86 
Brass . 
lead in the form of an insoluble sulphat, and the nitrated 
zinc may then be decomposed by a carbonated alkali. 
On this precipitation however there are several things 
to be observed. Copper, as Vauquelin remarks,* when 
dissolving in nitric acid absorbs nearly of its weight 
of oxygen, but lead under the same circumstance absorbs 
only -~A-. Hence one hundred parts of copper dissolved 
in nitric acid would require for their disoxygenation (a 
process which takes place whenever a metallic oxyd in 
solution is precipitated by the immersion of another metal 
in its metallic state) full 250 parts of lead, which last is of 
course oxydated in proportion as the copper is precipi- 
tated in the metallic form. But this large quantity of 
oxyd of lead cannot be held in solution by the nitric acid # 
except this is largely in excess, and this explains the rea- 
son of the appearance of a portion of oxyd of lead (as M„. 
Dize has observed) which forms at the latter end of the 
process and mixes with the newly precipitated metallic 
copper, so as to require a subsequent operation to sepa- 
rate them. Nor will an excess of nitric acid ensure the 
purity of the precipitated copper, for it happens here, as is 
now found to take place in very many of the reguline me- 
tallic precipitates, that the newly -separated metal is not 
pure, but largely alloyed with the metal added as a pre- 
cipitant. Therefore the loose flocculent metal which forms 
around the piece of lead is not pure copper, though it has 
a perfect cupreous appearance, but is copper largely al- 
loyed with lead. Vauquelin found that if 50 grains of 
pure copper are dissolved in an excess of nitric acid, and 
then entirely precipitated by metallic lead, of which about 
220 grains are required, the cupreous precipitate now 
weighs 138 grains instead of the original 50, and therefore 
is not pure copper but an alloy of 50 parts of copper with 
* An. China, tom, 28 , 
