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 mani- 

 festly unsuitable for food media. 



B. Sterilisation by Moist 

 Heat. 



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 



Fi. 2.- Hot-air steriliser. 



useful for sterilising distilled or tap water which may be re- 

 quired in various manipulations. To minimise rusting of knives 

 and steel instruments it is well to boil the g 



water for some time before placing them 

 in it. 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 

 bottom, and there ia'a tap-'at the bottom by FJG> 3> _ Koch > s steam 

 which water may bejsupplied[orjwithdrawn. steriliser. 



