REDUCTION OF METALS
BY
_LIGHT_.
The next substance, I shall treat of, is Light, which seems well
adapted to illustrate the theory of metallic reduction; as in
experiments made with this substance, water can be more effectually
excluded, than in trials with any other reducer of the metals.
Before any decisive experiments could be made on light, or indeed
any other reducer of the metals, it is obvious that we should first
ascertain the effects of water itself in the ordinary temperature of
the atmosphere: to determine this point, the two following experiments
were made.
EXP. 1. GOLD.
A piece of silk was immersed in an aqueous solution of nitro-muriate
of gold, and suspended in a phial over water: the phial was corked,
covered with black silk, and placed in a dark closet, to prevent the
action of light. The experiment was continued from the 20th of July
to the 20th of October, during which time the silk was frequently
observed, and found to continue moist: but no alteration could be
perceived, except that the yellow colour, which the solution of gold
gave the silk, was changed to a light brown.
The silk was now taken out of the phial, and a solution of phosphorus
in ether applied to it: shortly after, a white metallic film appeared:
the silk was then kept wet with water, and in some time, reduced gold
of its proper colour appeared.
EXP. 2. SILVER.
A bit of silk, which was dipped in an aqueous solution of nitrate of
silver, was suspended in a phial over water, and placed in a dark
closet, from the 20th of July to the 20th of October, and examined from
time to time; but no alteration could be observed; its white colour
continued pure, and unchanged.
The silk was now taken out of the phial, and the silver was immediately
reduced by means of a solution of phosphorus in ether.
Hence it appears, that water alone has not the power of reducing metals
in the ordinary temperature of the atmosphere.
EXP. 3. GOLD.
On the 24th of July a piece of silk was immersed in a solution of
nitro-muriate of gold in water, and dried by a gentle heat; it was
then suspended in a window, exposed to the sunbeams, as much as
possible: no change was perceived on it, till the 26th, when the margin
of the silk began to assume a purple tinge, which increased gradually,
and on the 29th exhibited a few obscure specks of reduced gold on the
side of the silk opposed to the light.
The purple tinge continued to increase, and the yellow, which the
solution of gold gave the silk, decreased gradually, till on the 27th
of August the colour was a mixture of purple, and light brown.
The reduction of the gold seemed to keep pace with these changes of
colour, and was very evident on some parts of the silk even in the gray
light; but much more so in sunshine: these changes were observed till
the 20th of October; the experiment was then discontinued; and scarce
any reduced gold could be seen on the side of the silk not opposed to
the light.
That this slight reduction of the gold, and changes of colour in the
silk, depended on water absorbed from the air of the chambre, will
appear from the following experiments.
EXP. 4. GOLD.
A bit of silk, which was dipped in the solution of gold employed in
the preceding experiment, was dried, and suspended in a crystal phial
over dry carbonate of potash: the phial was then corked, and further
secured from external humidity by covering the cork with wax: it was
now placed in a dark closet 24 hours, that the carbonate of potash
might have sufficient time to attract all the moisture, it could, from
the air of the phial, before exposing it to the action of light.
The phial was then placed in a window, exposed, as much as possible,
to the solar light, from the 24th of July to the 20th of October, and
carefully observed: the same side of the silk was always opposed to
the light, and had the yellow tinge, which the solution of gold gave,
altered to a brownish yellow: but the other side of the silk suffered
no visible change whatever; nor could any purple tint, or the smallest
vestige of reduced gold be perceived.
Another bit of silk, which was immersed in the same solution of gold,
and dried by a gentle heat, was suspended over some dry carbonate of
potash, contained in a crystal phial covered with black silk, and
placed in a dark closet, during the same space of time, underwent no
visible change whatever.
Both these bits of silk were now taken out of the phials, and the gold
was instantly restored to its metallic splendour by means of water and
a solution of phosphorus in ether: a proof that the nitro-muriate of
gold, in these bits of silk, suffered no change, during that space of
time.
EXP. 5. GOLD.
A piece of silk, which was dipped in the same solution of gold, and
placed on a china plate exposed to the sunbeams, was kept moderately
wet with water during the experiment: the first alteration observed
was, that the yellow colour of the silk began to change to a faint
green, succeeded by a purple tinge, which in about fifteen minutes
exhibited some particles of reduced gold: soon after the threads of the
silk, which acquired this purple tint, were gilded with gold; and, in
about an hour, the whole of the silk was covered with a superb coat of
reduced gold, exhibiting the texture of the silk in a beautiful manner.
The purple tinge, which attended the reduction verged in some parts on
red, and in others on blue.
This experiment, which is very amusing, I have repeated times
unnumbered: and when the sun is powerful, and the solution of gold
properly prepared, and of due strength, the coat of reduced gold is so
bright, and dazzling, as to distress the eye of the beholder.
I find, that the best way of preparing a solution of gold for
this purpose is to separate the oxid of gold from its solution in
nitro-muriatic acid by means of good ether.
The silk may be dipped in this solution, and when the ether
evaporates, kept moderately wet with distilled water.
It was with this solution in ether that I first reduced gold in the
fibres of silk, and it was by means of the same, I discovered, that
water was a necessary condition in these reductions by light.
EXP. 6. GOLD.
A bit of silk, which was dipped in a solution of nitro-muriate of gold
in alcohol, and placed on a china saucer, exposed to the sunbeams, was
kept wet with alcohol for the space of an hour; but no change even
in the colour, which the solution of gold gave the silk, could be
perceived.
The alcohol diffused the solution of gold unequally through the silk:
the rays of the sun being intercepted, suspended the experiment:
there was no sunshine on the following day: on the third day the silk
was again exposed to the sunbeams, and kept wet with alcohol, which
extracted a yellow tincture; a proof that the gold in the silk suffered
no degree of reduction: the sun shone pretty strong for an hour, or
more; and, at length, a faint tinge of purple, followed by some reduced
gold, appeared.
The preceding experiments leave us no room to doubt, that the purple
tinge and the small quantity of reduced gold, that appeared in this
experiment, depended on water attracted from the air, or deposited in
the silk by the alcohol during its evaporation.
Mr. Scheele reduced a solution of nitro-muriate of gold in water by
exposing it in a phial for a fortnight to the rays of the sun[23].
Mr. Lewis also made many experiments on the staining of marble, and
other substances with metallic solutions, and light[24].
EXP. 7. SILVER.
A piece of silk, which was immersed in a solution of nitrate of silver
in water, was dried by a gentle heat, and exposed to the light of the
sun as much as possible in the window of a chambre from the 20th of
July to the 20th of October.
In less than an hour the silk acquired a reddish brown colour: next day
the colour became more intense, and gradually increased, till, on the
third day, it bordered on black, which increased slowly: at length part
of it became gray, and a few minute particles of reduced silver could
be distinguished: the black tinge gradually disappeared, and the silk
was of a reddish brown colour: the reduced silver had a gray cast.
The following experiments demonstrate, that the changes of colour, and
the few particles of semireduced silver depended on water attracted
from the atmosphere.
EXP. 8. SILVER.
A bit of silk was immersed in part of the same solution of nitrate of
silver in water, and dried in the dark: the silk retained its white
colour: it was then suspended over dry carbonate of potash in a crystal
phial, which was corked, and secured from the ingress of moisture by
covering the cork with wax.
The phial was left in a dark closet 24 hours, that the salt might
imbibe as much moisture as possible from the silk, and air.
The phial was then placed in a window, exposed, as much as possible, to
the rays of the sun, from the 24th of July to the 20th of October: the
silk scarcely suffered any visible change, except a very faint tinge of
reddish brown; which was best seen by transmitted light; for the silk
viewed by reflected light appeared nearly white: but that even this
tinge, slight as it was, depended on a minute quantity of moisture,
which the carbonate of potash was unable to extract, appears from the
next experiment.
EXP. 9. SILVER.
A piece of silk was dipped in a solution of fused nitrate of silver in
alcohol, and carefully dried: it was then suspended over a quantity of
concentrated sulphuric acid in a crystal phial, which was coated with
a double fold of black paper, and gum arabic, reaching somewhat above
the level of the acid, to prevent the light from acting on it: the
phial was corked, and to exclude moisture more effectually, the cork
was covered with wax.
The phial was now placed in a dark closet 24 hours, that the humidity
of the enclosed silk and air might be attracted by the acid: it was
then placed in a window, exposed as much as possible to the rays of
the sun, from the 19th of July to the 26th of October: but the silk
underwent no visible change whatever: its white colour remained pure,
and unaltered.
The silk, at the end of this period, was taken out of the phial, and
wetted with water: a solution of phosphorus in ether was then applied
to it; and instantly the metallic splendour of the silver appeared.
Another bit of silk, which was immersed in a solution of nitrate
of silver in water, and dried in the dark, was suspended over dry
carbonate of potash in a phial, and placed in a dark closet about two
months; the silk underwent no visible alteration whatever; its white
colour remained pure.
I was desirous of knowing, if the nitrate of silver in this bit of silk
suffered any change, that might render it incapable of reduction: I
therefore took it out of the phial, and divided it into two parts: one
of these was wetted with water, and the silver was instantly reduced by
a solution of phosphorus in ether. The other part, which was suspended
in a window, exposed to the air of the chamber, and rays of the sun,
soon acquired a reddish brown colour.
Hence it is evident, that these changes of colour indicate partial
reductions of the metal; and that they never happen without the
presence of water.
EXP. 10. SILVER.
A bit of silk, which was dipped in a solution of nitrate of silver in
water, was placed on a China saucer, exposed to the rays of the sun,
and kept moderately wet with water: in a few minutes the white colour
of the silk was changed to a reddish brown, which by degrees became
darker; and, in about three or four hours, though the solar rays were
often languid, and intercepted, acquired a blackish gray colour,
most of the reddish brown having disappeared: next day there was no
sunshine; but towards the evening particles of reduced silver were
visible on the side of the silk opposed to the light.
EXP. 11. SILVER.
A bit of silk, which was immersed in a solution of nitrate of silver
in alcohol, and exposed to the beams of the sun, as in the preceding
experiment, was kept moderately wet with alcohol; but resisted the
action of the light much longer than the preceding: however, in some
time, specks, and lines, of a reddish brown, began to appear on some
parts of the silk: next day, though there was no sunshine, the reddish
brown colour increased a little: on the third day, it became more
intense; but no black, or gray colour, or particle of reduced silver,
could be observed.
Whoever compares this with the preceding experiments, can have no
doubt that the changes of colour, which appeared in this experiment,
depended on water attracted from the air, or deposited in the silk by
the alcohol during its evaporation.
The experiments detailed in this Chapter, prove beyond the power of
contradiction,
1. That water is essential to the reduction of metals by light: for
these experiments demonstrate, that the reduction can not take place
without water, and that it is always in proportion to the quantity of
that fluid present.
2. That light does not reduce metals by giving them phlogiston, as
Messrs. Macquer and Scheele supposed; for were this opinion true, light
should reduce them without the aid of water, and as well with alcohol
as with water.
3. Light does not reduce metals by fusing and expelling their oxygen,
as the antiphlogistians imagine; for were this its mode of agency, the
reduction should happen without the assistance of water, and as well
with alcohol as with water.
4. Light is a combustible body; for it acts like hydrogen, phosphorus,
sulphur, and charcoal, in the reduction of metals.
Since water then is essential to the reduction of metals by light,
and since light does not reduce metals by giving them phlogiston,
nor by fusing, and expelling their oxygen, it follows, that the
water is either decomposed, or that it unites with metallic earths,
and constitutes their phlogiston, or that it unites with, and
separates their oxygenous principle: but as the two last suppositions
are inadmissible, it is obvious that light reduces the metals by
decomposing water.
After explaining the manner in which other combustible bodies reduce
the metals by decomposing water; it will not be difficult to explain
how light too produces the same effect.
But in order to facilitate the explanation, it must be premised,
1. That it is a law of attraction, that, when any body is deprived, to
a certain degree, of another, for which it has a strong affinity, the
attraction of the former for the latter is much increased.
2. That light has a strong attraction for oxygen, or the base of vital
air.
3. That when oxygen is condensed, and fixed in any substance, it
contains much less light, and caloric, than it does in the gazeous
state.
4. That the oxygenous principle exists in water in this condensed
state, and consequently has a strong attraction for light.
Therefore, when light reduces the metals, it attracts the oxygen of the
water, while the hydrogen of the latter unites, in its nascent state,
to the oxygen of the metal, and reduces it, forming at the same time a
quantity of water equal to that decomposed.
Hence it follows that the light is oxygenated, and changed into vital
air, while the metal is restored to its combustible state.