50 A MANUAL OF BACTERIOLOGY 



which might be damaged by heating, filtration through 

 porous porcelain filters may be used. 



Various apparatus are needed for sterilisation of 

 vessels, etc., and for the preparation of sterile culture 

 media by heat. These will now be described. 



Hot-air steriliser (Fig. 2). This is a rectangular box of 



FIG. 2. Hot-air steriliser. 



sheet iron or copper with double walls, having an air- 

 space of nearly an inch between them, and furnished 

 with a door. The joints should be brazed, riveted, or 

 folded, not soldered. The outer skin at the bottom 

 should have a large hole cut in it in which a loose piece of 

 sheet iron or copper should be inserted to protect the 

 inner skin from oxidation, and this may be renewed as it 

 " burns " away. The top is perforated with a couple of 

 holes, through one of which a chemical thermometer, 



