MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
11 
alloying with the platina, rendered it fusible. When red lead was substituted 
for litharge, the same effect took place, due to the presence of particles of car- 
bonaceous and reducing matter. Both these substances also contained so 
much iron and other impurities, as to give a deep colour to the glass, far be- 
yond what was expected from the quantity of impurity present ; this was 
afterwards explained. Carbonate of lead was also found to be too impure. 
Finally, all the oxide of lead necessary was purified, by being converted into a 
nitrate, and crystallized once or twice, as occasion might require. 
15. For this purpose litharge is first washed, by which many black car- 
bonaceous and ferruginous particles are separated ; it is then dissolved in 
diluted nitric acid, so as to form a hot saturated solution, the operation being 
performed in clean earthenware vessels. Both the perfectly pure and the mo- 
derately pure acid have been tried without any sensible difference in the results : 
a little sulphuric acid does not seem injurious; and I find that sulphate of lead 
will dissolve perfectly in the glass; but muriatic acid has been always avoided. 
As the acid, water and litharge are made to act on each other by heat, either 
purposely applied or resulting from the chemical action going on, it will be 
found that when approaching towards neutrality the liquid will become very 
turbid. The hot saturated solution is then to be poured from the remain- 
ing litharge and undissolved nitrate of lead, and after standing a few mo- 
ments, again poured from the sediment, and set aside to crystallize in a cool 
place. Before it is left, however, it is to be examined as to its acidity: if strongly 
acid to litmus paper, it is in a right state ; if not, a little nitric acid should be 
added, for the crystals of nitrate have always been compact and pure under 
such circumstances, and more readily separable from insoluble matter. 
16. After eighteen or twenty-four hours, the basins of crystals are to be ex- 
amined; the clear mother liquor carefully poured off; the crystals broken up in 
the basins ; and then repeatedly washed in fresh clear portions of the mother 
liquor, that any insoluble deposited matter may be removed. There will ge- 
nerally be a portion of this deposit ; but if the process has been well performed, 
the crystals will be quite free. If they appear perfectly white or bluish white, 
they need not be recrystallized ; but if yellow, they must be dissolved in water, 
a little nitric acid added, and the crystallization repeated. The nitrate in 
the mother liquors and washings should be purified by repeated processes. 
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