180 BACTERIOLOGY. 



concentrated suiphuric acid in the desiccator. Do not 

 fix it in the flame. When dried, treat the cover-slip 

 with 1 drop of a mordant made by dissolving 25 

 grammes of tannic acid in 100 c.c. of a warm 1 per cent, 

 caustic potash solution, and add 4 or 5 drops of freshly- 

 prepared Ziehl-Neelsen carbol-fuchsin. Precipitation 

 at once occurs. Leave the stain on for twenty to 

 twenty-five minutes; wash in distilled water. The 

 thinner the preparation the better is the result. 



No matter which of the foregoing methods is selected, 

 there are certain features common to them all which 

 appear to be essential to satisfactory results, viz., per- 

 fectly clean cover-slips ; very great care in spreading 

 the bacteria on the slip, so as to avoid tearing off of 

 the flagella ; repeated dilution, so as to obtain relatively 

 few bacteria, and these well separated from one another; 

 and, finally, great care not to employ high temperatures 

 when either fixing or staining the preparation. 



STAINING IN GENERAL. 



The physics of staining and decolorization is hardly 

 a subject to be discussed at length in a book of this 

 character; but, as Kiihne has pointed out, it may be 

 briefly said that solutions which favor the production of 

 diffusion-currents facilitate intensity of staining, and by 

 a similar process increase the energy of decolorizing- 

 agents. For example, tissues which are transferred 

 from water into watery solutions of the coloring-mat- 

 ters are less intensely stained and more easily decolor- 

 ized than when transferred from alcohol into watcrj' 

 staining-fluids ; for the same reason tissues stained in 

 watery solutions of the dyes do not become decolorized 



