THE CULTIVATION OF BACTERIA. 



89 



Esmarch's Roll- tubes.— Either gelatin or agar may be 

 used, but if the agar is freshly made, it does not adhere well to 

 the walls of the tubes. The dilutions in tubes i, 2 and 3 are 

 made as above. Tubes containing a rather small amount of 

 the culture-medium are more convenient. A block of ice should 

 be at hand, and, with a tube filled with hot water and lying hori- 

 zontally, a hollow of the size of the test-tube should be melted 

 on the upper surface of the ice. In this hollow place the tube 

 of Kquefied gelatin or agar; roll it rapidly with the hand, taking 



Iff: - fi 



Fig. 33. — -Manner of Making Esmarch Roll-tube. 



care that the culture-medium does not run toward the neck 

 as far as the cotton plug. The medium is spread in a imiform 

 manner around the inside of the tube, where it becomes solidi- 

 fied. Gelatin roll-tubes must be kept in a place so cool that 

 there is no danger of their melting; in handhng them they are 

 to be held near the neck, so that the warmth of the hand may 

 not melt the gelatin. Agar roll- tubes should be kept in a 

 position a Uttle inclined from the horizontal, with the neck up, 

 for twenty-four hours, so that the agar may stick to the wall of 



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