only attainable in the maximum degree by simple polish.
An
antique engraving upon a presumable opal is a head of Sappho; and in
the collection of the Duke of Orleans was a head of Juba engraved upon
an opal. There is, too, in the national collection at Paris, an opal
engraved with a portrait of Louis XII.
THE HYDROPHANE.
The
hydrophane, composed of 93 in 100 parts of silica, 2 of alumina, and 5
of water, is a very celebrated stone, known from early antiquity.
In
its ordinary state the hydrophane is a white or reddish-yellow
substance, feebly translucent or completely opaque. But if it is
plunged into water it disengages little bubbles of gas, and at the same
time becomes transparent, and sometimes displays the colours of the
opal.
Taken
from the water this curious stone keeps its transparency for a short
time, but gradually as the water evaporates becomes once more opaque.
The ancient mineralogists, considering this stone an unexampled marvel, gave it the name of Oculus Mundi, the " eye of the world."
