CLINICAL BACTERIOLOGY AND HiEMATOLOGY 



the same way as flasks. Petri dishes are wrapped round with 

 tissue-paper or filter-paper before being sterilized. 



Before removing glass apparatus from the sterilizer remember 

 to let the temperature fall gradually, or the vessels may crack. 



If a sterilized test-tube be required in a hurry (as often 

 happens), plug a clean tube with cotton-wool, and hold it with 

 forceps, one blade being inside and one outside the open end. 

 Then heat every part of the tube thoroughly in the flame, taking 

 care the heat is great enough to scorch the plug. 



Cotton-wool is sterilized by being spread out in thin layers on 



the shelves of the apparatus, and 

 the heat is continued until the out- 

 side 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 mo- 

 ment at which they are to be used. 



Steam is chiefly used for the 

 sterilization of culture media before 

 use, and for the destruction of cul- 

 tures when they are done with. The 

 latter purpose, however, is accom- 

 plished more speedily and safely by 

 the addition of a few drops of com- 

 mercial formalin to each tube, or 

 the tubes may be filled with i in 20 

 carbolic, which, however, cannot be 

 relied on to kill spores. 

 The proper steam sterilizer consists of a metal cylinder with a 

 perforated diaphragm 6 or 8 inches from the bottom. It is en- 

 closed 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 



Fig. 7.— Steam Sterilizer. 



