STEAM. 47 



in clean water. The tubes and flasks are plugged with cotton 

 (raw cotton will do), which permits of the entrance of air but 

 is an effectual barrier to the entrance of bacteria. These 

 articles are then placed in the dry heat sterilizer for one hour. 

 After sterilization the cotton plug can be covered with a small 

 rubber cap as an additional safeguard against infection. 



The hot-air chamber (Figs. 11 and 12) is a single or double- 

 walled sheet-iron or copper chest having a door on one side 

 and several removable shelves on the inside. The top of the 

 chest is perforated by two holes, in one of which is placed a 

 thermometer to indicate the temperature of the inside of the 

 chest. The other opening is plugged with cotton. A large 



FIG. 12. 



Dry heat sterilizer. 



Bunsen burner is placed under this chest, which rests on an 

 iron frame. In order to distribute the heat evenly, a piece of 

 wire gauze is placed over the burner. 



The articles to be sterilized are placed on the shelves 

 within the chamber, but not until the temperature has 

 reached 150 C., so that they will remain in the chest exposed 

 to that temperature for one hour. Frequently the mistake is 

 made of counting the time from when the tubes, etc., are 

 placed in the chamber while it is still cold. 



Steam : All culture-media, woollen and cotton fabrics, wood, 

 and crockery must be sterilized by steam. Steam is very pene- 

 trating, and is, therefore, a most effective sterilizing agent. 



