Copyright
Michael Faraday.

Experimental researches in chemistry and physics online

. (page 39 of 49)
Online LibraryMichael FaradayExperimental researches in chemistry and physics → online text (page 39 of 49)
Font size
QR-code for this ebook


the latter will gradually become thinner andjthinner ; but at
any moment the process may be stopped, the cyanide washed
away by water, and the attenuated gold film left on the glass.
If towards the end a washing be made with alcohol, and then
with alcohol containing a little varnish, the gold film will be
left cemented to the glass f.

* The chlorine leaves a film of chloride of silver behind, the cyanide leaves
only metal.

f Air-voltaic circles are formed in these cases, and the gold is dissolved
almost exclusively under their influence. When one piece of gold-leaf was
placed on the surface of a solution of cyanide of potassium, and another,
moistened on both sides, was placed under the surface, both dissolved ; but
twelve minutes sufficed for the solution of the first, whilst above twelve hours
were required for the submerged piece. In weaker solutions, and with silver
also, the same results were obtained ; from sixty to a hundredfold as much
time being required for the disappearance of the submerged metal as for that
Which, floating, was in contact both with the air and the solvent. An action



1857.] of Gold (and other Metals) to Light. 395

In this manner the leaf may be obtained so thin, that I think
50 or even 100 might be included in a single progressive undu-
lation of light. But the character of the effect on light is not
changed, the light transmitted is green, as before ; and though
that green tint is due to a condition of the gold induced by
pressure, it as yet remains unchanged through all these varieties
of thickness and of proportion to the progressive or the lateral
undulation.

Gold-leaf, either fine or common, examined in the microscope,
appears as a most irregular thing. It is everywhere closely
mottled or striated, according as a part at the middle or the
edge of a leaf is selected, minute portions which are close to
other parts being four or five times as thick as the latter, if the
proportion of light which passes through may be accepted as
an indication. Yet this irregular plate does not cause any
sensible distortion of an object seen through it, that object being
the line of light reflected from a fine wire in the focus of a
moderate microscope. Nor perhaps was any distortion due
to consecutive convexities and concavities to be expected ; for
when the thicker parts of the leaf were examined they seemed
to be accumulated plications of the gold, the leaf appearing as
a most irregular and crumpled object, with dark veins running
across both the thicker and thinner parts, and from one to the
other. Yet in the best microscope, and with the highest power,
the leaf seemed to be continuous, the occurrence of the smallest
sensible hole making that continuity at other parts apparent,
and every part possessing its proper green colour. How such
a film can act as a plate on polarized light in the manner it
does, is one of the queries suggested by the phenomena which
requires solution.

When gold-leaf is laid upon glass and its temperature raised
considerably without disturbance, either by the blowpipe or
an ordinary Argand gas-burner, it seems to disappear, i. e.
the lustre passes away, the light transmitted is abundant and
nearly white, and the place appears of a pale brown colour.
One would think that much of the metal was dissipated, but all
is there, and if the heat has been very high (which is not neces-
sary for the best results), the microscope shows it in minute

of this kind has probably much to do with the formation of the films to be
described hereafter.



396 On the Experimental Relations [1857.

globular portions. A comparatively low heat, however, and one
unable to cause separation of the particles, is known to alter the
molecular condition of gold, and the gold-beater finds important
advantage in the annealing effect of a temperature that does not
hurt the skins or leaves between which he beats the metal.

It might be supposed that the annealed metal, in contracting
from the constrained and attenuated state produced by beating,
drew up, leaving spaces through which white light could pass,
and becoming itself almost insensible through the smallness of
its quantity ; and if gold-leaf unattached to glass be heated
carefully with oil in a tube, it does shrink up considerably even
before it loses its green colour, which finally happens. But if
the gold-leaf laid upon glass plates by water only be carefully
dried, then introduced into a bath of oil and raised to a tempe-
rature as high as the oil can bear for five or six hours, and then
suffered to cool, the plates, when taken out and washed, first
in camphine, and then in alcohol, present specimens of gold
which has lost its green colour, transmits far more light than
before and reflects less, whilst yet the film remains in form and
other conditions apparently quite unchanged. Being now exa-
mined in the microscope, it presents exactly the forms and
appearance of the original leaf, except in colour ; the same
irregularities appear, the same continuity, and if the destruction
of the green colour has not been complete, it will be seen that
it is the thicker folds and parts of the mottled mass that retains
the original state longest.

This change does not depend upon the substance in contact
with which the gold is heated*. If the leaf be laid upon mica,
rock-crystal, silver or platinum, the same result occurs ; the
surrounding medium also may change, and be air, oil or carbonic
acid, without causing alteration. Nor has the gold disappeared ;
a piece of leaf, altered in one part and not in another, was
divided into four equal parts, and the gold on each converted
by chlorine gas into crystallized chloride of gold ; the same
amount was found in each division.

When the gold-leaf is laid by water on plates of rock-crystal,

* The disappearance of gold-leaf as metal, when mingled with lime, alumina
and other bodies, and then heated, has been already observed ; and referred
to oxidation (J. A. Buchner). See Gmelin's * Chemistry/ vi. p. 206, " Purple
oxide of gold."



1857.] of Gold (and other Metals] to Light. 397

and then gradually heated in a muffle not higher than is neces-
sary, an excellent result is obtained. The gold is then of a
uniform pale brown colour by common observation; but when
examined by a lens, and an oblique light, all the mottle of the
original leaf appears. It adheres but very slightly to the rock-
crystal, and yet can bear the application of the pressure now
to be described.

When gold rendered colourless by annealing is subjected to
pressure, it again becomes of a green colour. I find a convex
surface of agate or rock-crystal having a radius of from a quarter
to half an inch very good for this purpose, the metal having
very little tendency to adhere to this substance. The greening
is necessarily very imperfect, and if examined by a lens it will be
evident that the thinner parts of the film are rarely reached by
the pressure, it being taken off by the thicker corrugations ;
but when reached they acquire a good green colour, and the
effect is abundantly shown in the thicker parts. At the same
time that the green colour is thus reproduced, the quantity of
light transmitted is diminished, and the quantity of light
reflected is increased. When the gold-leaf has been heated on
glass in a muffle, it generally adheres so well as to bear streak-
ing with the convex rock-crystal, and then the production of
the reflecting surface and the green transmission is very striking.
In other forms of gold film, to be described hereafter, the
greening effect of pressure (which is general to gold) is still
more strikingly manifested, and can be produced with the
touch of a card or a finger. In these cases, and even with gold-
leaf, the green colour reproduced can be again taken away by
heat, to appear again by renewed pressure.

As to the essential cause of this change of colour, more in-
vestigation is required to decide what that may be. As already
mentioned, it might be thought that the gold-leaf had run up
into separate particles. If it were so, the change of colour by
division is not the less remarkable, and the case would fall into
those brought together under the head of gold fluids. On the
whole, I incline to this opinion ; but the appearance in the micro-
scope, the occurrence of thin films of gold acting altogether
like plates and yet not transmitting a green ray until they are
pressed, and their action on a polarized ray of light, throw
doubts in the way of such a conclusion.



398 On the Experimental Relations [1857.

It may be thought that the beating has conferred a uniform
strained condition upon the gold, a difference in quality in one
direction which annealing takes away ; but when the gold is
examined by polarized light, there is no evidence as yet of such
a condition, for the green and the colourless gold present like
results ; and there is a little difficulty in admitting that such
an irregular corrugated film as gold-leaf appears to be can pos-
sess any general compression in one direction only, especially
when it is considered that it is beaten amongst tissues softer
than itself, and made up with it into considerable masses. The
greening effect of pressure occurs with the deposited particles
of electric discharges, and here it appears either amongst the
larger particles near the line of the discharge, or amongst the
far finer ones at a considerable distance. Such results do not
suggest a dependence upon either the size of the particles or
their quantity, but rather upon the relative dimensions of the
particles in the direction of the ray and transverse to that di-
rection. One may imagine that spherical or other particles,
which, being disposed in a plane, transmit ruby rays or violet
rays, acquire the power, when they are flattened, of transmitting
green rays, and such a thought sends the mind at once from
the wave of light to the direction and extent of the vibrations of
the ether. For it does not seem likely that pressure can pro-
duce its peculiar result by affecting the relation of the dimension
of the particle to the length-dimension of a progressive undu-
lation of light, the latter being so very much greater than the
former ; but the relation to the dimension of the direct or lateral
vibration of the particles of the ether may be greatly affected,
that being probably very small and much nearer to, if not even
less than, the size of the particles of gold.

Silver-leaf, as usually obtained by beating, is so opake, as
perfectly to exclude the light of the sun. When this is laid by
water on plates of rock-crystal and heated in a muffle, it begins
to change at a temperature lower than that required for gold,
and becomes very translucent, losing at the same time its re-
flective power : it looks very like the film of chloride produced
when a leaf of silver is placed in chlorine gas. When examined
by a lens or an ordinary microscope, the leaf seems to be as
continuous as in its original state ; the finest hole, or the finest
line drawn by a needle point, appears only to prove the con-



1857.] of Gold (and other Metals} to Light. 399

tinuity of the metallic film up to the very edges of these real
apertures. When pressure is applied to this translucent film,
the compressed metal becomes either opake or of a very dark
purple colour, and resumes its high reflective power. If a
higher heat than that necessary for this first change be applied,
then the leaf, viewed in the microscope, assumes a mottled
appearance, as if a retraction into separate parts had occurred.
At a still higher temperature this effect is increased ; but the
heat, whether applied in the muffle or by a blowpipe, which is
necessary to fuse the metal and make it run together in globules,
is very much higher than that which causes the first change of
the silver : the latter is, in fact, below such a red heat as is
just visible in the dark. Whatever the degree of heat applied,
the metal remains as metallic silver during the whole time.
When many silver leaves were laid loosely one upon another,
rolled up into a loose coil, introduced into a glass tube, and
the whole placed in a muffle and heated carefully for three or
four hours to so low a degree that the glass tube had not been
softened or deformed 5 it was found that the silver-leaf had
sunk together a little and shaped itself in some degree upon the
glass, touching by points here and there, but not adhering to
it. But it was changed, so that the light of a candle could be
seen through forty thicknesses : it had not run together, though
it adhered where one part touched another. It did not look
like metal, unless one thought of it as divided dead metal, and
it even appeared too unsubstantial and translucent for that ;
but when pressed together, it clung and adhered like clean
silver, and resumed all its metallic characters. ^

When the silver is much heated, there is no doubt that the
leaf runs up into particles more or less separate. But the
question still remains as to the first effect of heat, whether it
merely causes a retraction of the particles, or really changes
the optical and physical nature of the metal from the beaten or
pressed state to another from which pressure can return it back
again to its more splendid condition. It seems just possible
that the leaf may consist of an infinity of parts resulting from
replications, foldings and scales, all laid parallel by the beating
which has produced them, and that the first action of heat is
to cause these to open out from each other ; but that supposi-
tion leaves many of the facts either imperfectly explained or



400 On the Experimental Relations [1857.

untouched. The Arts do not seem to furnish any process
which can instruct us as to this condition, for all the operations
of polishing, burnishing, &c. applied to gold, silver and other
metals, are just as much fitted to produce the required state
under one view as under the other.

To return to gold : it is clear that that metal, reduced to
small dimensions by mere mechanical means, can appear of
two colours by transmitted light, whatever the cause of the
difference may be. The occurrence of these two states may pre-
pare one's mind for the other differences with respect to colour,
and the action of the metallic particles on light, which have yet
to be described.

Many leaves of gold, when examined by a lens and transmitted
light, present the appearance of red parts ; these parts are
small, and often in curved lines, as if a fine hair had been there
during the beating. At first I thought the gold was absolutely
red in these parts, but am inclined to believe that in the greatest
number of cases the tint is subjective, being the result of the
contrast between the white light transmitted through bruised
parts, and the green light of the neighbouring continuous
parts. Nevertheless, some of these places, when seen in the
microscope, appeared to have a red colour of their own, that is,
to transmit a true red light. As I believe that gold in a certain
state of division can transmit a ruby light, I am not prepared
to say that gold-leaf may not, in some cases, where the effect
of pressure in a particular direction has been removed, do the
same.

Many of the prepared films of gold were so thin as to have
their reflective power considerably reduced, and that in parts
which, under the microscope and in other ways, appeared to be
quite continuous : this agrees with the transmission of all the
rays already mentioned, but it seems to imply that a certain
thickness is necessary for full reflexion ; therefore, that more
than one particle in depth is concerned in the act, and that the
division of gold into separate particles by processes to be de-
scribed, may bring them within or under the degree necessary
for ordinary reflexion.

As particles of pure gold will be found hereafter to adhere
by contact, so the process of beating may be considered as
one which tends to weld gold together in all directions, and



1857.] f GM ( and other Metals) to Light. 401

especially in that transverse to the blow, a point favourable to
continuity in that direction, both as it tends to preserve and
even reproduce it.

If a polarized ray be received on an analyser so that no light
passes, and a plate of annealed glass, either thick or thin, be
interposed vertically across the ray, and no difference is observed
on looking through the analyser, the image of the source of light
does not appear ; but if the plate be inclined until it makes an
angle of from 30 to 45, or thereabouts, with the ray, the light
appears, provided the inclination of the glass is not in the plane
of polarization or at right angles to it, the effect being a
maximum if the inclination be in a plane making an angle of
45 with that of polarization. This effect, which is common to
all uncrystallized transparent bodies, is also produced by leaf-
gold, and is one of the best proofs of the true transparency of
this metal according to the ordinary meaning of the term. In
like manner, if a leaf of gold be held obliquely across an ordi-
nary ray of light, it partly polarizes it, as Mr. De la Rue first
pointed out to me. Here again the condition of true trans-
parency is established, for it acts like a plate of glass or water
or air. But the relations of gold and the metals in different
conditions to polarized light shall be given altogether at the
close of this paper.

Deflagrations of Gold (and other metals) heat pressure, fyc.
Gold wire deflagrated by explosions of a Leyden battery
produces a divided condition, very different to that presented
by gold leaves. Here the metal is separated into particles,
and no pressure in any direction, either regular or irregular,
has been exerted upon them in the act of division. When the
deflagrations have been made near surfaces of glass, rock-
crystal, topaz, fluor-spar, card-board, &c., the particles as they
are caught are kept separate from each other and in place, and
generally those which remain in the line of the discharge have
been heated by the passage of the electricity. The deposits
consist of particles of various sizes, those at the outer parts of
the result being too small to be recognized by the h'ighest
powers of the microscope. Beside making these deflagrations
over different substances, as described above, I made them in
different atmospheres, namely, in oxygen and hydrogen, to



402 On the Experimental Relations [1857.

compare with air ; but the general effects, the colours produced,
and the order of the colours, were precisely the same in all the
cases. These deposits were insoluble in nitric acid and in
hydrochloric acid, but in the mixed acids or in chlorine solution
were soluble, exactly in the manner of gold. There is no reason
to doubt that they consisted of metallic gold in a state of ex-
treme division.

Now as to the effects on light, i. e. as to the coloured rays
reflected or transmitted by these deposited particles, and first,
of those in the line of the discharge where the wire had been.
Here the mica was found abraded much, the glass less, and
the rock-crystal and topaz least. Where abraded, the gold
adhered ; in all the other parts it could be removed with the
slightest touch. The gold deposited in this central place was
metallic and golden by reflected light, and of a fine ruby colour
by transmitted light. On each side of this line the deposit had
a dark colour, but when particularly examined gave a strong
golden metallic reflexion, and by transmission a fine violet colour,
partaking of green and ruby in different parts, and sometimes
passing altogether into green. Beyond this, on each side,
where the tints became paler and where the particles appeared
to be finer, the transmitted tint became ruby or violet-ruby,
and this tint was especially seen when the deposit was caught
on a card. As to the reflected light, even at these faintest
parts it is golden and metallic. This is easily observed by
wiping off a sharp line across the deposit on glass in the very
faintest part, and then causing the sun's rays collected in the
focus of a small lens to travel to and fro across that edge ; the
presence of the metallic gold on the unwiped part is at once
evident by the high illumination produced there. It is evident
that all the colours described are produced by one and the
same substance, namely gold, the only apparent difference being
the state of division and different degrees of the application of
heat. The thickest parts of these deposits are so discontinuous,
that they cannot conduct the electricity of a battery of two or
three pairs of plates, i. e. of a battery unable to produce a spark
among the particles.

When any of these deposits of divided gold are heated to
dull redness, a remarkable change occurs. The portions which
before were violet, blue, or green by transmitted light, now



1857.] of Gold (and other Metals) to Light. 403

change to a ruby, still preserving their metallic reflecting power,
and this ruby is in character quite like that which is presented
in the arts by glass tinged by gold. This change is often far
better shown in the more distant and thinner parts of the
deposit, than in those nearest to the line of discharge ; for near
the latter place, where the deposit is most abundant, the metal
appears to run up into globules, as with gold-leaf, and so
disappears as a film. I believe that the ruby character of the
deposit in the line of discharge, is caused by the same action of
heat produced at the moment by the electricity passing there.
In the distant parts, the deposit, rubified by after-heat, is not
imbedded or fused into the glass, rock-crystal, topaz, &c., but
is easily removed by a touch of the finger, though in parts of
the glass plate which are made very hot, it will adhere.

If the agate pressure before spoken of, in respect of gold-leaf,
be applied to ruby parts not too dense, places will easily be
found where this pressure increases the reflective power con-
siderably, and where at the same time it converts the transmitted
ray from ruby to green ; making the gold, as I believe, then
accord in condition with beaten gold-leaf. On the other hand,
if parts of the unheated electric deposit, where they are purple-
grey, and so thin as to be scarcely visible without care, be in
like manner pressed, they will acquire the reflective power,
and then transmit the green ray ; and I think I am justified by
my experiments in stating, that fine gold particles, so loosely
deposited that they will wipe off by a light touch of the finger,
and possessing one conjoint structure, can in one state transmit
light of a blue-grey colour, or can by heat be made to transmit
light of a ruby colour, or can by pressure from either of the
former states be made to transmit light of a green colour ; all
these changes being due to modifications of the gold, as gold,
and independent of the presence of the bodies upon which for
the time the gold is supported ; for I ought to have said, if I
have not said so, that these changes happen with all the deposits
upon glass, mica, rock-crystal, and topaz, and whatever the
atmosphere in which they were formed.

When gold is deflagrated by the voltaic battery near glass
(I have employed sovereigns laid on glass for the terminals), a
deposit of metallic gold in fine particles is produced. The
densest parts have a dark slate-violet colour passing into violet



404 On the Experimental Relations [1857.

and ruby-violet in the outer thinner portions ; a ruby tint is
presented occasionally where the heat of the discharge has
acted on the deposit. The deposited gold was easily removed
by wiping, except actually at the spot where the discharge had
passed. When these deposits were heated to dull redness they
changed and acquired a ruby tint, which was very fine at the
outer and thinner parts. The portions nearer the place of
discharge presented ruby-violet and then violet tints, suggesting
that accumulation of that which presented a fine ruby tint
would, by stopping more and more light, transmit a ruby-violet
or violet ray only. Pressure with the agate surface had a like
effect as before, both with the heated and the unheated portions,
i. e. with the violet and the ruby particles ; but the effect was
not altogether so good, and the tint of the transmitted ray was
rather a green violet than a pure green. Still the difference
produced by the pressure was very remarkable. The unheated
particles at the surface, away from the glass, presented by re-
flexion almost a black ; being heated, they became much more



Online LibraryMichael FaradayExperimental researches in chemistry and physics → online text (page 39 of 49)