THEORY OF STAINING

incompatibility of certain fixing agents with certain staining
agents. Thus chromic acid and osmic acid, the two best
fixing agents known, are to a great degree incompatible with
staining by carmine, the most trustworthy of staining agents,
often rendering the operation of staining with carmine so
difficult that it is better to abandon it and employ some
other stain. A few hints on this distressful matter may here
be useful.
Hematoxylin is the best stain to use after chromic acid;
but some anilins (safranin, magdala, dahlia, for instance)
give good results when employed by the Hermann Bbttcher
process (see Anilins, No. 136).
Cochineal may also be used after chromic acid.
After osmic acid, picro-carmine or alum-carmine, or heema-
toxylin. But osmic acid preparations generally stain well
only after bleaching (see BLEACHING).
All stains take well after fixation with alcohol or with
corrosive sublimate, or with nitric acid.
It should be noted that, as found both by Flemming and
by Mayer, objects that have been too long in alcohol, or that
have been treated with very strong alcohol, no longer take a
precise nuclear stain, either with anilins or with other
colouring agents.
After fixation with any of the picric-acid fluids, and clue
washing out with alcohol, all objects stain well with any of
the usual stains.
45. Carmine.-Carmine, which is probably the most valu-
able and certainly is the most widely employed of histological
colouring agents, was first proposed as an aid in the examina-
tion of animal tissues by Gerlach in 1858, since which time,
notwithstanding the discovery of numerous other substances
that have proved most useful in many kinds of research, it
has held its place, with a firmness that shows few signs of
yielding, as the staining agent par excellence.
It is not a definite chemical substance. As is well known,

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