On these words of Pliny a great dispute has arisen. Some think that onyx is the material described, a conjecture founded on the variety of colors which that stone presents. To this it is objected, that onyx and murrha, onyx vases and murrhine vases are alike mentioned by Latin writers, and never with any hint as to their identity; nay, there is a passage in which Heliogabalus is said to have onyx and murrhine vases in constant use. Others, as we have said, think that they were variegated glass; others that they were the true Chinese porcelain, a conjecture in some degree strengthened by a line of Propertius:
"Murrheaq. in 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.