That Tyre possessed peculiar advantages for the manufacture, is very clear from geographical and geological data, the sand upon the shore at the mouth of the river Belus being pure silica, and well adapted to the manufacture. The extensive range of Tyrian commerce, too, gave ample facilities for the exportation and sale of the staple; and for some ages it must have constituted almost the only article, or at least the prominent article, of trade. Doubtless the rich freights of "the ships of Tyre," mentioned in Scripture, may in part have been composed of a material now as common as any of its original elements. From Tyre and Sidon the art was transferred to Rome. Pliny states it flourished most extensively during the reign of Tiberius, entire streets of the city being then occupied by the glass manufactories. From the period of Tiberius the progress of the art seems more definite and marked, both as relates to the quantity and mode of manufacture. It was during the reign of Nero, so far as we can discover, that the first perfectly clear glass, resembling crystal, was manufactured. Pliny states that Nero, for two cups of ordinary size, with handles, gave six thousand sestertia, equal in our currency to about two hundred and fifty thousand dollars; and that rich articles of glass were in such general use among the wealthy Romans as almost to supersede articles of gold and silver. The art, however, at that period, seems to have been entirely devoted to articles of luxury, and from the great price paid, supported many establishments,--all however evidently upon a comparatively small scale, and confined, as it would appear, to families. Up to this period, no evidence appears to prove that any other than colored articles in glass-ware were made. It is clear, too, that the furnaces and melting-pots then in use were of very limited capacity, the latter being of crucible shape; and it was not until the time of Nero that the discovery was made that muffled crucibles or pots, as at the present day, were required in order to make crystal glass. (Without them, it is well known, crystal glass cannot be perfected.) It appears, further, that a definite street in the city of Rome was assigned to the manufacturers of this article; and that in the reign of Severus they had attained such a position, and accumulated wealth to such a degree, that a formal tax was levied upon them. Some writers take the ground that this assessment was the primary cause of the transfer of the manufacture to other places.