On the Manufacture of Optical Glass. [1829. 



from the quantity of impurity present ; this was afterwards ex- 

 plained. 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 oc- 

 casion might require. 



15. For this purpose litharge is first washed, by which many 

 black carbonaceous and ferruginous particles are separated ; it 

 is then dissolved in diluted nitric acid, so as to form a hot sa- 

 turated solution, the operation being performed in clean earth- 

 enware vessels. Both the perfectly pure and the moderately 

 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 pur- 

 posely 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 remaining litharge and undissolved 

 nitrate of lead, and, after standing a few moments, 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 cry- 

 stals are to be examined ; 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 generally 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. 



17. The good crystals are to be washed in three or four 

 waters, to remove the last portion of deposit and adhering so- 



