PRECIOUS STONES. 71
thallium glass is about 0.050, and of the Diamond 0,057,
so that as regards the greatest charm of the Diamond—its dispersion of
light—thallium glass is almost its equal. This is why imitation
diamonds of good quality look so well at night. But what this glass has
in dispersive powers it lacks in hardness; the facets lose their
lustre, become scratched, and owing to chemical alteration, it goes "
off colour" and takes on an opacity that renders it in time absolutely
worthless. Glass made specially for imitation diamonds must have a high
index of refraction and dispersive power, even though it lack hardness
; and to obtain such a glass a considerable amount of lead is used in
its manufacture. Increase of lead means increase of dispersive power
but decrease in hardness. So far as is known, lead seems to be the only
suitable element that will impart brilliancy to glass. This property
was known to the Romans; but after the fall of Rome, it seems to have
been lost and not re-discovered until about the seventeenth century,
when it again came into use in England, and English glass was
considered the finest. The art of glass-making for all decorative
purposes had reached a very high standard of perfection in ancient
Koine, so high, in fact, that it is doubtful if we equal it to-day with
all our modern methods and improvements.
M.
Feil, of Paris, was one of the first men to produce a good quality
strass, and as a result imitations are now made so well that their
detection is exceedingly difficult without applying suitable scientific
tests.
The
darkening or opacity taken on by paste with age is due to the
sulphiding of the lead. This sulphiding is accelerated in large towns
and cities where there is an
