_White lead_, which is the carbonate of the metal, is in the form of a
heavy snow-white powder, or in white chalk-like masses. It consists of
variable proportions of the hydrated oxide and neutral carbonate; those
specimens are the whitest which contain most carbonate; and the best
English white lead I find to contain four equivalents of carbonate and
one of hydrated protoxide. The grayer variety, formed by the action of
distilled water on metallic lead, consists of only two of the former to
one of the latter.[1221] It may be known by its being blackened like the
two former compounds by sulphuretted-hydrogen,—by being soluble with
effervescence in nitric acid,—and by becoming permanently yellow when
heated to redness, in consequence of the expulsion of its carbonic acid,
and its conversion into protoxide. These tests, however, apply with
exactness only to the pure carbonate, in which state white lead is not
often met with in the shops. It is generally adulterated with sulphates,
in consequence of which it is only partially acted on by nitric acid,
and does not become distinctly yellow under a strong red heat. Dutch
white-lead contains no less than between 78·5 and 25 per cent. of
impurities insoluble in nitric acid, Venetian white-lead from 11 to 14·5
per cent., Munich white-lead between 1 and 7·5 per cent.[1222] I have
met, however, with perfectly pure specimens in the shops of this city.