_a._ By analysis:--
1. Silica, 72·80%; potassa, 16·80%; lime (with a trace of alumina), 9·68%;
magnesia, 40%; traces of oxide of manganese and iron (and loss) ·32%. This
is the difficultly fusible 'Bohemian tube-glass,' so valuable in chemical
manipulations.
2. Silica, 69·3%; potassa, 15·8%; soda, 3%; lime, 7·6%; alumina, 1·2%;
magnesia, 2%; oxide of iron, ·5%; oxide of manganese (and loss), ·6%.
English chemical glass (without lead). More fusible than the last.
_b._ Materials used:--
1. Quartz (hyalin, in powder), 60%; calcined purified pearlash, 30%;
fresh-burnt lime (very pure), 9%; nitre (dried), 3/4%; arsenious acid or
peroxide of manganese, 1/4%. Said to be the proportions used in the
production of _a_, 1 (_above_).
2. (M. Peligot.) Quartz, 71-1/2%; carbonate of potassa (or its equiv.,
dry), 20%; quicklime, 8-1/2%; (manganese, a little). Said to be the
formula for the hardest and least fusible 'Bohemian tube-glass.' It is
very intractable and infusible, except at a very high temperature; but the
addition of an exceedingly small quantity of boracic acid, borax, or
arsenious acid, causes it to flow into a glass possessing great brilliancy
and hardness, and capable of being wrought at the highest heat of the
ordinary furnace.