LEAD. (_Plomb_, Fr.; _Blei_, Germ.) This is one of the metals most
antiently known, being mentioned in the books of Moses. It has a gray
blue colour, with a bright metallic lustre when newly cut, but it
becomes soon tarnished and earthy looking in the air. Its texture is
close, without perceptible cleavage or appearance of structure; the
specific gravity of common lead is 11·352; but of the pure metal, from
11·38 to 11·44. It is very malleable and ductile, but soft and destitute
of elasticity; fusible at 612° Fahr., by Crighton, at 634° by Kupfer,
and crystallizable on cooling, into octahedrons implanted into each
other so as to form an assemblage of four-sided pyramids.
There are four oxides of lead. 1. The suboxide of a grayish blue colour,
which forms a kind of crust upon a plate of lead long exposed to the
air. It is procured in a perfect state by calcining oxalate of lead in a
retort; the dark gray powder which remains is the pure suboxide. 2. The
protoxide is obtained by exposing melted lead to the atmosphere, or,
more readily, by expelling the acid from the nitrate of lead by heat in
a platinum crucible. It is yellow, and was at one time prepared as a
pigment by calcining lead; but is now superseded by the chromate of this
metal. Litharge is merely this oxide in the form of small spangles, from
having undergone fusion; it is more or less contaminated with iron,
copper, and sometimes a little silver. It contains likewise some
carbonic acid. The above oxide consists of 104 of metal, and 8 of
oxygen, its prime equivalent being 112, upon the hydrogen scale; and it
is the base of all the salts of lead. 3. The plumbeous suroxide of
Berzelius, the sesquioxide of some British chemists, is the well-known
pigment called RED LEAD or _minium_. It consists of 100 parts of metal
and 10 of oxygen. 4. The plumbic suroxide of Berzelius, or the peroxide
of the British chemists, is obtained by putting red lead in chlorine
water, or in dilute nitric acid. It is of a dark brown, almost black
colour, which gives out oxygen when heated, and becomes yellow oxide. It
kindles sulphur when triturated with it. This oxide is used by the
analytical chemist to separate, by condensation, the sulphurous acid
existing in a gaseous mixture.
Among the ores of lead some have a metallic aspect; are black in
substance, as well as when pulverized; others have a stony appearance,
and are variously coloured, with usually a vitreous or greasy lustre.
The specific gravity of the latter ores is always less than 5. The whole
of them, excepting the chloride, become more or less speedily black,
with sulphuretted hydrogen or with hydrosulphurets; and are easily
reduced to the metallic state upon charcoal, with a flux of carbonate of
soda, after they have been properly roasted. They diffuse a whitish or
yellowish powder over the charcoal, which, according to the manner in
which the flame of the blowpipe is directed upon it, becomes yellow or
red; thus indicating the two characteristic colours of the oxides of
lead.
We shall not enter here into the controversy concerning the existence of
native lead, which has been handled at length by M. Brongniart in the
_Dictionnaire des Sciences Naturelles_, article _Plomb, Mineralogie_.