FOOTNOTES:
[2] The manufacture of glass was known very early, but glass perfectly transparent and colourless was reckoned so valuable, that Nero is said to have given a sum equal to £25 for two moderate sized colourless drinking glasses.—Starke’s Oriental Letters.
Glass for optical uses is heavy, homogeneous, free from streaks and veins. More expensive chemical substances are employed in its manufacture than are used in making common glass.
[3] Pebbles, again, are proved by grinding the edges briskly on a moderately smooth file, or porous stone; they will resist the action of these, and emit sparkles of light as the velocity of the friction is increased, while glasses, on the contrary, yield, and are ground without difficulty.