_Litharge_ is the protoxide of lead in a state of semivitrification.
_Red lead_ is a compound of two equivalents of protoxide and one of
deutoxide. The former is generally in the form of a grayish-red heavy
powder, sometimes partly crystalline; the latter in the form of a bright
red powder approaching in colour to vermilion. They may be known by
their colour;—by their becoming black when suspended in water and
treated with a stream of sulphuretted-hydrogen gas;—and by litharge
being entirely, and red lead partly, soluble in nitric acid, and forming
a solution which possesses the properties to be mentioned presently for
solutions of the acetate. The chemical actions concerned in these
changes are obvious, except in the instance of nitric acid on red lead.
Here the acid dissolves the protoxide only, and the deutoxide, which
seems to act the part of an acid in the pigment, is separated in the
form of a brown powder.