Large plate glass for looking-glasses is made by suffering the mass, in a state of complete fusion, to flow upon a casting table, with iron ledges. These confine the melted matter, and, as it cools, a metallic roller is passed over it, to reduce it to an uniform thickness.
Glass utensils, unless very small and thin, require to be gradually cooled in an oven. This operation is called annealing, and is necessary in order to prevent them from cracking by change of temperature, wiping, or slight accidental scratches.
It appears that the manufacture of glass was known very early; but glass perfectly transparent was esteemed of extremely high value. It is stated that the Emperor Nero purchased two glass cups with handles for a sum which was equivalent to 50,000l. of our money. The windows of some of the houses of the ancient city of Pompeii, which was buried by an eruption of Mount Vesuvius, in the year 79, were glazed, but the glass was thick, and not transparent.
By many persons flint is used as a test for ascertaining the purity of silver coins. This is done by rubbing them upon the flint; and if the mark which they leave be not perfectly white, they are rejected as counterfeit.
91. CALCEDONY is a species of quartz, generally of whitish, bluish, or smoke-grey colour; and, when broken, it appears internally dull, and somewhat splintery.
It is generally found in a massive state, is harder than flint, generally semi-transparent, and 2½ times heavier than water.
The name of this stone is derived from Chalcedon, in Upper Asia, whence it appears to have been originally obtained, and where it is still found in considerable abundance. Several superb specimens of calcedony have been found in Britain, and particularly in some of the tin and copper mines of Cornwall. It occurs in several parts of Scotland; and in many of the countries of the Continent. In the Leverian Museum there was a specimen of calcedony, which weighed more than 200 pounds. Its whole surface appeared such that, at first sight, one might imagine it to have formerly been in a liquid state: it had much the appearance that thick oil has while boiling.
Few stones are susceptible of a higher or more beautiful polish than calcedony. Hence the different varieties of it are cut into ring and seal stones, necklaces, ear-pendants, small vases, cups, and snuff-boxes.
92. ONYX is a kind of calcedony, generally marked alternately with stripes of white and black, or white and brown.
Its name is derived from the Greek language, and has been given on account of its resemblance in colour to the whitish band at the base of the human nail. The distinction which appears to be made betwixt onyx and sardonyx, arises from the colours of the former being arranged either concentrically, or in a somewhat confused manner, and those of the latter in regular stripes or bands.