Pliny mentions having seen images of Augustus cast in this substance. It probably was some coarse kind of glass resembling the ammonitrum, or such as that in which the scoriæ of our iron furnaces abound. Glass was worked either by blowing it with a pipe, as is now practiced, by turning in a lathe, by engraving and carving it, or, as we have noticed, by casting it in a mould. The ancients had certainly acquired great skill in the manufacture, as appears both from the accounts which have been preserved by ancient authors, and by the specimens which still exist--among which we may notice, as pre-eminently beautiful, that torment of antiquaries, the Portland vase, preserved in the British Museum. We have already adverted to another vase of the same kind, and of almost equal beauty, found in one of the tombs near the Gate of Herculaneum. A remarkable story is told by Dion Cassius, of a man who, in the time of the Emperor Tiberius, brought a glass cup into the imperial presence and dashed it on the ground. To the wonder of the spectators, the vessel bent under the blow without breaking, and the ingenious artist immediately hammered out the bruise, and restored it whole and sound to its original form; in return for which display of his skill, Tiberius, it is said, ordered him to be immediately put to death. The story is a strange one, yet it is confirmed by Pliny, who both mentions the discovery itself, and gives a clue to the motives which may have urged the emperor to a cruelty apparently so unprovoked. He speaks of an artificer who had invented a method of making flexible glass, and adds that Tiberius banished him, lest this new fashion should injure the workers in metal, of whose trade the manufacture of gold, silver, and other drinking-cups, and furniture for the table, formed an extensive and important branch. The Romans were also well acquainted with the art of coloring glass, as appears, among other proofs, from the glass mosaics, of which mention has been made. Pliny speaks of a blood-red sort, called hæmatinum, from blood, of white glass, blue glass, etc. The most valuable sort, however, was the colorless crystal glass, for two cups of which, with handles on each side, Nero gave 6,000 sesterces, about $240.