lead, taking its place, and throwing down the lead in the form of a
crystalline arborescence. This is a very characteristic test; and also
one of much delicacy; for I have found a small thread of zinc will very
easily detect a twentieth part of a grain of lead dissolved in the form
of acetate in 20,000 parts of water. It acts also on the nitrate of
lead. Its action is impaired or prevented by an excess of acetic or
nitric acid.
These tests are amply sufficient for determining the presence of lead in
a solution, provided they act characteristically. Others have been also
used, however; and it is therefore right to notice them cursorily.
The _alkaline carbonates_ throw down a white precipitate in a very
diluted solution of lead. This test is ineligible, because the alkaline
carbonates cause a white precipitate with many other salts. It might be
rendered decisive, however, by washing the precipitate thoroughly,
suspending it in pure water and transmitting sulphuretted-hydrogen,
which blackens it. No other white carbonate is similarly altered except
those of bismuth and silver, which are rare.
The _soluble sulphates_ likewise cause with solutions of lead a white
precipitate, the sulphate of lead. To this test the same objections
apply as to the carbonates of the alkalis.
The _ferro-cyanate of potash_ causes a white precipitate, the
ferro-cyanate of lead. This is an objectionable test, as many other
substances besides lead are similarly acted on by it.