56 CALCEDONY. 
cylindrical shape. This being cut longitudinally by 
scissars or shears, is gradually bent back until it be- 
comes a flat plate. 
Large plate glass for looking-glasses is made by suf- 
fering the mass, in a state of complete fusion, to flow 
upon a casting table, with iron ledges. These con- 
fine 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 3 000/. 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 ascer- 
taining 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 
whitishy bluish, or smoky-grey colour; and, when Iroken, it 
appears internally dull, and somewhat splintery. 
It is generally found in a massive state, is harder than flinty 
generally semi-transparent, and 2J times heavier than water. 
The name of this stone is derived from Chalcedon, 
in Upper Asia, whence it appears to have been ori- 
ginally obtained, and where it is still found in consider- 
able abundance. Several superb specimens of calce- 
dony have been found in Britain, and particularly in. 
