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PRECIOUS STONES.
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cases is colourless, or what is ordinarily called white glass.
But if to these substances metallic oxides, or metals in a divided
state, are added, even in minute quantities, the result is coloured
glass.
Chemical
analysis shows us that the elements of glass are found in all
vegetables. If, then, a fire consumes a certain quantity of wood,
gathered together at a single spot, vitrifications will be found in
the residuum. When silicious stones are subjected to an intense heat,
the bases contained in the stones and in the cinders combine and
produce glass. This is what may be seen every day in an examination of
the interior walls of a lime-kiln or brick-kiln. It is evident, then,
that the discovery of glass belongs to the earliest period of man's
existence. If it be remarked, besides, that the glass thus obtained is
always coloured, and therefore in harmony with the pronounced taste of
primitive people for brilliant objects, we understand how these
vitreous substances produced by conflagrations and, above all, by the
action of fire upon silicious stones, must have excited, in the most
lively manner, the attention of men from the first ages of our species.
Had
this book been written a dozen or fifteen years ago, it could have
furnished but little information on this head; but, thanks to the
researches
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