92 PRACTICAL BACTERIOLOGY. 



tial, if tlie preparation is to be of any use, to refrain 

 from heating it in order to dry it more quickly. If 

 left alone, the bacteria do not practically alter their 

 shape at all during the process of drying ; but they 

 do not adhere so firmly to the cover- glass that they 

 cannot be easily washed off with water, and hence it 

 is necessary to "fix " the layer which contains them. 

 The simplest way to do this is to pass the cover-glass, 

 with the side on which the bacteria are uppermost, 

 three times through the flame of a Bunsen burner, at 

 the same rate as the pendulum of a middle-sized regu- 

 lator swings. 



This fixing requires great care ; if the cover-glass 

 is not sufficiently heated, the bacteria do not subse- 

 quently stain so readily, and also are apt to become 

 detached from its surface j if, on the other hand, it is 

 too much heated, they almost entirely lose their power 

 of absorbing the stain. The safest plan is to take up 

 the cover-glass by its edges between the forefinger 

 and thumb, and to pass it quickly through a low flame 

 of the Bunsen burner, until it becomes unpleasantly 

 warm. 



Particles of gelatine and agar-agar must never be 

 transferred with the bacterium material on to the 

 cover-glass, for, although they dry well, these nutrient 

 media do not become securely fixed, and hence are apt 

 to become detached later, especially when warm stain- 

 ing solutions are added, when as a rule they carry the 



