86 APPLIED BACTERIOLOGY 



detached and carry the bacteria off the cover-glass with 

 them. Only the smallest possible quantity of material 

 should be used, otherwise the bacteria will be found to be 

 too crowded ; the right quantity to use is soon found by a 

 little practice. The cover-glass is now allowed to dry 

 either spontaneously, taking care to protect from dust, 

 or by holding between the fingers some distance over a 

 fiame. 



Now, in order to prevent the bacteria from becoming 

 detached from the cover-glass during the subsequent wash- 

 ing, it is necessary to ' fix ' the layer containing the bacteria. 

 To ' fix ' the bacteria, the cover-glass is held in a pair of 

 forceps, and is passed (the side on which the bacteria are 

 uppermost) through the flame of the Bunsen burner three 

 times, at the same rate as the swing of the pendulum of an 

 ordinary clock. 



This fixing must be done with great care ; if the cover- 

 glass is not sufficiently heated, the bacteria come off during 

 the washing, and if, on the other hand, the cover-glass 

 is overheated, the bacteria lose their power of absorbing 

 the stain. 



The preparation is now stained by transferring a few 

 drops of the stain on to the cover-glass by means of a 

 pipette; or the cover-glass is laid face downwards upon 

 the surface of the stain, which is contained in a watch- 

 glass or small dish, in such a way that the cover-glass 

 floats upon the surface of the liquid. It is best to hold 

 the cover-glass by the edge between the thumb and first 

 finger, and then to bring it as close as possible to the 

 surface of the stain, and drop it suddenly. 



The second method gives the best and most evenly- 

 stained preparations after a little practice, but the first is 

 somewhat easier. When using the first method, the cover- 

 glass should be quite covered with the stain. If it is 



