METHODS OF CULTIVATION 



45 



Various methods of destroying and of getting rid of 

 organisms are known, such as the use of chemical " germi- 

 cides," heat, and filtration through porous porcelain. 

 The addition of chemical germicides, such as carbolic acid 

 or corrosive sublimate, is out of the question ; for although 

 the vessels and media might be rendered sterile thereby, 



FIG. 2. Hot-air steriliser. 



the growth of the organisms which are being investigated 

 would equally be prevented, so that the two last, viz. heat 

 and filtration, are those which are employed, the former 

 being used for vessels, instruments, and culture media, 

 solid and fluid, the latter for fluid culture media only. 



Various apparatus are needed for sterilisation and the 

 preparation of culture media. These will now be described. 



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

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



