THE CULTIVATION OF BACTERIA. 99 



Tubes Nos. 2 and 3 are to be treated in the same manner. 

 Burn the plugs, and fill the empty tubes with 5 per cent. 

 solution of carbolic acid. They should be sterilized for an 

 hour in the steam sterilizer on each of three days. 



The culture-medium in the Petri dish will soon solidify. 

 Colonies develop usually in from one to two days. In 

 plate No. i they will be very numerous, in plate No. 2 less 

 numerous, and in plate No. 3 still less numerous. Where 

 the "number is small the colonies will be widely separated 

 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 microscope and the colonies be inspected with the 

 low power. The iris diaphragm should be partly closed 

 and the concave mirror should be used. Dilution-cultures 

 prepared as described in the next paragraph, where the 

 principle is the same, are shown in Fig. 31. In tube No. 

 i the colonies are so numerous as to look like fine white 

 dust. In tubes 2 and 3 they become 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 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 

 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 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 solidified. 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 may not melt the 



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