_Reduction of Metals by Sulphur._
Though Sulphur, as far as my reading extends, has seldom been
considered as a reducer of the metals, yet as it holds a distinguished
rank among combustible bodies, analogy led me to examine its powers in
reducing and fixing the metals in the fibres of silk.
The vapour of sulphur appeared to be the most simple form, in which
it could be applied to this purpose; and may be obtained by placing a
phial containing flowers of sulphur in hot sand: as the sulphur melts,
it assumes the form of vapour, which soon fills the phial, expels the
atmospheric air, and bears a considerable heat, before it inflames; a
bit of silk, prepared for the experiment, may be held over the vapour,
as it issues from the phial, or immersed in it. The neck of the phial
should be of a convenient size for this purpose.
But these experiments are more conveniently made by means of a
sulphurous match, and a glass funnel, in which the silk imbued with the
metallic solution may be suspended by a thread passed through it, and
made fast with a cork, which also serves to confine the vapour.
The glass is then placed on a table, and by moving it a little beyond
the verge of the same, a lighted match is readily introduced, which, as
soon as the glass is filled with vapour, may be withdrawn; the vapour
is confined by making the glass glide back on the table; and thus the
phenomena of the experiment can be easily observed.
Though the vapour obtained in this manner be chiefly sulphurous acid
gas; yet its effects are not less interesting on that account.
EXP. 1. GOLD.
A bit of silk was dipped in a solution of gold in ether, and dried:
it was then suspended in the glass funnel, and exposed for some time
to the vapour obtained from a burning match: but no change could be
perceived, except that the silk became a little brown.
Another bit of silk, prepared in the same manner, was immersed in the
vapour of sulphur, formed in a phial placed in hot sand, with the same
result.
EXP. 2. GOLD.
A piece of silk was immersed in a solution of nitro-muriate of gold in
water, suspended in the glass funnel, and exposed, while wet, to vapour
of sulphur, formed by a burning match; no sooner did the vapour touch
the silk, than the reduction commenced; and in a few seconds the whole
piece was covered with a splendid coat of reduced gold, permanent, and
retentive of its lustre; but had a few specks of a dull violet hue.
The silk viewed by transmitted light appeared of a beautiful blue
colour; and being removed from the vapour, and suspended in the air,
began in about ten minutes to exhale a vapour, which continued about
two hours, and smelled acid, and pungent.
Another bit of silk, dipped in the same solution of gold, and dried,
was wetted with alcohol, and exposed to the same vapour: the silk
acquired a brownish hue; and a small white metallic film appeared on
its lower end, where the alcohol most abounded: the silk was then
wetted with water, and replaced in the vapour; instantly a lively
purple with a bright pellicle of reduced gold appeared.
EXP. 3. SILVER.
A piece of silk was immersed in a solution of nitrate of silver in
water, and suspended in the air of a dark closet to dry; the silk
retained its white colour, though it remained in the air twenty-four
hours; it was then exposed fourteen hours to the vapour obtained from a
burning match; but suffered no change, except that it acquired a brown
tinge: it was now wetted with alcohol, and replaced in the vapour for
some time; no signs of reduction appearing, it was wetted again with
the alcohol, and exposed to the vapour; but still no signs of reduction
could be perceived: I then wetted the silk with distilled water,
replaced it in the sulphureous vapour; and in about a minute reduced
silver appeared.
EXP. 4. SILVER.
I dipped a bit of silk in a solution of nitrate of silver in distilled
water, and exposed it, while wet, to the vapour of sulphur, as in the
preceding experiment; in a few seconds the silver appeared in its
metallic form, attended with a variety of lively colours: the most
remarkable of these were a pleasant blue, orange, purple, and yellow,
which soon disappeared: the reduced silver also disappeared in a
great measure, some faint traces only remaining. The silk was removed
from the vapour into the air, but exhaled no vapour, as happened in
experiment the second.
A small bit of sulphur was suspended in a phial containing a solution
of nitrate of silver in water; and after some weeks the sulphur was
coated with reduced silver of no great lustre.
EXP. 5. PLATINA.
A bit of silk was immersed in a solution of nitro-muriate of the ore
of platina in distilled water, and dried in the air; it was then
suspended in the glass funnel, and exposed to the vapour of a burning
match: but no signs of reduction could be observed: the silk retained
the colour, the solution gave it.
Another bit of silk was dipped in the same solution of platina, and
exposed, while wet, to the sulphureous vapour; in a few seconds the
reduction was very evident. The silk was immerged in a glass of clear
water, and transferred bright films of reduced platina to the surface
of that fluid; most of them were of the same colour as the spangles
in the ore of platina; and some were distinguished by lively blue and
purple colours.
Another bit of silk, dipped in the same solution of platina, was
immersed, while wet, in the vapour of sulphur, formed in a phial placed
in hot sand; the reduction soon commenced, and was much more perfect,
and permanent, than in pieces exposed to the vapour obtained from
ignited matches.
It is remarkable that sulphur reduced this metal much better, than
phosphorus or hydrogen gas did: but the reduced platina disappears
after some time, and leaves nothing behind but a brown stain.
If the films be transferred from the silk to water, they may be
preserved in their metallic form.
EXP. 6. MERCURY.
A bit of silk was immersed in a solution of nitrate of mercury in
distilled water, and dried; parts of the silk immediately acquired
a slate colour: the silk was then exposed to the vapour of sulphur,
obtained from a burning match; but it suffered no change, except that
the colour became a little more intense.
Another bit of silk was dipped in the same solution of mercury, and
exposed, while wet, to the same vapour: the reduction instantly
commenced in a very evident manner, accompanied with several colours,
as blue, purple, and yellow.
The silk was removed from the vapour, and soon lost most of its lustre,
which was succeeded by a slate colour, through which some particles of
reduced mercury were observed to shine.
EXP. 7. MERCURY.
A bit of silk was immersed in a solution of oxygenated muriate of
mercury in water, and dried in the air: it was then exposed to the
sulphureous vapour obtained from a burning match; but the silk
underwent no visible change. This solution of mercury does not change
the white colour of silk, as that of nitrate does.
Another bit of silk was dipped in the same solution of oxygenated
muriate of mercury, and exposed, while wet, to the vapour; in a few
seconds reduced mercury appeared, unattended by any colour, except a
slight tinge of citron yellow on part of the silk. After some time the
whole of the reduced mercury vanished.
EXP. 8. COPPER.
A piece of silk was immersed in a solution of sulphate of copper, and
dried; it was then suspended in the glass funnel, and exposed to the
vapour obtained from a burning match; but the silk suffered no change,
retaining the colour, which the solution gave it.
Another bit of silk was dipped in the same solution of copper, and
exposed, while wet, to the vapour of an ignited match; in a short
time a white metallic film appeared, accompanied with a brown tinge;
the silk becoming dry, was wetted with water, and exposed to a brisk
vapour from another match: the film became more evident, and parts of
it verged on yellow, bordering on a copper colour. After some time this
white metallic film disappears, and nothing remains but a light brown
stain.
EXP. 9. LEAD.
A bit of silk was immersed in a solution of acetite of lead in
distilled water, and dried in the air: it was then exposed to the
sulphureous vapour of a burning match; but underwent no visible change
whatever.
Another bit of silk was dipped in the same solution of lead, and
exposed, while wet, to the vapour; reduced lead soon appeared all over
the silk; but after some time this reduced lead disappears, unless it
be transferred to the surface of water.
EXP. 10. TIN.
A bit of silk was immersed in a solution of muriate of tin in distilled
water, and dried in the air: it was then exposed to the vapour obtained
from a burning match, which soon filled the glass; but produced no
visible change on the silk.
Another bit of silk was dipped in the same solution of tin, and
exposed, while wet, to the sulphureous vapour; and in a few seconds
reduced tin appeared all over the silk; but the tin soon disappears, if
not transferred to water.
EXP. 11. ARSENIC.
A bit of silk was immersed in a solution of muriate of arsenic in
distilled water, and dried in the air; it was then exposed to the
vapour of sulphur, obtained from a burning match; but the silk suffered
no apparent alteration.
Another bit of silk was dipped in the same solution of arsenic, and
exposed, while wet, to the vapour: in a few seconds the arsenic was
reduced, but not in a very evident manner; the silk was immerged in
water, and several bright films of reduced arsenic floated on the
surface of that fluid.
EXP. 12. BISMUTH.
A bit of silk was immersed in a solution of nitrate of bismuth in
distilled water, and dried in the air; it was then exposed to the
vapour of sulphur obtained from a burning match; but no signs of
reduction appeared.
Another bit of silk was dipped in the same solution of bismuth, and
exposed, while wet, to the sulphureous vapour; the bismuth was soon
reduced, accompanied with a brown stain: but the metallic lustre soon
disappeared.
EXP. 13. ANTIMONY.
A piece of silk was immersed in a solution of tartarite of antimony in
water, and dried; it was then exposed to the vapour of a burning match;
but no change could be observed; nor did the silk immerged in water
deposit any films on its surface.
Another piece of silk was dipped in the same solution of antimony, and
exposed, while wet, to the vapour: the silk, in a few seconds, acquired
a yellow colour, and a bright bluish film of reduced antimony appeared
on the lower end of the silk, where the water was most abundant: the
silk was then immerged in a glass of water, and transferred a large
pellicle of reduced antimony to the surface of that fluid.
EXP. 14. IRON.
A bit of silk was immersed in a largely diluted solution of sulphate of
iron in distilled water, and dried in the air; it was then exposed to
the vapour of sulphur obtained from a burning match; after some time
the colour of the silk became a little brown; but no reduced iron could
be seen; nor did the silk immerged in a glass of clear water transfer
any thing metallic to its surface.
Another bit of silk was dipped in the same solution of iron, and
exposed, while wet, to the sulphureous vapour; in a short time the silk
was withdrawn, and a minute shining film, of a livid white colour, was
visible on its lower margin, where the water most abounded: the silk
was then immerged in a glass of water, and transferred a large film of
reduced iron to its surface.
This experiment succeeded also with the vapour of sulphur formed in a
phial placed on hot sand. The films of reduced iron were seldom visible
on the silk; but were soon rendered visible, by transferring them to
water.
EXP. 15. ZINC.
A piece of silk was dipped in a diluted solution of sulphate of zinc,
and dried; it was then exposed to the vapour obtained from a burning
match; no change whatever could be perceived: the silk was immerged in
a glass of water, but deposited no film on its surface.
Another piece of silk was dipped in the same solution of zinc, and
immersed while wet in the vapour of sulphur formed in a phial placed
on hot sand; in about half a minute the silk was withdrawn from the
vapour, and some parts of it had a shining livid appearance, which I
took for reduced zinc: in order to determine if the reduction were
real, the silk was immerged in a glass of clear water, and transferred
to its surface a bright film of reduced zinc.
If the silk be kept too long in the vapour, some sulphur will be
condensed on its surface, and give it a yellow colour: if it be then
dipped in a glass of water, it will deposit both the sulphur, and
the metallic films on its surface; but the appearance of the sulphur
is so different from that of the reduced metal, that they are easily
distinguished.
Another bit of silk was immersed in a largely diluted solution of
muriate of zinc, and exposed, while wet, to the vapour obtained from a
burning match: the silk being withdrawn exhibited a few minute shining
films on its lower end, and on immersion in water, left bright films of
reduced zinc floating on the surface of that fluid.
Very often these films cannot be seen, until they are transferred to
water, which I found a very useful test in doubtful cases: they are
better seen in the gray light than in sunshine.
These experiments on the reduction of metals by sulphur were often
repeated with nearly the same result: but some variety occurs depending
on the quantity of water present, the strength of the metallic
solution, and sulphureous vapour, and also on the time the silk is
exposed to the vapour.
When the experiments are made in a phial placed in hot sand, part of
the metallic solution frequently drops from the silk, and falling on
the bottom, or sides of the phial, is reduced, covering these parts
with a metallic crust; on which the sulphur soon reacts, and changes
the greater part into a sulphure. The phial commonly cracks.
The following conclusions are deducible from the experiments related in
this chapter.
1. Water is essential to the reduction of metals by sulphur; for this
effect is always in proportion to the quantity of water present.
2. Alcohol does not promote these reductions without the aid of water.
3. Sulphur does not reduce the metals by giving them phlogiston;
nor by uniting with and separating their oxygen; for were either of
these opinions just, the sulphur, so minutely divided by heat, should
reduce metallic solutions in alcohol as effectually as it does metallic
solutions in water.
When we compare these reductions by sulphur, with those effected by
phosphorus, and consider that water is essential to both; we must
conclude, that the reduction is effected in the same manner, viz. by
the decomposition of water; which may be thus explained.
The sulphur attracts the oxygen of the water, while the hydrogen of the
latter unites, in its nascent state, to the oxygen of the metal, and
restores it to the metallic form.
Hence it follows, that the sulphur is oxygenated by the oxygen of the
water, while the metal is restored to its combustible state.
It also follows, that a quantity of water equal to that consumed is
formed by the hydrogen of the water, and the oxygen of the metal.
This explanation is supported by a fine experiment of Dr. Priestley,
who obtained inflammable air by passing the vapour of water through
sulphur heated in an earthen tube[15]; and farther confirmed by the
experience of some judicious makers of oil of vitriol, who always
sprinkle the sulphur with a certain proportion of water, before they
inflame it.
The decomposition of water in these reductions must be granted, or it
must be supposed, that a single takes place in preference to a double
affinity; which cannot be admitted.
These experiments point out several errors in M. Lavoisier’s Table of
the Affinities of the Oxygenous Principle; for he has placed sulphur
at a much greater distance from that principle than any of the metals
treated of in this chapter, except gold; and even in this instance his
table is erroneous, unless it be proved, that sulphur reduces metals
by directly uniting with, and separating their oxygen; a supposition
which implies, that the reduction is effected by a single affinity; and
therefore inadmissible.
I shall close this chapter with a short extract from the chemical
essays of Bishop Watson; as it seems to have some relation to the
experiments related here.
His lordship says, that his experiments on “the Derbyshire lead ore
instruct us to believe, that the lead in this kind of ore is in its
metallic state; as the ore was changed into lead without the addition
of any substance containing the inflammable principle[16].”
But the reduction of lead by sulphur and water, related in this
chapter, points out an evident source of the inflammable principle;
for the ore itself must contain some water, a constituent part of
which is hydrogen, or the base of inflammable air: beside his lordship
thinks, no appearance of lead would have taken place, had there been no
communication with the external air: but as air always holds water in
solution, an ample source of the inflammable principle is evident.