STERILTSA TION B Y MOIST HE A T. 37 



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 1 70 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 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 would not reach it at the 

 ordinary pressure. 



B. Sterilisation by Moist Heat. 



(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, 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. 



(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 is a tap at the 

 bottom by which water may be supplied or withdrawn. If 



