28 METHODS OF CULTIVATION OF BACTERIA 



be insufficient. 



This means of sterilisation is used for the glass 

 flasks, test-tubes, plates, Petri's 

 dishes, the use of which will 

 be described. Such pieces of 

 apparatus are thus obtained 

 sterile and dry. It is advisable 

 to put glass vessels into the 

 chamber before heating it, and 

 to allow them to stand in it 

 after sterilisation till the tem- 

 perature falls. Sudden heating 

 or cooling is apt to cause 

 glass to crack. The method is 

 manifestly unsuitable for food 

 media. 



B. Sterilisation by Moist 

 Heat. 



FIG. 2. Hot-air steriliser. B. (1) By Boiling. The 



boiling of a liquid for five 



minutes is sufficient to kill ordinary germs if no spores be 

 present, and this method is useful for sterilising distilled or tap 

 water which may be required in various manipulations. It is 

 best to sterilise knives and instruments 

 used in autopsies by boiling in water to 

 which a little sodium carbonate has been 

 added to prevent rusting. Twenty minutes' 

 boiling will here be sufficient. The boiling 

 of any fluid at 100 C. for one and a half 

 hours will ensure sterilisation under almost 

 any circumstances. 



B. (2) By Steam at 100 C. This is by 

 far the most useful means of sterilisation. 

 It may be accomplished in an ordinary 

 potato steamer placed on a kitchen pot. 

 The apparatus ordinarily used is "Koch's 

 steam steriliser " (Fig. 3). This consists of 

 a tall metal cylinder on legs, provided with 

 a lid, and covered externally by some bad 

 conductor of heat, such as felt or asbestos. 

 A perforated tin diaphragm is fitted in 



the interior at a little distance above the T ^ , , 



T, , ,, . , FIG. 3. Koch s steam 



bottom, and there is a tap at the bottom steriliser. 



