_a._ By analysis:--
1. (Berthier.) Silica, 59·19%; oxide of lead, 28·68%; potassa, 12·13%;
oxides of iron and manganese, traces. Finest colourless English crystal.
2. (Brande; Faraday.) Silica, 52%; oxide of lead, 34%; potassa, 34%.
Crystal.
3. (Faraday.) Silica, 44·30%; oxide of lead, 43·05%; potassa, 11·75%;
alumina, ·50%; oxides of iron and manganese, ·12%; (loss 28%). Heaviest of
three samples of flint glass examined.
_b._ Materials used:--
1. Finest Lynn-sand (calcined, sifted, and washed), 51%; litharge
(purest), 28% (or red lead, 29%), refined pearlashes (calcined before
being weighed), 16%; nitre (purified), 4-3/4% arsenious acid and peroxide
of manganese, of each, 1/8%. Very fine crystal.
2. (M. Payen.) Fine sand, 46%; red lead, 31%; purified carbonate of
potash, 23%. French crystal.
3. (Geddes.) White Lynn-sand, 51%; red lead or litharge, 33%; refined
pearlashes, 13%; nitre, 3%; a very little arsenious acid and peroxide of
manganese. Ordinary English flint-glass. Crystal 'cullet' may be added at
will to the above. This glass was originally prepared from powdered
flints, a fact to which it owes its common name.
GLASS, OPTICAL. 1. (Crown glass.) Purest siliceous sand, 55%; carbonate of
soda (dry), 12%; chalk (dry), 11%; carbonate of baryta, 22%.