PREPARATION OF NUTRIENT MEDIA. 49 



it should be pei'fectly clear, and of a very light yellow 

 colour ; when it is solid, it should be yellowish brown 

 in colour, translucent, and, in thin layers, trans- 

 parent. 



Milk and bouillon are, in a similar manner, poured 

 into test-tubes, plugged with cotton-wool, and then, 

 like agar-agar, sterilised. The potato is one of the most 

 useful of the solid media for the culture of bacteria, 

 only it is not easy to sterilise it thoroughly. Of all 

 the different methods which may be employed, the 

 following appears to have proved itself the best. 

 Firm, waxy potatoes are taken, which do not break up 

 when boiled ; they are thoroughly washed, left soak- 

 ing in corrosive sublimate solution (1 : 1000) for one 

 or two hours, and are then peeled. They are next 

 placed again into the sublimate solution for five 

 minutes, after which they are taken out, washed with 

 sterilised water, and cut up into slices about 1 cm. 

 thick, with a knife which has been thoroughly heated 

 and is still warm. These slices are immediately put 

 into sterilised cultivation dishes, which are then 

 covered up with their tight-fitting glass lids, and 

 placed in the steam steriliser. These cultivation 

 dishes are flat, about 9 or 10 cm. in diameter, and \\ 

 to 2 cm. deep ; they are supplied with lids which fit 

 over the sides (Fig. 5) . They are at present used more 

 frequently than anything else for the so-called plate 

 cultivations. They are sterilised beforehand in the hot- 



