1. _Galena_, sulphuret of lead. This ore has the metallic lustre of lead
with a crystalline structure derivable from the cube. When heated
cautiously at the blowpipe it is decomposed, the sulphur flies off, and
the lead is left alone in fusion; but if the heat be continued, the
coloured surface of the charcoal indicates the conversion of the lead
into its oxides. Galena is a compound of lead and sulphur, in equivalent
proportions, and therefore consists, in 100 parts, of 86-2/3 of metal,
and 13-1/3 of sulphur, with which numbers the analysis of the galena of
Clausthal by Westrumb exactly agrees. Its specific gravity, when pure,
is 7·56. Its colour is blackish gray, without any shade of red, and its
powder is black; characters which distinguish it from _blende_ or
sulphuret of zinc. Its structure in mass is lamellar, passing sometimes
into the fibrous or granular, and even compact. It is brittle. The
_specular_ galena, so called from its brightly polished aspect, is
remarkable for forming the _slickensides_ of Derbyshire--thin seams,
which explode with a loud noise when accidentally scratched in the mine.
The argentiferous galena has in general all the external characters of
pure galena. The proportions of silver vary from one-fifth part of the
whole, as at Tarnowitz, in Silesia, to three parts in ten thousand, as
in the ore called by the German miners Weisgültigerz; but it must be
observed, that whenever this lead ore contains above 5 per cent. of
silver, several other metals are associated with it. The mean proportion
of silver in galena, or that which makes it be considered practically as
an argentiferous ore, because the silver may be profitably extracted, is
about two parts in the thousand. See SILVER. The above rich silver ores
were first observed in the Freyberg mines, called Himmelsfürst and
Beschertglück, combined with sulphuret of antimony; but they have been
noticed since in the Hartz, in Mexico, and several other places.
The antimonial galena (_Bournonite_) exhales at the blowpipe the odour
peculiar to antimony, and coats the charcoal with a powder partly white
and partly red. It usually contains some arsenic.
2. The _Seleniuret of lead_, resembles galena, but its tint is bluer.
Its chemical characters are the only ones which can be depended on for
distinguishing it. At the blowpipe, it exhales a very perceptible smell
of putrid radishes. Nitric acid liberates the selenium. When heated in a
tube, oxide of selenium of a carmine red rises along with selenic acid,
white and deliquescent. The specific gravity of this ore varies from 6·8
to 7·69.
3. _Native minium_ or _red lead_, has an earthy aspect, of a lively and
nearly pure red colour, but sometimes inclining to orange. It occurs
pulverulent, and also compact, with a fracture somewhat lamellar. When
heated at the blowpipe upon charcoal, it is readily reduced to metallic
lead. Its specific gravity varies from 4·6 to 8·9. This ore is rare.
4. _Plomb-gomme._ This lead ore, as singular in appearance as in
composition, is of a dirty brownish or orange-yellow, and occurs under
the form of globular, or gum-like concretions. It has also the lustre
and translucency of gum; with somewhat of a pearly aspect at times. It
is harder than fluor spar. It consists of oxide of lead, 40; alumina,
37; water, 18·8; foreign matters and loss, 4·06; in 100. Hitherto it has
been found only at Huelgoët, near Poullaouen, in Brittany, covering with
its tears or small concretions the ores of white lead and galena which
compose the veins of that lead mine.
5. _White lead, carbonate of lead._ This ore in its purest state, is
colourless and transparent like glass, with an adamantine lustre. It may
be recognized by the following characters:--
Its specific gravity is from 6 to 6·7; it dissolves with more or less
ease, and with effervescence, in nitric acid; becomes immediately black
by the action of sulphuretted hydrogen, and melts on charcoal before the
blowpipe into a button of lead. According to Klaproth, the carbonate of
Leadhills contains 82 parts of oxide of lead, and 16 of carbonic acid,
in 98 parts. This mineral is tender, scarcely scratches calc-spar, and
breaks easily with a waved conchoidal fracture. It possesses the double
refracting property in a very high degree; the double image being very
visible on looking through the flat faces of the prismatic crystals. Its
crystalline forms are very numerous, and are referrible to the
octahedron, and the pyramidal prism.
6. _Vitreous lead_, or _sulphate of lead_. This mineral closely
resembles carbonate of lead; so that the external characters are
inadequate to distinguish the two. But the following are sufficient.
When pure, it has the same transparency and lustre. It does not
effervesce with nitric acid; it is but feebly blackened by sulphuretted
hydrogen; it first decrepitates and then melts before the blowpipe into
a transparent glass, which becomes milky as it cools. By the combined
action of heat and charcoal, it passes first into a red pulverulent
oxide, and then into metallic lead. It consists, according to Klaproth,
of 71 oxide of lead, 25 sulphuric acid, 2 water, and 1 iron. That
specimen was from Anglesea; the Wanlockhead mineral is free from iron.
The prevailing form of crystallization is the rectangular octahedron,
whose angles and edges are variously modified. The sulphato-carbonate,
and sulphato tri-carbonate of lead, now called _Leadhillite_, are rare
minerals which belong to this head.
7. _Phosphate of lead._--This, like all the combinations of lead with an
acid, exhibits no metallic lustre, but a variety of colours. Before the
blowpipe, upon charcoal, it melts into a globule externally crystalline,
which, by a continuance of the heat, with the addition of iron and
boracic acid, affords metallic lead. Its constituents are 80 oxide of
lead, 18 phosphoric acid, and 1·6 muriatic acid, according to Klaproth’s
analysis of the mineral from Wanlockhead. The constant presence of
muriatic acid in the various specimens examined is a remarkable
circumstance. The crystalline forms are derived from an obtuse rhomboid.
Phosphate of lead is a little harder than white lead; it is easily
scratched, and its powder is always gray. Its specific gravity is 6·9.
It has a vitreous lustre, somewhat adamantine. Its lamellar texture is
not very distinct; its fracture is wavy, and it is easily frangible. The
phosphoric and arsenic acids being, according to M. Mitscherlich,
isomorphous bodies, may replace each other in chemical combinations in
every proportion, so that the phosphate of lead may include any
proportion, from the smallest fraction of arsenic acid, to the smallest
fraction of phosphoric acid, thus graduating indefinitely into arseniate
of lead. The yellowish variety indicates, for the most part, the
presence of arsenic acid.
8. _Muriate of lead._ _Horn-lead_, or _murio-carbonate_.--This ore has a
pale yellow colour, is reducible to metallic lead by the agency of soda,
and is not altered by the hydrosulphurets. At the blowpipe it melts
first into a pale yellow transparent globule, with salt of phosphorus
and oxide of copper; and it manifests the presence of muriatic acid by a
bluish flame. It is fragile, tender, softer than carbonate of lead, and
is sometimes almost colourless, with an adamantine lustre. Spec. grav.
606. Its constituents, according to Berzelius, are, lead, 25·84; oxide
of lead, 57·07; carbonate of lead, 6·25; chlorine, 8·84; silica, 1·46;
water, 0·54; in 100 parts. The carbonate is an accidental ingredient,
not being in equivalent proportion. Klaproth found chlorine, 13·67;
lead, 39·98; oxide of lead, 22·57; carbonate of lead, 23·78.
9. _Arseniate of lead._--Its colour of a pretty pure yellow, bordering
slightly on the greenish, and its property of exhaling by the joint
action of fire and charcoal a very distinct arsenical odour, are the
only characters which distinguish this ore from the phosphate of lead.
The form of the arseniate of lead when it is crystallized, is a prism
with six faces, of the same dimensions as that of phosphate of lead.
When pure, it is reducible upon charcoal, before the blowpipe, into
metallic lead, with the copious exhalation of arsenical fumes; but only
in part, and leaving a crystalline globule, when it contains any
phosphate of lead. The arseniate of lead is tender, friable, sometimes
even pulverulent, and of specific gravity 5·04. That of
Johann-Georgenstadt consists, according to Rose, of oxide of lead 77·5;
arsenic acid 12·5; phosphoric acid 7·5, and muriatic acid 1·5.
10. _Red lead_, or _Chromate of lead_.--This mineral is too rare to
require consideration in the present work.