STERILISATION BY MOIST HEAT 



27 



one hour to a temperature of 170 C., is sufficient to kill all the 

 organisms which usually pollute 

 articles in a bacteriological 

 laboratory, though circum- 

 stances might arise where this 

 would 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 heat- 

 ing it, and to allow them to 

 stand in it after sterilisation 

 till the temperature falls. Sud- 

 den heating or cooling is apt 

 to cause glass to crack. The 

 method is manifestly unsuitable 

 for food media. 



FIG. 2. Hot-air steriliser. 



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 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 



FIG. 3. Koch's steam of a tal1 metal cylinder on legs, provided 

 steriliser. with a lid, and covered externally by 



