114 



BACTERIOLOGY 



and can readily be studied. They may be examined with a hand- 

 lens, or the entire dish may be placed on the stage of the micro- 

 scope and the colonies be inspected with the low power. The 

 iris diaphragm should be nearly closed and the plane mirror should 

 be used. Dilution-cultures prepared as described in the next 

 paragraph, where the principle is the same, are shown in Fig. 45. 

 In tube No. i the colonies are so numerous as to look like fine white 

 dust. In tubes 2 and 3 they are less numerous and larger. 



Esmarch's Roll-tubes. — Use liquefied gelatin or agar. The 

 dilutions^^in tubes i, 2 and 3 are made as above. Tubes contain- 

 ing a rather small amount of the culture-me&ium are more con- 



If fp- 



MiyiiJ^ 



Fig. 44. — Manner of making Esmarch roll-tube. 



venient. A block of ice should be at hand, and, with a tube filled 

 with hot water and lying horizontally, 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 liquefied gelatin or agar; roll it rapid- 

 ly 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 sohdified. 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 near the neck, so that the warmth of the hand 



