ree-quarters of an inch. If the Canada is more viscous, so
that the thread does not break, the section when cemented by it will
most probably slip on the slide. On the other hand, if the balsam is
more brittle, it will crumble away during the grinding.

Assuming that the proper point has been reached, the section is
mounted with the usual precautions to avoid air bubbles, i.e. by
dropping one edge on the balsam first. When all is cold, the surface
of the section may be ground on an iron plate with emery passing the
80 sieve, till it is about 1/40 inch thick. From this point it must
be reduced on ground glass by flours of emery and water; the rough
particles of the former may be washed out for fine work.

The process of grinding should not take more than half an hour if the
section is properly cut, etc. Beyond this point the allowable
thickness must depend on the nature of the rock; a good general rule
is to get the section just so thin that felspars show the yellow of
the first order in a polarising, microscope. The section is then
finished with, say, two minutes emery or water of Ayr-stone dust. It
is better not to have the surface too smooth.

To transfer the section, the hard Canada round the sides is scraped
away, and the section itself covered with some fresh Canada from the
bottle. It is then warmed till it will slip off when a pin, or the
invaluable dentist's chisel, is pressed against one side. If the
section be very delicate, the cover slip should be placed over it
before it is moved to the proper slide. The Canada used for mounting
is not quite so hard as that employed in grinding, but it should be
hard when cold, i.e. not sticky.

The art of preparing Canada balsam appears to consist in heating it
under such conditions as will ensure its being exposed in thin layers.
I have wasted a good deal of time in trying to bake Canada in
evaporating basins, with the invariable result that it was either over
or under-baked, and got dark in colour during the process.

On reviewing the process of rock section-cutting and mounting as just
described, I cannot help thinking that, if properly systematised, it
could be made much more rapid by the introduction of proper automatic
grinding machinery. It also seems not improbable that a proper
overhaul of available gums and cements would be found to lead to a
cementing material less troublesome than Canada balsam.

Sec. 79. Cutting Sections of Soft Substances.