ill be,
effected.

Sec. 78. Grinding Rock Sections, or Thin Slips of any Hard Material.

A note on this is, perhaps, worth making, for the same reasons as were
given for note, Sec. 75, which it naturally follows. Just as
trout-fishing; is described by Mr. Francis as the "art of fine and far
off," [Footnote: In the Badminton Library, volume on Fishing.]
section grinding may be called "the art of Canada balsam cooking," as
follows. A section of rock having been cut from the lump as just
described, it becomes; necessary to grind it down for purposes of
microscopical investigation. For this purpose it is placed on a slip
of glass, and cemented in position by Canada, balsam. Success in the
operation of grinding the mounted section depends almost entirely on
the way in which the mounting is done, and this in its turn depends on
the condition to which the Canada has been brought.

To illustrate the operations, I will describe a specific case, viz.
that of grinding the section of "gabbro"' above described, for
microscopical purposes. One side of the section is probably
sufficiently smooth and plane from the operation of the diamond wheel;
if not, it must be ground by the finger on a slab of iron or gun-metal
with emery and water, the emery passing a sieve of 80 threads to the
inch. The glass base on which the section is to be mounted for
grinding is placed on a bit of iron or copper plate over a Bunsen
burner, and three or four drops of natural Canada balsam are placed
upon it. The section is placed on the plate to heat at the same time.

The temperature must not rise so high as to cause any visible change
in the Canada balsam, except a slight formation of bubbles, which rise
to the surface, and can be blown off. The heating may require to be
continued, say, up to twenty minutes. The progress of the operation
is tested by examining the balsam as to its viscous properties.

An exceedingly simple and accurate way of testing is to dip a pair of
ordinary forceps in the balsam, which may be stirred a little to
secure uniformity. The forceps are introduced with the jaws in
contact, and, as soon as withdrawn, the jaws are allowed to spring
apart, thus drawing out a balsam thread. In a few moments the thread
is cold, and if the forceps be compressed, this thread will bend.

The Canada must be heated until it is just in such a state that on
bringing the jaws together the thread breaks. The forceps may open to
about th