He relates, that for two moderate-sized colourless drinking-glasses the Emperor Nero paid 6000 sistertii, which is equivalent to 25_l._ of our money. [73] Job xxviii. 17. [74] Plinii Hist. Nat. xxxvi. 26. Pliny relates the story of the man who brought a vessel of malleable glass to the Emperor Tiberius, and who, after dimpling it by dashing it against the floor, restored it to its original shape and beauty by means of a hammer; Tiberius, as a reward for this important discovery, ordered the artist to be executed, in order, as he alleged, to prevent gold and silver from becoming useless. But though Pliny relates this story, it is evident that he does not give credit to it; nor does it deserve credit. We can assign no reason why malleable substances may not be transparent; but all of them hitherto known are opaque. Chloride of silver, chloride of lead and iron constitute no exception, for they are not malleable, though by peculiar contrivances they may be extended; and their transparency is very imperfect. Many specimens of the coloured glasses made by the ancients still remain, particularly the beads employed as ornaments to the Egyptian mummies. Of these ancient glasses several have been examined chemically by Klaproth, Hatchett, and some other individuals, in order to ascertain the substances employed to give colour to the glass. The following are the facts that have been ascertained: 1. _Red glass._ This glass was opaque, and of a lively copper-red colour. It was probably the kind of red glass to which Pliny gave the name of hæmatinon. Klaproth analyzed it, and obtained from 100 grains of it the following constituents: Silica 71 Oxide of lead 10 Oxide of copper 7·5 Oxide of iron 1 Alumina 2·5 Lime 1·5 ----- 93·5[75] [75] Beitrage, vi. 140. No doubt the deficiency was owing to the presence of an alkali. From this analysis we see that the colouring matter of this glass was _red oxide of copper_. 2. _Green glass._ The colour was light verdigris-green, and the glass, like the preceding, was opaque. The constituents from 100 grains were, Silica 65 Black oxide of copper 10 Oxide of lead 7·5 Oxide of iron 3·5 Lime 6·5 Alumina 5·5 ----- 98·0[76] [76] Ibid., p. 142. Thus it appears that both the red and green glass are composed of the same ingredients, though in different proportions. Both owe their colour to copper. The red glass is coloured by the red oxide of that metal; the green by the black oxide, which forms green-coloured compounds, with various acids, particularly with carbonic acid and with silica.