COVER-GLASS PREPARATIONS 85 



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 

 desirable to quicken the staining process, as is necessary in 

 the case of some organisms, by using hot staining solution, 

 the cover-glass, well covered with the staining reagent, is 

 held by means of a pair of forceps over a low gas-flame 

 until steam just begins to rise from the liquid ; when this 

 happens, the source of heat is removed. This treatment is 

 then repeated at frequent intervals. A better method is to 

 float the cover-glass face downwards upon the staining 

 liquid, which has just previously been heated in a small 

 dish, until the steam begins to rise. Great care must be 

 taken not to allow the staining solutions to boil, as this 



