By P. CARLES.
When tinned iron serves for containing alimentary matters, it is
essential that the tin employed should be free from lead. The latter
metal is rapidly oxidized on the surface and is dissolved in this form
in the neutral acids of vegetables, meat, etc. The most exact method
of demonstrating the presence of lead consists in treating the
alloy--so-called tin--with _aqua regia_ containing relatively little
nitric acid. The whole dissolves; the excess of acid is driven off by
evaporation at a boiling heat, and the residue, diluted with water, is
saturated with hydrogen sulphide. The iron remains in solution, while
the mixed lead and tin sulphides precipitated are allowed to digest for
a long time in an alkaline sulphide. The tin sulphide only dissolves; it
is filtered off and converted into stannic acid, while the lead sulphide
is transformed into sulphate and weighed as such.
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