salts of the dye bases, for instance, rosanilin acetate) form
combinations with acid dyes (_i.e._ salts of the dye acids, for
instance, ammonium picrate) which are to be regarded as neutral dyes,
such as rosanilin picrate. Their employment offers considerable
difficulties as they are very imperfectly soluble in water. A practical
application of them was first possible after Ehrlich had ascertained
that certain series of the neutral dyes are easily soluble in excess of
the acid dye, and so the preparation of solutions of the required
strength, readily kept, was made possible. Among the basic dyes which
are suitable for this purpose are those particularly which contain the
ammonium group, especially methyl green, methylene blue, amethyst
violet[5] (tetraethylsafraninchloride), and to a certain extent pyronin
and rhodamin also. In contradistinction to these, the members of the
triphenylmethan series, such as fuchsin, methyl violet, bismarck brown,
phosphin, indazine, are in general less suited for the purpose, with the
exception of methyl green already mentioned. The acid dyes specially
suited for the production of soluble neutral stains are the easily
soluble salts of the polysulpho-acids. The salts of the carbonyl acids
and other acid phenol dyes are but little suitable: and least of all,
the nitro dyes. Specially to be mentioned among the acid dye series are
those which can be used for the preparation of the neutral mixtures:
orange g., acid fuchsin, narcein (an easy soluble yellow dye, the sodium
salt of sulphanilic acid--hydrazo-[beta]-naphtholsulphonic acid).

If a solution of methyl green be allowed to fall drop by drop into a
solution of an acid dye, for instance orange g., a coarse precipitate
first results, which dissolves completely on the further addition of the
orange. No more orange should be added than is necessary for complete
solution. This is the type of a simple neutral staining fluid.
Chemically the above-mentioned example may be thus explained; in this
mixture all three basic groups of the methyl green are united with the
acid dye, so that we have produced a triacid compound of methyl green.

Simple neutral mixtures, which have one constituent in common, may be
combined together straight away. This is very important for triple
staining, which can only be attained by mixing together two simple
neutral mixtures, each consisting of two components. A chemical
decomposition need not be feared. We thus get