138 BACTERIOLOGY. 



the tubes containing them, and, without removing the 

 cotton plugs, solidified 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 organisms 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 a 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 1887 in the Pathological Labora- 

 tory of the Johns Hopkins University. In this method 

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

 and held in position by being placed upon a towel in 

 a dish. A horizontal groove is melted in the upper 

 surface of the ice with a test-tube 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 

 becomes adherent to the sides of the tube and is difficult 



