called Thumerite, from
the name of Mount Thumor, where it was first procured. It is found in
great agglomerations, and in somewhat different forms, which may,
however, all be reduced to the rhomboidal, that is, to an oblique
rhomboid or prism with four sides, so much compressed as to make the
angle so sharp that it resembles the edge of an axe. It is translucent,
and sometimes transparent ; it has single refraction ; its light is
vitreous and resinous ; its colour brown, violet-blue, grey, or yellow.
It scratches glass, but is scratched by the topaz ; it yields a whitish
powder ; its specific weight is from 3*27. It becomes electric when
warmed or rubbed ; under the action of the blowpipe it melts into a
brown-grey glass ; acids have no effect on it, and it is composed of
lime, alumina, and silex, with oxide of iron and manganese. It is found
principally in primitive rocks, but also in others of different
formations, in the Dauphiné, in the Pyrenees, in Norway, and at St.
Gothard. This mineral, especially that from the Dauphiné, takes a
beautiful polish.
XV.
BERYL.
The stone
which the ancients named beryllus is none other than that now called
aquamarina of Ceylon, of which we have already spoken.
Compared with other gems, the beryl is but little prized at the present day, being procured in great
