6o PRACTICAL BACTERIOLOGY. 



tinSj and hence of the germs, is not obtained, for they 

 naturally collect somewhat at the lower end of the 

 tube, which is in the water, and thus the coating 

 of gelatine becomes gradually thinner towards the 

 mouth ; but the advantage obtained by this method is 

 that the cotton-wool cannot become wetted either by 

 the water or by the gelatine, which can hardly be 

 prevented if the ordinary method of rolling the tube 

 horizontally be adopted. If, however, it is very im- 

 portant that the layer of gelatine should be as even as 

 possible, it is best to proceed as follows. In order to 

 protect the cotton-wool from the water, it can be 

 soaked in paraffin or covered with an india-rubber 

 cap ; the test-tube is then laid on the surface of the 

 water, and is rolled quite horizontally. 



If the operation has succeeded, the gelatine should 

 be spread in an even layer over the inner surface of 

 the tube. If agar-agar be used, the water should 

 not be cooled; on the contrary, it should as a rule 

 be somewhat warmed, otherwise the medium would 

 solidify too rapidly, and thus the layer would be un- 

 even. Bacteria colonies develop in these tubes in 

 the same way as in the cultivation dishes; their 

 specific characteristics, however, are not so easy to 

 recognise as in the latter, as observations cannot be so 

 easily made from above. 



