ful nitro body, the
hexa-nitro compound, is completely abolished. Connective tissue and bone
substance, however, behave differently with the picro-carmine mixture,
in as much as here the diffuse stain depends exclusively on the
concentration of the carmine, and is quite uninfluenced by the addition
of a chemical antidote. This staining can only be limited by dilution,
but not by the addition of opposed dyes. We must look upon the latter
kind of tissue stain not as a chemical combination, but as a mechanical
attraction of the stain on the part of the tissue. We may also say:
=chemical stains are to be recognised by the fact that they react to
chemical antidotes; mechanical stains to physical influences=; of course
always assuming, that purely neutral solutions are employed, and that
all additions, which alter the chemical relation of the tissues such as
alkalis and acids, or which raise or limit the affinity of the dye for
the tissues, are avoided. A further consequence of this view is, that
all successive double staining may be serviceably replaced by
simultaneous multiple staining, if the chemical nature of the staining
process is settled. In contradistinction, in all double stains, which
can only be effected by successive staining, mechanical factors are
concerned.

In the staining of the dry blood specimen, purely chemical staining
processes are concerned, and therefore the polychromatic combination
stain is possible in all cases.

The following combinations are possible for the blood:

1. Combined staining with acid dyes. The best known example is the
eosin-aurantia-nigrosin mixture, in which the haemoglobin takes on an
orange, the nuclei a black, and the acidophil granulations a red hue.

2. Mixtures of basic dyes. It is possible straight away to make mixtures
consisting of two basic dyes. As specially suitable we must mention
fuchsin, methyl green, methyl violet, methylene blue. On the other hand,
mixtures of three bases are fairly difficult to prepare, and the
quantitative relations of the constituents must be exactly observed. For
such mixtures, fuchsin, bismarck brown, chrome green, may be used.

3. Neutral mixtures. These have played an important part in general
histology, from the time that they were first introduced by Ehrlich into
the histology of the blood up to the present day; and deserve before all
others a full consideration.

Neutral staining rests on the fact, that nearly all basic dyes (_i.e._