104 Manual of Veterinary Microbiology. 



stances whicli act the part of mordant, that is, which 

 increase the penetrating power of the coloring sub- 

 stance and render its fixation more energetic; such 

 are, for example, alum, potash, soda, lithia, carbonate 

 of ammonia, aniline, etc., all of which we will find 

 in the various liquids used in histology and micro- 

 biology. 



Heat is calculated to expedite the staining of mi- 

 crobes, as well as of organic tissues in general; we 

 shall see that this property has been taken advantage 

 of in order to obtain rapid staining of various bacilli, 

 and especially those of tuberculosis. 



Two groups of aniline colors are recognized. In 

 the first {basic colors) the coloring substance is asso- 

 ciated with a colorless acid. In the second {acid 

 colors) it plays the part of acid and is combined with 

 a colorless base. 



This distinction should be noted because the colors 

 of the first class have a special afiinity for cell nuclei 

 and for bacteria, whilst those of the second give dif- 

 fuse staining, that is, fix themselves indifferently on 

 all parts of the cells and on the intercellular sub- 

 stance. 



The basic colors include gentian and methyl violet, 

 methyl green, methylene blue, fuchsin, bismark brown^ 

 vesuvin, and safranin. 



The principal acid colors are eosin and Jluorescine. 



Single and double staining. — In any preparation con- 

 taining microbes we can give to all the elements, mi- 

 crobic and non-microbic, the same coloration. In the 

 case of certain micro-organisms we ought, indeed, to 

 limit ourselves to this method of staining, such as 

 the germs of fowl cholera, pneumo-enteritis of the 



