CULl'lVATION ON PLATES AND IN TUBES. 59 



have to be employed ia order to recognise and classify 

 them. Before discussing this further it is necessary 

 to describe another method of cultivation, which may 

 be used instead of the cultivation on plates, and which 

 in many cases is preferable. For ordinary u'se, how- 

 ever, the plates are the best, for they are, at any rate, 

 more easily prepared, and, with careful handling, yield 

 just as good results as Esmarch's tubes, whilst they 

 afiFord greater facilities for observation. 



Yon Esmarch's method. — In this case the gelatine or 

 agar-agar is left in the test-tubes, and, after it has 

 been liquefied, the material containing the bacteria is 

 quickly introduced into it. The tubes are then 

 plugged with wadding and tipped several times from 

 side to side, in order to thoroughly mix their contents. 

 Especial care must be taken to prevent air- bubbles 

 from forming, as these would greatly add to the 

 difficulties of the later investigations. The tube is 

 now plunged into water, which, if gelatine is used, had 

 better be cooled with ice, and is rolled horizontally 

 backwards and forwards on the surface, until the gela- 

 tine has solidified. Any vessel will do for the water, 

 provided it is large enough : but a tin dish, filled to 

 the brim, answers the purpose best. Many prefer to 

 place the test-tube in the dish, with its mouth stretch- 

 ing over the edge, whilst the remainder of the tube 

 floats on the surface of the water. It is quite true 

 that by this means an even distribution of the gela- 



