go MANUAL OF BACTERIOLOGY. 



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



which has dried out partially. 



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

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

 glass was laid on another larger plate of glass, 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 Uttle benches enclosed 

 within a sterile chamber. The more convenient Petri dish has displaced the 

 original glass plate to a large extent. 



The isolation of bacteria may sometimes be effected by 

 drawing 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 sterihzing, over the surface of 

 another, perhaps over several in succession. 



Another very convenient method of obtaining isolated colonies 

 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, fiat and spreading. If the 

 colony consists of bacteria which have the property of liquefying 

 gelatin, a httle funnel-shaped pit or depression forms at the site 

 of the colony. The appearance of colonies may be of great as- 



