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INHERENT QUALITIES
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115
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It
has long been known that some diamonds absorb light. Robert Boyle in
1664 described this property of shining or phosphorescing in the dark,
after being exposed to the sunlight. Late experiments have again
demonstrated this peculiar power. The same result is obtained by
exposing diamonds to a high-tension current of electricity in a vacuum,
the light produced being of different colors, though South African
stones emit in a majority of cases, a bluish light. Exposed to radium,
diamonds glow with varying degrees of light and in various colors.
Colorless crystals which Sir William Crookes kept embedded in radium
bromide for a period of 12 months, were found to have assumed a bluish
tint which resisted both fire and acids. They had also become
radio-active, and heating to dull redness did not destroy the acquired
power. Diamond is transparent to the X-rays, while glass is practically
opaque.
The
somewhat general idea that this quality of shining in the dark is
common to all diamonds is an error founded on the statement by careless
educators, of the truth that some do so. Isolated cases have been
mentioned in such a way that they have been understood as typical, and
some descriptions of phosphorescent stones have been quite imaginative.
Reading in a dark room by the light of a phosphorescent diamond is so
rare that no person other than the narrator would be likely to meet
with a similar case. Experiments show that very few diamonds, either by
exposure to sunlight or rubbing, will show any light in a dark room.
The diamond is ranked as a non-conductor of electricity and though, on rubbing, it becomes positively elec-
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