FIXING AGENTS

Od11ai  tissues that are put into them   in the fresh state; but in
asart  reality none of these liquids has such a property.       It is
possible to obtain permanent preparations of the most delicate
l4tion organic elements; but in order to attain that end it is neces-
Oeasia  sary to employ not one single preservative liquid, but a series
means  of reagents, of which some serve to fix the elements in their
ml W  form and render them     more or less unchangeable, others to
101 as  stain them, and others, finally, to preserve them from ulterior
To   changes " (Ranvier).
Od  5. A fixing agent should possess the following qualities:
od'005 h  It must kill quickly, without exerting any injurious chemical
11 '   action on the tissues; it must not be dehydratant, or it will
Mhich  cause shrinking; it must not interfere with subsequent pro-
atural  cesses, as by dyeing the tissues, or by preventing them  from
moin staining well, or by making them brittle. There is, I believe,
she  only one reagent that is free from all these defects, viz. vapour
With.  of osmium; choice must be made amongst the others, when-
tting  ever it is necessary to employ one of these, of that which has
a ell  the least defects for the particular case.
ls aro   Fixing agents may be conveniently grouped into two classes:
on o those which do not seriously interfere with subsequent stain-
sp~o   ing by carmine, and those which do. In the former class we
have-
Alcohol.
S           Osmium vapour.
reste Corrosive sublimate.
Wei   Picric acid (either pure or in the form  of Kleinenberg's
d in         fluid).
avail.      Merkel's solution.
Un in       Nitric acid.
rtions      Silver nitrate (in weak solutions).
going In the latter class we have-
mater Chromic acid.
com.  Chromate of ammonia.
tlV Potassium bichromate.

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