ing a bad conductor of electricity: it becomes positively electrified
by friction. The electrical resistance is about that of ordinary glass,
and is diminished by one-half during exposure by Rontgen rays; the
dielectric constant (16) is greater than that which should correspond to
the specific gravity.

The phosphorescence produced by friction has been known since the time
of Robert Boyle (1663); the diamond becomes luminous in a dark room
after exposure to sunlight or in the presence of radium; and many stones
phosphoresce beautifully (generally with a pale green light) when
subjected to the electric discharge in a vacuum tube. Some diamonds are
more phosphorescent than others, and different faces of a crystal may
display different tints. The combustibility of the diamond was predicted
by Sir Isaac Newton on account of its high refractive power; it was
first established experimentally by the Florentine Academicians in 1694.
In oxygen or air diamond burns at about 850 deg., and only continues to do
so if maintained at a high temperature; but in the absence of oxidising
agents it may be raised to a much higher temperature. It is, however,
infusible at the temperature of the electric arc, but becomes converted
superficially into graphite. Experiments on the combustion of diamond
were made by Smithson Tennant (1797) and Sir Humphry Davy (1816), with
the object of proving that it is pure carbon; they showed that burnt in
oxygen it yields exactly the same amount of carbon dioxide as that
produced by burning the same weight of carbon. Still more convincing
experiments were made by A. Krause in 1890. Similarly Guyton de Morveau
showed that, like charcoal, diamond converts soft iron into steel.
Diamond is insoluble in acid and alkalis, but is oxidised on heating
with potassium bichromate and sulphuric acid.

Bort (or Boart) is the name given to impure crystals or fragments
useless for jewels; it is also applied to the rounded crystalline
aggregates, which generally have a grey colour, a rough surface, often a
radial structure, and are devoid of good cleavage. They are sometimes
spherical ("shot bort"). Carbonado or "black diamond," found in Bahia
(also recently in Minas Geraes), is a black material with a minutely
crystalline structure somewhat porous, opaque, resembling charcoal in
appearance, devoid of cleavage, rather harder than diamond, but of less
specific gravity; it sometimes displays a rude cubic crystalline form.
The