The speaker described in detail the various operations, exhibited the
different kinds of apparatus employed, showed the operations, and
exhibited the finished sections. In some rocks a thin chip can be
broken off, others require to be sawn, and for the latter purpose the
diamond saw is best. Having obtained the chip, it is first polished on
one side, then cemented to a little square of glass, and the other
side polished in the same way. The sections must not be too thick, nor
too thin; they are usually made from a hundredth to a thousandth of an
inch thick. Lathes employed in polishing minerals require to be
provided with conical spindles, so that the wear, due to grit and
emery dust getting on them, may be readily taken up. The grinding
wheel may be either horizontal or vertical; the former has the
advantage that the mineral can be held in either hand; with the latter
only the right hand can be employed, and that in an awkward and
tiresome position. Mr. Julian then referred briefly to the kinds of
emery, its preparation by elutriation, etc., and cautioned operators
against using rouge or tin putty powder in polishing rock sections,
although they may be employed in polishing certain minerals and gems.
The object of making the rock sections being to study their
constituents and determine what minerals enter into their composition,
it is important that no foreign substance, liable to adhere to the
specimen and to be mistaken for one of its ingredients, be placed on
the section while grinding. Lastly, the minerals are mounted on glass,
with or without covers, by means of Canada balsam. Square glasses are
to be preferred to the long and narrow strips, usually employed, as
less liable to break in the center, and more easily revolved on the
stage of a microscope.
Mr. L.H. Landy then exhibited, by means of the gas microscope, several
beautiful rock sections, both American and German. The same gentleman
also showed the effect of passing polarized light through certain
crystal sections, the black cross and rainbow-hued rings revolving
like so many wheels as the polarizer was turned.
At the conclusion of this brilliant exhibition, Dr. P.T. Austen made
some remarks on