IOO MANUAL OF BACTERIOLOGY. 



melt the gelatin. Agar roll-tubes should be kept in a posi- 

 tion a little inclined from the horizontal, with the mouth up, 

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

 of the tube. For reasons stated above, it is best to employ 

 agar which has partially dried out. 



By the plate-method as originally devised by Koch, instead of using Petr 

 dishes, the gelatin was poured upon a sterile plate of glass. This plate of 

 glass was laid on another larger plate of gla-s, which formed a cover for a dish 

 of ice-water, the whole being provided with a leveling -apparatus. The plate 

 was kept perfectly level until it had solidified, which took place rapidly on the 

 cold surface. The glass plates were placed on little benches enclosed within a 

 sterile chamber. This method is now seldom or never used. 



The isolation of bacteria may sometimes be effected by draw- 

 ing a platinum wire containing material to be examined rapidly 

 over the surface of a Petri dish containing solid gelatin or agar; 

 or over the surface of. the slanted culture-medium in a test- 

 tube; or by drawing it over the surface of the medium in 

 one test-tube, then without sterilizing, over the surface of 

 another, perhaps over several in succession. 



Another very convenient method of obtaining isolated colo- 

 nies is to introduce a very small amount of material into the 

 water squeezed out in the bottom of a slant agar tube, then 

 flood this over the surface of the agar. 



Appearance of the Colonies. The colonies obtained in 

 the Petri dishes or roll-tubes (Fig. 32) may be studied with a 

 hand-lens or with a low -power microscope. In the latter case, 

 use the concave mirror with the iris diaphragm partly closed. 

 The colonies present various appearances. Some of them are 

 white, some colored; some are quite transparent and others are 

 opaque; some are round, some are irregular in outline; some 

 have a smooth surface, others appear granular, and others 

 present a radial striation. Surface colonies often present 

 different appearances from those occurring more deeply. Sur- 

 face colonies are likely to be broad, flat and spreading. If the 



