STERILISATION BY MOIST HEAT. 



29 



Fig. 2. — Hot-air steriKseir. 



A. (2) Sterilisation by Dry Heat in a Hot-air Chamber. — 



The chamber (Fig. 2) consists of an outer and inner case of 

 sheet iron. In the bottom of the outer there is a large hole. 

 A Bunsen is lit beneath this, and 

 thus plays on the bottom of the 

 inner case, round all of the sides of 

 which the hot air rises and escapes 

 through holes in the top of the 

 outer case. A thermometer passes 

 down into the interior of the cham- 

 ber, halfway up which its bulb 

 should be situated. It is found, as 

 a matter of experience, that an ex- 

 posure in such a chamber for one 

 hour to a temperature of 170° C, 

 is sufficient to kill all the organisms 

 which usually pollute articles in a 

 bacteriological laboratory, though 

 circumstances 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 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. 



B. (i) 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 rust- 

 ing. Twenty minutes' boiling will here be sufficient. The boil- 

 ing 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 ordi- 



