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golden and metallic in appearance.

I prepared an apparatus by which many of the common metals
could be deflagrated in hydrogen by the Leyden battery, and
being caught upon glass plates couldbe examined as to reflexion,
transmission, colour, &c., whilst in the hydrogen and in the
metallic, yet divided state. The following are briefly the
results ; which should be considered in connexion with those
obtained by employing polarized light. Copper : a fine deposit
presenting by reflexion a purplish red metallic lustre, and by
transmission a green colour, dark in the thicker parts, but
always green ; agate pressure increased the reflexion where it
was not bright, and a little diminished the transmission, render-
ing the green deeper, but not changing its character as in the
case of gold. Tin gave a beautiful bright white reflexion, and
by transmission various shades of light and dark brown ; agate
pressure diminished the transmission and increased the reflexion
in places before dull or dead; the effect appeared to be due
simply to the lateral expansion of the separate particles filling
up the space. Iron presented a fine steel grey, or slate metallic
reflexion and a dark brown transmission ; agate pressure gave
the same effect as, with tin, but no change of colour. Lead: a
bright white reflexion, the transmission a dark smoky brown ,



1857.] of Gold (and other Metals) to Light. 405

agate pressureTappeared to change this brown towards blue.
Zinc : the reflexion bright white and metallic ; the transmission
a dark smoky colour with portions of blue-grey, brown-grey
and pale brown ; agate pressure tended to change the blue-grey
to brown. Palladium: the reflexion fine metallic and dark
grey ; the transmitted light, where most abundant, sepia-brown ;
agate pressure converted the tint in the thinner places from
brown towards blue-grey. Platinum : the reflexion white,
bright and metallic ; the transmission brown or warm grey with
no other colours ; agate pressure increased the reflexion and
diminished the transmission as with tin. Aluminium: the
reflexion metallic and white, very beautiful ; the transmitted
light was dark brown, bluish brown, and occasionally in the
thinner parts orange ; agate pressure caused but little change.

Films of Gold (and other metals) by Phosphorus, Hydrogen,
Sfc. effect of heat pressure.

The reduction of gold from its solution by phosphorus is
well known. If fifteen or twenty drops of a strong solution of
gold, equal to about 1J grain of metal, be added to two or
three pints of water, contained in a large capsule or dish, if
four or five minute particles of phosphorus be scattered over
the surface, and the whole be covered and left in quietness for
twenty-four or thirty-six hours, then the surface will be found
covered with a pellicle of gold, thicker at the parts near the
pieces of phosphorus, and possessing there the full metallic
golden reflective power of the metal ; but passing by gradation
into parts, further from the phosphorus, where the film will
be scarcely sensible except upon close inspection. If plates
of glass be introduced into the fluid under the pellicle, and
raised gradually, the pellicle will be raised on them ; it may
then be deposited on the surface of pure distilled water to
wash it ; may be raised again on the glass ; the water allowed
to drain away, and the whole suffered to dry. In this way the
pellicle remains attached to the glass, and is in a very convenient
condition for preservation and examination.

If phosphorus be dissolved in two or three times its bulk of
sulphide of carbon, and a few drops of the fluid be placed on
the bottom of a dry basin, vapour of the phosphorus will soon
rise up and bring the atmosphere in the basin to a reducing



406 On the Experimental Relations [1857.

state. If a plate of glass large enough to cover the basin
have six or eight drops of a strong neutral solution of chloride
of gold placed on it, and this be spread about by a glass
stirrer, so as to form a flowing layer on the surface, the glass
may then be inverted and placed over the dish. So arranged,
the gold solution will keep its place, but will have a film of
metal reduced on its under surface. The plate being taken off
after twenty, thirty, or forty minutes, and turned with the gold
solution upwards, may then gradually be depressed in an in-
clined position into a large basin of pure water, one edge
entering first, and the gold film will be left floating. After
sufficient washing it may be taken up in portions on smaller
plates of glass, dried, and kept for use. Mr. Warren De la
Rue taught me how to make and deal with these films : they
may by attention be obtained very uniform, of very different
degrees of thickness, from almost perfect transparency to
complete opacity, and by successive application of the same
collecting glass plate may be superposed with great facility.

These films may be examined either on the water or on the
glass. When thick, their reflective power is as a gold plate,
full and metallic; as they are thinner, they lose reflective
power, and they may be obtained so thin as to present no
metallic appearance, all the coloured rays of light then passing
freely through them. As to the transmitted light, the thinner
films generally present one kind of colour ; it appears as a
feeble grey-violet, which increases in character as the film
becomes thicker and sometimes approaches a violet ; a greenish
violet also appears ; and the likeness of the grey-violet tint of
these films to the stains produced by a solution of gold on the
skin or other organic reducing substance, or the stain produced
on common pottery, cannot be mistaken. Superposition of
several grey-violet films does not produce a green tint, but
only a diminution of light without change of colour. In those
specimens made by particles of phosphorus floating on the
solution of gold, very fine green tints occur at the thicker and
golden parts of the film. The colour of the gold here may
depend in some degree on the manner in which these films are
formed : the thicker parts are not produced altogether by the
successive addition of reduced gold from the portion of fluid
immediately beneath them. When a particle of phosphorus



1857.] of Gold (and other Metals) to Light. 407

is placed on pure water, it immediately throws out a film
which appears to cover the whole of the surface ; in a little
while the film thickens around the particle and is easily di-
stinguished by its high reflective power. It is this film which
reduces the gold in solution, being itself consumed in the
action ; the result is a continued extension from the phosphorus
outwards, which, after it has covered the solution with a thin
film of gold, continues to cause a compression of the parts
around the phosphorus and an accumulation there, rendering
the gold at a distance of half an inch from the phosphorus so
thick, that it is brilliant by reflexion and nearly opake by
transmission ; whilst near to the phosphorus the forming film
is so thin as to be observed only on careful examination, and
is still travelling outwards and compressing the surrounding
parts more and more. The phosphorus is very slowly con-
sumed ; a particle not weighing ylhyth of a grain will remain for
four or five days on the surface of water before it disappears.

Though the particles of these films adhere together strongly,
as may be seen by their stiffness on water, still the films cannot
be considered as continuous. If they were, those made by
vapour of phosphorus could not thicken during their formation,
neither could they dry on glass in the short time found
sufficient for that purpose. Experimentally also, I find that
vapours and gases can pass through them. Very thin films
without folds did not sensibly conduct the electricity of a
single pair of Grove's plates ; thicker films did conduct ; yet
with these proofs that these films could not be considered as
continuous, they acted as thin plates upon light, producing
the concentric rings of colours round the phosphorus at their
first formation, though their thickness then could scarcely be
the i-Jijth, perhaps not the Tnhfth of a wave undulation of light.
Platinum, palladium, and rhodium produced films, showing
these concentric rings very well.

Many of these films of gold, both thick and thin, which,
being of a grey colour originally, were laid on a solution
of cyanide of potassium to dissolve slowly, changed colour as
they dissolved and became green ; if change occurred, it
was always towards green. On the other hand, when laid on
a solution of chlorine, the change during solution was towards
an amethyst or ruby tint. The films were not acted upon by



408 On the Experimental Relations [ 1 857.

pure nitric, or hydrochloric, or sulphuric acids, or solutions of
potassa or brine. They dissolved in damp chlorine gas, not
changing in colour during the solution. I believe them to
consist of pure gold.

When these gold films were heated to dull redness they
changed. The reflexion, though not much altered, was a
little more metallic and golden than before; more light was
transmitted after the heating and the colour had altered from
greenish to violet, or from grey-green to ruby or amethyst ;
and now two or three films superposed often gave a very ruby
colour. This action is like that of heat on the particles
separated by electric explosions. If not overheated, the
particles were not fused to the glass, but could be easily wiped
off. Whenever these heated particles were pressed by the
convex agate, they changed in character and transmitted green
light. Heat took away this character of the gold, the heat of
boiling oil, if continued, being sufficient ; but on applying
pressure at the same spot, the power of transmitting green
light was restored to the particles. In many cases, where the
gold adhered sufficiently to the glass to bear a light drawing
touch from the finger or a card, such touch altered the light
transmitted from amethystine to green ; so small is the pressure
required when the particles are most favourably disposed.

Heating injured the conducting power for electricity of these
films, no doubt by retraction of the particles, though there
was no such evident appearance in these cases, as in the un-
attached gold-leaf of the particles running up into globules.

A given film, examined very carefully in the microscope by
transmitted lamp-light, with an aperture of 90 and power of
700 linear, presented the following appearances. The un-
heated part was of a grey colour, and by careful observation
was seen to be slightly granular. By very close observation
this grey part was often resolvable into a mixture of green and
amethystine stria?, it being the compound effect of these which in
general produce the grey sensation in the eye. When a part
of such a film was heated, the transmitted colour was changed
from grey to purple, as before described, and the part thus
heated was evidently more granular than before. This dif-
ference was confirmed in other cases. That the heated part
should thus run up, seems to show that many of the particles



1857.] of Gold (and other Metals] to Light. 409

must have been touching though they did not form a con-
tinuous film ; and on the other hand, the difference between
the effect here and with unattached gold-leaf, shows that the
degree of continuity as a film must be very small. When these
heated films were greened by agate pressure, or the drawing
pressure of a card, the green parts remained granulated,
apparently in the same degree as when purple. The green
was not subjective or an effect of interference, but a positive
colour belonging to the gold in that condition. Every touch of
the agate was beautifully distinct as a written mark. The parts
thus greened and the purple parts appeared to transmit about
the same amount of light. Though the film appeared gra-
nulated, no impression was made upon the mind that the
individual particles of which the film consisted were in any
degree rendered sensible to the eye.

The unheated gold films when pressed by agate often
indicated an improved reflective power, and the light trans-
mitted was also modified ; generally it was less, and occasionally
tended towards a green tint ; but the effect of pressure was
by no means so evident as in particles which had been heated.
Films of some other metals were reduced by phosphorus in
like manner, the results in all these cases being of course
much affected by the strength of the solution and the time of
action ; they are briefly as follows : Palladium : a weak solu-
tion of the chloride gave fine films, apparently very continuous
and stiff;, the reflexion was strong and metallic, of a dark grey
colour ; the transmission presented every shade of Indian ink.
Platinum chloride gave traces of a film excessively thin, and
very slow in formation. Rliodium chloride in three or four
hours gave a beautiful film of metal in concentric rings, varying
in reflecting and transmitting power over light and also in
colour; those which reflected well, transmitted little light;
and those which transmitted, reflected little light; one migh
have thought there was no metal in some of the rings betwee
other rings that reflected brilliantly, but the metal was ther
of transmitting thickness ; the transmitted colour of rhodium
varied from brown to blue. Silver : a solution of the nitrate
gave films showing the concentric rings ; the light transmitted
by the thinner parts was of a warm brown, or sepia tint ; the
film becomes very loose and mossy in the thicker parts and is



410 On the Experimental Relations [1857.

wanting in adherence; pressure brings out the full metallic
lustre in every part, and in the thin places converts the colour
from brown to blue, being in that respect like the result with
pale gold-leaf, in which the silver present dominates over the
colour of the gold. I do not think there is phosphorus com-
bined with this silver ; I did not find any, and considering the
surface action on metals which float as films between air and
water, it seems improbable that it should be there.

Hydrogen was employed to reduce some of the metals, their
solutions being placed in an atmosphere of the gas. The
action differed considerably from that of phosphorus, as might
be expected. Gold produced a very thin film, too thin to be
washed; it had a faint metallic reflexion, and transmitted a
slate-blue colour like the former films. Platinum chloride was
acted on at once ; minute spots appeared here and there on
the surface ; these enlarged, became rough and corrugated at
the middle, though brilliant at the edges, and at last formed
an irregular coat over the fluid ; at the part where the film
was flat and brilliant, it resembled that produced by the
electric explosion, and by transmission gave a dark grey colour.
Iridium required much time, and formed a crust from centres
like the platinum. Palladium gave an instant action, but most
of the reduced metal sank in a finely divided state ; a film may
be obtained, but it has very little adhesion. Rhodium is
reduced, but the film consists of floating particles, having so
little adhesion that it cannot be gathered up. Silver is reduced,
but the film is very thin and has no tenacity.

A copper film of very beautiful character may be obtained
as follows in all varieties of thickness. Let a little oxide of
copper be dissolved in olive-oil to form a bath, and having
immersed some plates of glass, for which purpose microscope
plates 3x1 inches are very convenient, let the whole be heated
up to the decomposing temperature of the oil ; being left to
cool, and the plates then drained and washed successively in
camphine and alcohol, they will be found covered with a film
of copper, having the proper metallic lustre and colour by
reflexion ; and by transmission, presenting a green colour,
which, though generally inclining to olive, is in the thinner
films often more beautiful than the green presented by pressed
gold.



1857.] of Gold (and other Metals) to Light. 41 1

Diffused particles of gold -production proportionate size
colour aggregation and other changes.

Agents competent to reduce gold from its solution are very
numerous, and may be applied in many different ways, leaving
it either in films, or in an excessively subdivided condition.
Phosphorus is a very favourable agent when the latter object
is in view. If a piece of this substance be placed under the
surface of a moderately strong solution of chloride of gold,
the reduced metal adheres to the phosphorus as a granular
crystalline crust. If the solution be weak and the phosphorus
clean, part of the gold is reduced in exceedingly fine particles,
which becoming diffused, produce a beautiful ruby fluid.

This ruby fluid is well obtained by pouring a weak solution
of gold over the phosphorus which has been employed to
produce films, and allowing it to stand for twenty-four
or forty-eight hours ; but in that case all floating particles
of phosphorus should be removed. If a stronger solution
of gold be employed, the ruby fluid is formed, but it soon
becomes turbid and tends to produce a deposit. When the
gold is in such proportion that it remains in considerable
excess, still the ruby formation is not prevented, and being
formed it mingles unchanged with the excess of gold in
solution. If an exceedingly weak solution of gold be em-
ployed the production of ruby appears to be imperfect and
retarded. The nearer the solution is to neutrality at the
commencement the better ; when a little hydrochloric acid was
added the effect was not so good, and the colour of the fluid
was more violet than ruby.

If a pint or two of the weak solution of gold before described
be put into a very clean glass bottle, a drop of the solution of
phosphorus in sulphide of carbon added, and the whole well
shaken together, it immediately changes in appearance, becomes
red, and being left for six or twelve hours, forms the ruby
fluid required ; too much sulphide and phosphorus should not
be added, for the reduced gold then tends to clot about the
portions which sink to the bottom.

Though the sulphide of carbon is present in such processes
and very useful in giving division to the phosphorus, still it is
not essential. A piece of clean phosphorus in a bottle of



On the Experimental Relations [1857.

the gold solution gradually produces the ruby fluid at the
bottom, but the action is very slow. If the phosphorus be
attached to the side of the bottle, but always beneath the
surface of the solution, the streams of ruby fluid may be seen
moving both upwards and downwards against the side of the
glass, and forming films in the vicinity of the phosphorus
perfect in their golden reflexion, and yet transmitting light of
ruby, violet, and other tints, thus giving, first a proof that the
particles are gold, and then connecting the present condition
of the gold with that of the films already described. On the
other hand, the phosphorus may be excluded and the sulphide
of carbon employed alone ; for when it and the solution of gold
are shaken together, the gold is reduced and the ruby fluid
formed ; but it soon changes to purple or violet.

A quick and ready mode of producing the ruby fluid, is to
put a quart of the weak solution of gold (containing about 0*6
of a grain of metal) into a clean bottle, to add a little solution
of phosphorus in ether, and then to shake it well for a few
moments : a beautiful ruby or amethystine fluid is immediately
produced, which will increase in depth of tint by a little time.
Generally, however, the preparations made with phosphorus
dissolved in sulphide of carbon, are more ruby than those
where ether is the phosphorus solvent. The process of
reduction appears to consist in a transfer of the chlorine from
the gold to the phosphorus, and the formation of phosphoric
or phosphorous acids and hydrochloric acid, by the further
action of the water.

The fluids produced may easily be tested for any gold yet
remaining unreduced, by trial of a portion with solution of
protochloride of tin. If any be found, it is easily reduced by
the addition of a little more of the phosphorus in solution.
After all the gold is separated as solid particles, the fluid may
be considered in its perfected state. Occasionally it may
smell of phosphorus in excess, even after it has been poured
off from the deposited particles of it and the sulphide. In
that case it is easy to deprive it of this excess by agitation in
a bottle with air. When kept in closed vessels mouldiness
often occurs. If this be in groups it is collected with facility
at the end of a splinter of wood and removed, or the whole
fluid may be poured through a wet plug of cotton in the neck



1857.] of Gold (and other Metals) to Light. 413

of a funnel, the reduced gold passing freely. All the vessels
used in these operations must be very clean ; though of glass,
they should not be supposed in proper condition after wiping,
but should be soaked in water, and after that rinsed with
distilled water. A glass supposed to be clean, and even a new
bottle, is quite able to change the character of a given gold
fluid.

Fluids thus prepared may differ much in appearance. Those
from the basins, or from the stronger solutions of gold, are
often evidently turbid, looking brown or violet in different
lights. Those prepared with weaker solutions and in bottles,
are frequently more amethystine or ruby in colour and ap-
parently clear. The latter, when in their finest state, often
remain unchanged for many months, and have all the appear-
ance of solutions. But they never are such, containing in fact
no dissolved, but only diffused gold. The particles are easily
rendered evident, by gathering the rays of the sun (or a lamp)
into a cone by a lens, and sending the part of the cone near
the focus into the fluid ; the cone becomes visible, and though
the illuminated particles cannot be distinguished because of
their minuteness, yet the light they reflect is golden in cha-
racter, and seen to be abundant in proportion to the quantity
of solid gold present. Portions of fluid so dilute as to sho,w
no trace of gold, by colour or appearance, can have the pre-
sence of the diffused solid particles rendered evident by the
sun in this way. When the preparation is deep in tint, then
common observation by reflected light shows the suspended
panticles, for they produce a turbidness and degree of opacity
which is sufficiently evident. Such a preparation contained in
a pint bottle will seem of a dull pale-brown colour, and nearly
opake by reflexion, and yet by transmission appear to be a fine
ruby, either clear or only slightly opalescent.

That the ruby and amethystine fluids hold the particles in
suspension only, is also shown by the deposit which occurs
when they are left at rest. If the gold be comparatively abun-
dant, a part will soon settle, i. e. in twenty-four or forty-eight
hours ; but if the preparation be left for six or eight months,
a part will still remain suspended. Even in these portions,
however, the diffused state of the gold is evident ; for where,
as in some cases, the top to the depth of half an inch or more



414 On the Experimental Relations [1857.

has become clear, it is seen that the ruby portion below is as a
cloud sinking from it ; and in the part which has apparently
been cleared from colour by the settling of the particles,
the lens and cone of light still show the few, or rather the
fine diffused particles yet in suspension, though the proto~
chloride of tin can show no gold in solution. The mould or
mucus before spoken of, often collects the larger, heavier
particles, and becomes of a dark blue colour ; it may then be
taken out by a splinter of wood, and being shaken in water,
disengages the particles, which issue from it in clouds like the
sporules from a ripe puff-ball.

A gradual change goes on amongst the particles diffused
through these fluids, especially in the cases where the gold is
comparatively abundant. It appears to consist of an aggrega-
tion. Fluids, at first clear or almost clear to ordinary obser-
vation, become turbid ; being left to stand for a few days, a
deposit falls. If the supernatant fluid be separated and left
to stand, another deposit may be obtained. This process may
be repeated, and whilst the deposition goes on, the particles
in the fluid still seem to aggregate ; it is only when the fluid
is deprived of much gold that the process appears to stop.
Even after the fluid has attained a fine marked ruby tint, if



Online LibraryMichael FaradayExperimental researches in chemistry and physics → online text (page 40 of 49)