6 CLINICAL BACTERIOLOGY AND H^MATOLOGY 



on the shelves of the apparatus, and the heat is continued until 

 the outside is singed. 



Metal instruments (knives, scissors, etc.) may be sterilized 

 in the same way and at the same time. They should be 

 wrapped loosely in cotton-wool, and should not be removed 

 from their wrapping" until the moment at which they are to 

 be used. 



STEAM is chiefly used for the sterilization of culture media 

 before use, and for the destruction of cultures when they are 

 done with. The latter purpose, however, is accomplished more 

 speedily and safely by the addition of a few drops of commer- 

 cial formalin to each tube, or the tubes may be filled with I in 

 20 carbolic; neither method can be relied on to kill spores. 



The proper steam sterilizer consists of a metal cylinder with 

 a perforated diaphrag'm 6 or 8 inches from the bottom. It is 

 enclosed in a thick layer of felt or other non-conductor of heat, 

 and is provided with a lid. The space between the bottom and 

 the diaphragm is partly filled with water, which is boiled by 

 means of a Bunsen flame or Fletcher's burner, the apparatus 

 to be sterilized being' placed in the chamber above so as to be 

 exposed to the steam. 



An ordinary steamer (such as is used for cooking potatoes 

 or fish) will answer every purpose. In procuring such a 

 steamer for bacteriological use it is best to choose one that 

 will accommodate a litre flask holding a funnel, as it is often 

 a great convenience in the filtration of fluids which become 

 solid on cooling to carry out the process in an atmosphere of 

 steam. 



Exact details of the way it is used will be given subsequently. 



PREPARATION OF CULTURE MEDIA 



Bacteria are grown in the same way as other plants. A 

 gardener who wishes to grow a plot of a particular plant will 

 first prepare a soil suitable for the growth of that plant, and 

 free it as far as possible of all seeds, roots, etc. He will then 

 sow it with the seeds of the plant in question, and do what 

 he can to expose them to a suitable temperature. An exactly 

 similar process is adopted when we wish to cultivate the 

 smallest of all plants. The soil which we prepare is called 



