Though this art is fully treated of in books on practical biology, it
is occasionally of use to the physicist, and the following note treats
of that part of the subject which is not distinctly biological.

Soft materials, of which thin sections may be required, generally
require to be strengthened before they are cut. For this purpose a
variety of materials are available. The one most generally used is
hard paraffin. The only point requiring attention is the embedding.
The material must be dry.

This is accomplished by soaking in absolute alcohol, i.e. really
absolute alcohol made by shaking up rectified spirit with potassium
carbonate, previously dried, and then digesting for a day with large
excess of quick-lime, making use of an inverted condenser and finally
distilling off the alcohol without allowing it to come in contact with
undried air. After soaking for some time in absolute alcohol, the
material may be transferred to oil of bergamot, or oil of cloves, or
almost any essential oil. After soaking in this long enough to allow
the alcohol to diffuse out, the material may be lifted into a bath of
melted paraffin (melting at, say, 51 deg. C.). The process of soaking is
in some cases made to go more rapidly by exhausting, and, if the
material will stand it, by raising the temperature over 100 deg. C. The
soaking process may require minutes, hours, or days, according to the
size and density of the material; but a few hours are usually
sufficient.

When cold, the sections may be cut in any of the ordinary forms of
microtome.

Another way of embedding is to soak in collodion, and then precipitate
the latter in the material and around it by plunging into nearly
absolute alcohol. The collodion yields a harder matrix than the
paraffin.

Whatever form of cutting machine is employed, the art of sharpening
the knife is the only one requiring any particular notice. The
easiest way of obtaining a knife hard enough to sharpen, is to use a
razor of good quality. If it has to be ground, it is best to do this
on a fine Turkey stone which is conveniently rested on two bits of
rubber tubing, to avoid jarring the blade. Many stones are slightly
cracked, but on no account must the razor be dragged across a crack,
or the edge will suffer.

The necessary and sufficient condition is that the razor must be
worked in little sweeps over the stone, and pressed against the latter
by little more than its own weight, and