24 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



being taken not to allow any medium to run down the sides. 

 The test tubes are plugged with cotton wool, stacked in wire 

 cages and sterilised in the steam bath for twenty minutes on 



FIG. 5. APPARATUS FOB BACTERIOLOGICAL CULTURE. 



three successive days. The medium should remain perfectly 

 clear after sterilisation, and the tubes are then ready for use. 

 Agar medium. For cultures to be grown at a high tem- 

 perature, agar agar, a Japanese product made from a species 

 of marine alga, is used instead of gelatine in the above process. 

 This medium can be heated to 40 C. without melting. 



