28 METHODS OF CULTIVATION OF BACTEKIA 



be insufficient. This means of sterilisation is used for the glasB 

 flasks, test-tubes, plates, Petri dishes, the use of which will be 



described. Such pieces of 

 j apparatus are thus obtained 



II 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. 



ITS. 



_riE 



^o 



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 re- 

 quired in various manipulations. To minimise rusting of knives 



and steel instruments it is well to boil the 



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 is a tap at the bottom by _„ „ 



. . . ' , K. , .... J Fig. 3. — Koch s steam 



which water may be supplied or withdrawn, steriliser in section 



