metallic
ingot is attacked with hot nitro-hydrochloric acid until no more iron
is dissolved. The bulky residue consists chiefly of graphite, together
with translucent chestnut-coloured flakes of carbon, black opaque
carbon of a density of from 3'0 to 3'5, and hard as diamonds—black
diamonds or carbonado, in fact—and a small portion of transparent
colourless diamonds showing crystalline structure. Besides these, there
may be carbide of silicon and corundum, arising from impurities in the
materials employed.
"
The residue is first heated for some hours with strong sulphuric acid
at the boiling point, with the cautious addition of powdered nitre. It
is then well washed and for two days allowed to soak in strong
hydrofluoric acid in cold, then in boiling acid. After this treatment
the soft graphite disappears, and most, if not all, the silicon
compounds have been destroyed. Hot sulphuric acid is again applied to
destroy the fluorides, and the residue, well washed, is attacked with a
mixture of the strongest nitric acid and powdered potassium chlorate,
kept warm—but not above 60° C, to avoid explosions. This treatment must
be repeated six or eight times, when all the hard graphite will
gradually be dissolved, and little else left but graphitic oxide,
diamond, and the harder carbonado and boart. The residue is fused for
an hour in fluorhydrate of fluoride of potassium, then boiled out in
water, and again heated in sulphuric acid. The well-washed grains which
resist this energetic treatment are dried, carefully deposited on a
slide, and examined under the microscope. Along with numerous pieces of
black diamond are seen transparent colourless pieces, some amorphous,
others with a crystalline appearance. Although many fragments of
crystals occur,
