THE MICROTOMIST'S VADE-MECUM

taken for mixture with alcohol, as if chromic acid be dissolved
directly in alcohol a very violent reaction takes place.)
215. Palladium Chloride (F. E. Schulze's method ').-Single
palladium chloride (Pd Cl) may be prepared by dissolving
palladium-ore in hydrochloric acid containing a certain quan-
tity of nitric acid, and evaporating to dryness. To dissolve
the chloride water acidulated with HC1 must be employed;
to dissolve the chloride of commerce take for 10 grammes of
the chloride 1 litre of water and 4 to 6 drops HCL. The solu-
tion will be complete in twenty-four hours. Such a solution
may be conveniently kept in stock and diluted as required for
use.   The 1-100 solution should be dark red-brown, a 1-800
solution pale yellow, like a 0,2 per cent. chromic-acid solution.
The free acid of the solution is necessary to the reaction it
is intended to produce in the tissues.
To harden tissues containing much connective tissue, as,
for example, the ciliary body, a 1-800 solution should be
taken. A piece of the tissue of the size of a bean may be
placed in 30 c.c. of the solution, care being taken to remove
all structures that would be an obstacle to the penetration of
the chloride, which, for many tissues, has far less penetrating
power than chromic acid. This is particularly the case with
structures that are poor in connective tissue, as certain nerves,
brain, and spinal cord, and deeply-stratified epithelia, such as
human epidermis. Very small pieces, therefore, of such
tissues must be employed; of nerves such may be chosen as
are rich in connective tissue, as the anterior part of the N.
opticus, for the solution has great penetrating power in regard
to all connective tissue.
Hardening is generally complete at the normal temperature
in two or three days, but the specimens may remain in the
solution for weeks or months without hurt. The consistency
of the hardened tissues is far more favorable for cutting
than that obtained by chromic acid or Miller's solution,
I ' Arch. Mik. Anat.,' iii (1867), p. 477.

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