_a._ By analysis:--
1. (Dumas.) Silica, 75·9%; soda, 17·5%; lime, 3·8%; alumina, 2·8%. French
mirror-glass.
2. (Mitscherlich.) Silica, 60%; potassa, 25%; lime, 12·5%; loss, 2·5%(?).
Finest Bohemian plate.
_b._ Materials used:--
1. Finest siliceous sand, 45%; dried carbonate of soda, 25%; lime, 5%;
nitre (purified), 2%; plate-glass cullet, 23%; peroxide of manganese and
cobalt azure, a very little. Ordinary English plate.
2. Whitish quartz sand, 60%; purified carbonate of soda (dried), 20%; lime
(slaked by exposure to the air), 9%; plate-glass cullet, 11% (or more).
Sometimes as much cullet as sand is used; but in all cases 1% to 1-1/2% of
its weight in carbonate of soda is added with it, besides that ordered in
the formula, to compensate for loss of alkali by remelting. Used at the
celebrated plate-glass works at Saint-Gobain, France. The product
possesses an amount of excellence which British manufacturers have yet
failed to equal.