142 



BACTERIOLOGY 



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 — ^hori- 

 zontal 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 difiicult to remove. The tube 



Fig. 27 



Demonstrating Booker's method of rolling Esmarch tubes on a block of ice. 



is then placed in the groove in the ice and rolled until its 

 contents are solid. 



There is an erroneous impression that Esmarch tubes are 

 not a success when made from ordinary nutrient agar-agar 

 because of the tendency of this medium to shrink and slip 

 to the bottom of the tube. This slipping down of the agar- 

 agar is due to the water, which is squeezed from it during 

 solidification, getting between the medium and the walls 

 of the tube. This can easily be overcome by allowing the 



