THE MICROTOMIST I VADE-MECUM

employed as an examination and mounting medium. Dilution
with water is sometimes advisable from an optical point of
view, on account of the increased visibility that it gives to
many structures by lowering the index of refraction of the
glycerin. But from the point of view of efficacious preserva-
tion, it is always advisable to use undiluted glycerin, the
strongest that can be procured.
Long soaking of tissues in glycerin of gradually increased
strength is a necessary preliminary to mounting in all cases
in which it is desired to obtain the best possible preparations
and to ensure that they shall keep well. If this soaking is
done on the slide (the cover being removed and the object
treated with fresh glycerin every one or two days), it is well
to take the precaution recommended by Beale, of luting the
edges of the cover so as to make the preparation airtight, as
glycerin is so highly hygroscopic that a drop of it exposed to
the air rapidly diminishes in strength to a very considerable
degree. In order to facilitate the removal of the cover in
this process, the slide may be gently warmed by passing it
two or three times through the flame of a spirit-lamp. No
preparation can be considered to be made secundum artem
until every part of the object has been thoroughly impregnated
with strong pure glycerin.
The shrinking that frequently occurs when delicate struc-
tures are brought into glycerin may generally be cured by this
treatment; cells which at first appear hopelessly collapsed
gradually swell out to their normal forms and dimensions.
For closing glycerin mounts, the edges of the cover should
first (after having been cleansed as far as possible from
superfluous glycerin) be painted with a layer of glycerin-jelly;
as soon as this is set a coat of any of the usual cements may
be applied.
Glycerin dissolves carbonate of lime, and is therefore to be
rejected in the preparation of calcareous structures that it is
wished to preserve.

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