§ 79 Cutting Sections of Soft Substances.
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° 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° 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 the grinding must be regular. The edge may be inspected under a microscope, and it must be perfectly smooth and even before it will cut sections. A finishing touch may be given on a leather strap, but it must be done skilfully, otherwise it is better omitted.
The necessity for providing exceptionally keen and sharp edges arose in the manufacture of phonographs, where the knife used to turn up the wax cylinders must leave a perfectly smooth surface. In 1889 this was being accomplished on an ivory lap fed with a trace of very fine diamond dust.
I have had this method in mind as a possible solution of the difficulty of razor-grinding, but have not tried it. I imagine one would use a soft steel or ivory slip rubbed over with fine diamond dust and oil by means of an agate. The lap used in the phonograph works was rotated at a high speed.