STERILIZATION OF APPARATUS, ETC. 3 



regularly backward and forward and up and down, so that all 

 parts of its surface may be examined systematically. A simple 

 form is all that is required, the verniers being quite unneces- 

 sary for most purposes. It can be fitted to most stands. 



STERILIZATION OF APPARATUS, ETC. 

 Requisites. i. A thermometer graduated to 200 C. 



2. A hot-air sterilizer; or a cubical biscuit tin, the soldering 

 of which has been replaced by brazing this must be mounted 

 upon a tripod stand; or a kitchen oven, preferably a gas oven. 



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 

 bacteriological 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 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 in- 

 struments. The heat must be continued for at least half an 

 hour, and must not fall below 130 C. as indicated by the 

 thermometer. Another method, which is less reliable than the 

 use of the thermometer, but. which may be resorted to in an 

 emergency, is to wrap the apparatus loosely in cotton-wool, 

 and to proceed with the heating (allowing the temperature to 

 rise gradually) until the outer part of the wool is slightly 

 singed over the whole of the exposed surface. 



The special sterilizer which is used in the bacteriological 

 laboratory consists of a copper or iron oven with double walls 

 and perforated metal shelves. There is a hole in the top, which 

 is fitted with a perforated cork, through which the thermometer 

 passes. The oven is mounted on a stand, and heated by 

 means of a large Bunsen or Fletcher's burner. 



