MOIST HEAT 35 



the instruments, dry carefully and at once, and return 

 them to their store cases. 



Streaming steam i. e., steam at 100 C. destroys 

 the vegetative forms of bacteria in from fifteen to 

 twenty minutes, and the sporing forms in from one 

 to two hours. This method is chiefly used for the 

 sterilisation of the various nutrient media intended for 

 the cultivation of bacteria, and is carried out in a 

 steam kettle of special construction, known as Koch's 

 steam steriliser (Fig. 28) or in one of its many modifi- 

 cations, the most efficient of which is Arnold's (Fig. 29). 



The steam steriliser in its simplest form consists of 

 a tall tinned-iron or copper cylindrical vessel, divided 

 into two unequal parts by a movable perforated metal 

 diaphragm, the lower, smaller portion serving for a 

 water reservoir, and the upper part for the reception of 

 wire baskets containing the articles to be sterilised. 

 The vessel is closed by a loose conical lid, provided 

 with handles, and perforated at its apex by a tubu- 

 lure ; it is mounted on a tripod stand and heated 

 from below by a Bunsen burner. The more elaborate 

 steriliser is cased with felt or asbestos board, and pro- 

 vided with a water gauge, also a tap for emptying the 

 water compartment. 



To USE THE STEAM STERILISER. 



1. Fill the water compartment to the level of the 

 perforated diaphragm, place the lid in position, and 

 light the Bunsen burner. 



2. After the water has boiled, allow sufficient time 

 to elapse for steam to replace the air in the sterilising 

 compartment, as shown by the steam issuing in a 

 steady, continuous stream from the tubulure in the lid. 



3. Remove the lid, quickly lower the wire basket 

 containing media tubes, etc., into the sterilising com- 

 partment until it rests on the diaphragm, and replace 

 the lid. 



