THE BACTERIOLOGICAL LABORA TORY 31 



Sterilisers. In a study like bacteriology, all 

 vessels, instruments, etc,, used in the cultivation 

 of microbes, must, before use, be rendered perfectly 

 sterile. It cannot be too firmly impressed upon 

 the mind that the only way to obtain pure cultiva- 

 tions of microbes, is the complete sterilisation of 

 all vessels arid instruments used by the experi- 

 menter. For the accomplishment of this object 

 steam, hot-air, Btmsen or spirit flames, and germi- 

 cides are used as sterilising agents. 



Fig. 9 represents Koch's steam steriliser ; and is 

 used for sterilising test-tubes, flasks, and for cook- 

 ing potatoes. It is a cylindrical vessel of stout 

 tin plate, with a copper bottom, provided with a 

 conical lid, brass tubulure for the insertion of a 

 thermometer, a grating, water gauge, tap, and a 

 receiver with perforated bottom for cooking potatoes 

 (b). The cylinder (which is 20 in. high and 10 

 in. diameter) is divided into two compartments 

 (a and c). The lower one contains boiling water, 

 while the steam therefrom passes into the upper or 

 sterilising compartment. The cylinder is heated 

 from below by a Bunsen's or Fletcher's burner. 

 Steaming is usually kept up for from fifteen to 

 twenty minutes ; and this operation is repeated on 

 three successive days each time for twenty minutes. 

 By such steaming the various cultivation media, 

 etc., are rendered sterile, i.e. free from microbes. A 

 later form of this steriliser contains three compart- 

 ments instead of two. Two of these are used as 

 sterilising compartments, while the lowest one con- 

 tains the boiling water, which is always kept at a 



