50 TEXT-BOOK OF BACTERIOLOGY. 



spreading out the liquid. Then lay a second cover-glass over the 

 first, and produce between the two a perfectly even, extremely 

 thin layer. Now carefully draw the upper glass away from the 

 lower one, and there are at once two preparations, both of which 

 can be used. It is necessary to wait till they are perfectly air-dried 

 and all traces of moisture have disappeared. Then seize the cover- 

 ^lass with the forceps and draw it three times through the flame. 

 If we wish to remove any haemoglobin that may be in the layer 

 TDefore staining it, and thus isolate the bacteria as much as possible, 

 we must lay the preparation for a few seconds in a 1^ to 5^ solution 

 of acetic acid (as recommended by Gunther), remove this with dis- 

 tilled water, and dry the cover-glass anew before proceeding. 



Generally, however, this precaution is unnecessary, and we may 

 therefore drop some of the dilute alcoholic anilin solution from a 

 pipette upon the preparation immediately after the heating process 

 in the flame. 



When the stain has operated a few minutes, wash awaj' what is 

 superfluous with distilled water and the process is complete. 



The time to be allowed for the action of the stain cannot be 

 definitely fixed. It depends upon the nature of the preparation 

 itself and on the staining power of the dye emploj'ed. 



Methyl-blue, for instance, always requires much more time than 

 fuchsin or gentian-violet. 



The preparation is now ready to be examined in water. Dry 

 the upper surface of the cover-glass with some blotting-paper or 

 with the finger (since it has to receive the drop of oil for the im- 

 mersion lens), and then the wet preparation is laid on a slide. The 

 evaporating fiuid must be added to from time to time, to avoid 

 difficuities in the refractive conditions — the so-called dry blots — 

 which would otherwise occur and make investigation impossible. 



In examining such objects in water, too, one often sees Brown's 

 molecular movement, even in the stained bacteria. A few bacilli 

 or cocci which have freed themselves from their surroundings may 

 be seen to merrily dance about in the fluid. 



Of course the cover-glasses will bear all kinds of staining mat- 

 ter. We can with equal satisfaction use the simple anilin staining 

 solutions, or the alkaline bacteria stains, or the anilin-water mix- 

 tures. If it is desired to obtain particularly' intense and rapid 

 stainings, the coloring matter is warmed on the cover-glass, or the 

 latter is allowed to float, preparation side downward, on the surface 

 of a hot solution of the dj'e. 



After the more active staining processes, it is often necessary 

 to employ the stronger decoloring agents, alcohol, acids, etc. 



