Parthis pocula cocta focis."

At the same time this quotation is not so conclusive as it might have
been, since Pliny speaks of murrha as "hardened in the earth by heat,"
and the poet may only have meant the same thing, though the expression
in that case would be somewhat strained. To us, Pliny's description
appears to clearly point to some opaline substance; the precious opal
has never in modern times been found in masses approaching to the size
necessary to make vessels such as we have spoken of. The question is
not likely to be settled, and it is not improbable that the material
of these murrhine vases is entirely unknown to us, as the quarries of
many marbles used by the ancients have hitherto eluded our research,
and the marbles themselves are only known by their recurrence among
ancient buildings.

We may here notice one or two facts connected with glass, which show
that the ancients were on the verge of making one or two very
important discoveries in physical science. They were acquainted with
the power of transparent spherical bodies to produce heat by the
transmission of light, though not with the manner in which that heat
was generated by the concentration of the solar rays. Pliny mentions
the fact that hollow glass balls filled with water would, when held
opposite to the sun, grow hot enough to burn any cloth they touched;
but the turn of his expression evidently leads to the conclusion that
he believed the heat to become accumulated in the glass itself, not
merely to be transmitted through it. Seneca speaks of similar glass
balls, which magnified minute objects to the view. Nay, he had nearly
stumbled on a more remarkable discovery, the composition of light, for
he mentions the possibility of producing an artificial rainbow by the
use of an angular glass rod. At a far earlier period Aristophanes
speaks of "a transparent substance used to light fires with," usually
translated glass. The passage is curious, as it shows a perfect
acquaintance with the use of the burning glass.

With the laws of reflection the ancients, as we know from the
performances ascribed to Archimedes, were well acquainted. It is
singular that being in possession of such remarkable facts connected
with refraction, they should never have proceeded to investigate the
laws by which it is governed.

[Illustration: GLASS VESSELS (_of Pompeii_).]

The first object figured _h_, in the annexed block, is a glass funnel,
_infundibulum_;