THE CULTIVATION OF BACTERIA. 



99 



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

 for roll-tubes, but if the agar is freshly made, it does not adhere 

 well to the walls of the tuber. 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. The 

 tubes may be capped with a rubber cap and revolved in a dish 

 of ice water, or under the water tap or, a still better method 

 employed first by W. D. Booker, a block of ice should be at 

 hand, and, with a tube filled with hot water and lying horizon- 



FIG. 35. Manner of making Esmarch roll-tube. 



tally, 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 liquified gelatin or agar; roll it rapidly with the hand, taking 

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

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

 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 handling them they 

 are to be held by the portion of the tube into which the cot- 

 ton plug projects, so that the warmth of the hand may not 



