MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
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20. Silica. — This material is in its most convenient state when it forms part 
of a combination consisting of two proportions silica, and one oxide of lead. 
As yet, the silica I have used has been the flint glass-maker’s sand, obtained 
from the coast of Norfolk, well washed and calcined. The silicate has been 
prepared by mixing two by weight of this sand with one of litharge, or with 
such quantity of nitrate of lead as is equivalent to one of litharge (16) ; the 
mixture is put into a large Hessian or Cornish crucible, which being covered 
over, has been put into a furnace and raised to a bright red heat for eighteen 
or twenty-four hours. On taking out the crucible, the charge has been found 
diminished somewhat in bulk, and of a porous structure and appearance like 
loaf-sugar. It has been freed from the crucible, the outside portions removed, 
and the pure parts carefully pulverized in a clean Wedgwood mortar. The 
powder has then been washed over in water, so as to obtain the whole in a fine 
state of division ; after which it has been dried, and preserved in bottles. No 
sieve should be used in these comminuting operations, nor any reducing or 
metallic matter brought in contact with the substance. Every care should be 
taken to avoid contamination : 24 parts by weight of the silicate are equiva- 
lent to 16 parts, or one proportional of silica, and 8 parts of protoxide of lead. 
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21. The advantage of the silica in this combined state depends upon the 
known composition of the substance, its comparatively easy pulverization, and 
ready fusion with the other materials. That there is iron in the silica (and the 
litharge when used) is objectionable; and the trials for its removal have only 
been delayed that the investigation of a more important point, namely, a suc- 
cessful process, might proceed. From some brief experiments, I am led to 
believe that an unexceptionable source of silica will be obtained by acting 
upon this silicate, in a state of fine division, by nitric acid and water, or else by 
the use of rock crystal. 
22. On some occasions I used pulverized flint glass, as the source of silica, 
conceiving that being already in a fusible state, it must possess an advantage 
over other silica, in allowing rapid mixture with the other materials. Allow- 
ance was made for the oxide of lead present, and the alkali was permitted to 
pass, as a substance that would probably do no harm. But a striking effect took 
place, which at once showed the necessity of perfectly pure materials. The 
glass when finished and cold was of a deep purple colour : this was immediately 
