CLEARING AGENTS

CHAPTER XXV.
CLEARING AGENTS.
277. Clearing agents are liquids whose primary function it
is to make microscopic preparations transparent by pene-
trating amongst the highly refracting elements of which the
tissues are composed, the clearing liquids themselves having
i an index of refraction not greatly inferior to that of the
tissues to be cleared. Hence, all clearing agents are liquids
of high index of refraction. The same substances have
generally a second function which consists in getting rid of
the alcohol in which preparations are generally preserved and
facilitating the penetration of the balsam or other resinous
medium in which preparations are, in most cases, finally
mounted. Hence, all of the group of bodies here called
"clearing agents," must be capable of expelling alcohol from
tissues and must be at the same time solvents of Canada
balsam and the other resinous mounting media.
It is important to note the manner of employing these
agents. The old plan was to take the object out of the
alcohol and float it on the surface of the clearing medium in
a watch-glass. This plan was faulty, because the alcohol
i escapes from the surface of the object into the air quicker (in
most instances) than the clearing agent can get into it;
hence the object must shrink. To avoid or lessen this cause
of shrinkage, clearing is now generally done by the method
suggested by Giesbrecht, which consists in putting the
clearing medium   under the alcohol containing the object.
This is done in the following manner. Take a test-tube, and

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