STERILISA TTON B Y MOIST HE A T. 37 



of the outer case. A thermometer passes down into the 

 interior of the chamber, half-way up which its bulb should 

 be situated. It is found as a matter of experience, that an 

 exposure 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 temperature falls. Sudden 

 heating or cooling is apt to cause glass to crack. The 

 method is unsuitable for food media. Solid media would 

 be scorched by such a temperature, and fluid media would 

 not reach it at the ordinary pressure. 



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 manipula- 

 tions. It is best to sterilise knives and instruments used in 

 autopsies by boiling in water, as dry heat frequently spoils 

 the temper of the steel. 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. A perforated tin diaphragm is fitted in 

 the interior at a little distance above the bottom, and there 



