sterilizer ; or a large kitchen steamer — this should 

 be deep enough to contain a litre flask holding a funnel. 



4. A large Bunsen burner or spirit-lamp. 



Bacteria and their spores are ubiquitous, and it is necessary to 

 sterilize all vessels and other apparatus and all culture media 

 before use. The methods which are adopted all depend upon the 

 action of heat ; chemical antiseptics are rarely used in the bacterio- 

 logical laboratory for the sterilization of apparatus, for it would be 

 difficult to remove them completely, and the traces which might 

 remain would prevent the development of those germs which we 



Fig. 6. — Hot-air Sterilizer. 



wished to cultivate. Two chief methods are in use, sterilization 

 by dry heat and by steam; we exclude sterilization by steam under 

 pressure, as this requires special and expensive apparatus, and is 

 never absolutely necessary, though often convenient. 



Dry Heat is used to sterilize all glass vessels (flasks, Petri 

 dishes, test-tubes, pipettes, etc.), cotton-wool, and metal instru- 

 ments. The heat must be continued for at least half an hour, 

 and must not fall below 150° C. as indicated by the thermometer. 

 Another method, which is less reliable than the use of the ther- 

 mometer, but which may be resorted to in an emergency, is to 

 wrap the apparatus loosely in cotton-wool, and to proceed with the 



