ESMABCM TUBES. 121 



plates or into dishes, spread over the inner surface of 

 the tube containing them, and without removing the 

 cotton plugs are caused to solidify in this position. The 

 tubes then present a thin cylindrical lining of gelatin 

 or agar-agar, upon which the colonies develop. In all 

 other respects the conditions for the growth of the organ- 

 isms are the same as in flat plates. 



Esmarch directs that after completion of the dilu- 

 tions the tops of the cotton plugs in the test-tubes 

 should be cut off flush with the mouths of the tubes and 

 sterilized rubber caps be placed over them. They are 

 then to be held in the horizontal position and twisted 

 between the fingers upon their long axis under ice- 

 water. The gelatin becomes solidified thereby and 

 adheres to the sides of the tube. When the gelatin is 

 quite hard the tubes are removed from the water, wiped 

 dry, the rubber caps removed, and the tubes set aside 

 for observation. 



For some time past we have deviated from the direc- 

 tion given by v. Esmarch for this part of his method, 

 and instead of rolling the tubes under ice- water, we roll 

 them upon a block of ice (Fig. 29), after the method 

 devised by Booker in the Pathological Laboratory of 

 the Johns Hopkins University in 1887. In this method 

 a small block of ice only is needed. It is arranged 

 nearly level, and is held in position by being placed in 

 a dish upon a towel. A horizontal groove is melted in 

 the surface of the ice with a test-tube full of hot water. 

 The tubes to be rolled are then held in an almost, not 

 quite, horizontal position and twisted between the fingers 

 until the sides are moistened by the contents to within 

 about 1 cm. of the cotton plug, care being taken that 

 the gelatin does not touch the cotton; otherwise the latter 



