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 carbonaceous 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 moderately pure acid have been tried without any sensible difference in the results: a little sulphuric acid does not seem injurious; andl 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 ou, 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 crystals 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 las 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-
