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colour. This change will occur for any number of times, as
often as the jelly is wetted and dried. Here the gold remains
in the same metallic state through this great change of colour,
the association or the absence of water being the cause : and
the effect strengthens in my mind the thought before expressed,
that in the ruby fluids the deposited particles are frequently
associates of water and gold. It is a striking case of the joint
effect of the media and the gold in their action on the rays of
light, and the most striking case amongst those where the
medium may be changed to and fro.

When a ruby jelly is prepared with salt, and being warm is
poured out in thin layers on to glass or porcelain, it first gela-
tinizes and then dries up ; in which case the salt is excluded
and crystallizes. When the dry jelly is put into cold water, the
salt dissolves and can be removed. The jelly then swells to a
certain amount, after which it can be left soaking in water for a
week or longer, until everything soluble is separated. No
change takes place in the ruby tint, no gold is removed. When
the last water is poured off and the remaining jelly warmed, it
melts, forming a fine ruby fluid, which can either be dissolved
in more water, or regelatinized, or be dried and preserved for

1857.] of Gold (and other Metals) to Light. 435

any length of time. It is perfectly neutral ; gives no signs of
dissolved gold by any of the tests of the metal ; is not changed
by sulphuretted hydrogen, gallic acid, pyrogallic acid, dilute
caustic alkalies, or carbonated alkalies or lime-water ; or by
dilute sulphuric, hydrochloric or nitric acids, the actions being
continued for fourteen days : being boiled with zinc filings, it
does not change ; and even when dilute sulphuric or hydro-
chloric acid is added to evolve nascent hydrogen, still the ruby
character undergoes no alteration. Strong sulphuric, or nitric,
or hydrochloric acid does not alter it whilst cold ; but when
warmed, the first causes the gold to separate as dark aggre-
gated metallic particles, and the two latter gradually cause the
change to amethyst and blue formerly described. Chlorine, or
a mixture of hydrochloric and nitric acids, dissolves the gold,
the ruby colour disappears, and the ordinary solution of gold
is produced. In all these cases the ruby gold behaves exactly
as metallic gold would do with the same agents, and quite unlike
whatwould be expected from any possible combination of oxygen
and gold.

In some of these jellies the ruby particles are so determinate
as to give the brown reflexion by common observation ; in others
they are so fine as to look like ruby solutions, unless a strong-
sunlight and a lens be employed ; and the impression again
arises, that gold may exist in particles so minute as to have
little or no power of reflecting light. Ruby particles of extreme
fineness, when present in small amount in water, appear to re-
main equally diffused for any length of time ; if in larger amount,
that which settles to the bottom will remain for weeks and
months as a dense ruby fluid, but without coming together : both
circumstances seem to imply an association of the particles of
gold with envelopes of water. Many circumstances about the
ruby jellies imply a like association with that animal substance,
and many of the stains of gold upon organic substances pro-
bably include an affinity of the metal of the like kind.

Relations of Gold (and other metals) to polarised Light.

It has been already stated, that when a ray of common light
passes through a piece of gold-leaf inclined to the ray, the light
is polarized. When the angle between the leaf and the ray is
small, about 15, nearly all the light that passes is polarized;

436 On the Experimental Relations [1857.

but as the leaf is really very irregular in thickness, and ill-
stretched as a film, parts inclined at different angles are always
present at once. The light transmitted is polarized in the
same direction as that transmitted by a bundle of thin plates
of glass, inclined in the same direction. The proportion of
light transmitted is small, as might be expected from the high
reflective power of the metal. The polarization does not seem
due to any constrained condition of the beaten gold, for it is
produced, as will be shortly seen, by the annealed colourless
leaf-gold, and also by deposits of gold particles ; but is common
to it with other uncrystallized transparent substances. It would
seem that a very small proportion of the gold-leaf can be occu-
pied by apertures, since the light which passes is nearly all
polarized. On subjecting thin gold-leaf, or heated gold-leaf,
or films of gold, or any preparations which required the support
of glass, results of polarization were obtained, but the observa-
tions were imperfect because of the interfering effect of the

Proceeding to employ a polarized ray of light, it was found
that a leaf of gold produced generally the same depolarizing
effect as other transparent bodies. Thus, if a plate of glass be
held perpendicular to the ray, or inclined to it either in the
plane of polarization or at right angles to it, there is no depo-
larization ; but if inclined in the intermediate positions, the ray
is more or less depolarized. So it is with gold-leaf; the same
effects are produced by it. Further, the depolarization is
accompanied by a rotation of the ray, and in this respect the
quadrants alternate, the rotation being to the right-hand in two
opposite quadrants, and to the left in the intervening quadrants.
So it is with gold-leaf; the same effects are produced by it,
and the rotation is in the same direction with that produced by
glass, when inclined in the same quadrant.

As further observation in this direction was stopped by the
necessity of employing glass supports for the leaves, films, &c.,
I sought for a medium so near glass in its character, as should
either reduce its effect to nothing, or render it so small as to
cause its easy elimination. Either camphine or sulphide of
carbon was found to answer the purpose with crown-glass ;
but the latter, as it possesses no sensible power of rotation
under ordinary circumstances, is to be preferred. Should a

1857.] of Gold (and other Metals) to Light. 437

medium of higher optic force be required, it would probably
be supplied by the use of that dangerous fluid, phosphorus
dissolved in sulphide of carbon. A rectangular glass cell being
provided, which did not itself affect the polarized ray, was placed
in its course and filled to a certain height with sulphide of car-
bon. A plate of crown-glass was then introduced perpendicu-
larly to the ray ; it did not affect it ; being inclined as before
described, the effect on the ray was still insensible, the glass
appearing to be, for all ordinary observations such as mine,
quite as the medium about it. I could now introduce gold-leaf
attached to glass into the course of the polarized ray, its con-
dition as a flat film or plane being far finer than when stretched
on a wire ring as before. It proved to be so far above the
sulphide of carbon, as to have powers of depolarization appa-
rently as great as those it had in air, and being inclined, brought
in the image at the analyser exceedingly well. It was indeed
very striking to see, when the plate was moved parallel to itself,
the darkness when mere glass intervened, and the light which
sprung up when the gold-leaf came into its place ; the opake
metal and the transparent glass having apparently changed cha-
racters with each other. By care I was able to introduce a
stretched piece of gold-leaf (without glass) into the sulphide of
carbon ; its effects were the same with those just described.

In all the experiments to be described, the plane of
polarization and the plane of inclination had the same
relation to each other : the figure shows the position
of the polarizing Nicol prism, as the eye looks through
it at the light, and a, b represents the vertical axis,
about which the plates were inclined. Whether they
were inclined in one direction or the other, or had the g
glass face or the metal face towards the eye, made no
difference. In all cases with gold-leaf, it was found that the
ray had been rotated ; that it required a little direct rotation
of the analyser to regain the minimum light ; that short of that
red tints appeared, and beyond it blue or cold, these being
necessarily affected in some degree by the green colour of the
gold-leaf. Thinned gold-leaf produced the same results; but
as holes appeared in those that were thinnest, the results were
interfered with, because the light passing through them was
affected by the analyser in a different manner, and yet mingled

438 On the Experimental Relations [1857.

its result with that of the light which had passed through
the gold.

The gold-leaf plates, deprived of green colour by heating in
oil, were found with the glass in such good annealed condition,
as not to affect the ray ; but when they were moved, until the
oblique colourless gold came into the course of the ray, it was
depolarized ; a red image appeared ; direct rotation of the
analyser reduced this a little in intensity and then changed the
colour to blue. The reduction was not much, and both in
that and the first appearance of the red image, there is a
difference between the heated and the unheated gold: pro-
bably the green tint of the latter, which would tend to extin-
guish the red and produce a minimum, may be sufficient to
account for the effect. Gold which had been re-greened by
agate pressure acted in like manner on the polarized ray, but
the experiments were imperfect.

A glass plate having gold-leaf on one part of it, had a
second glass plate put over it and gummed at the edges. In
the sulphide of carbon, therefore, it represented in one part a
plate of air, and in another a compound plate of air and gold ;
both acted in the same direction, but the air and gold much
more than the air. Gold on glass in this medium, or gold in
air, or glass in air, all gave results in the same direction, i. e.
required direct rotation of the analyser to compensate for

I proceeded to examine the other forms of gold ; and first, the
deposits on glass obtained by electric deflagration. These af-
fected the ray of polarized light exactly in the manner of gold-
leaf, and that even at the distant parts of the deposit. It was
most striking to contrast the thinnest and faintest portion of
such a film with the neighbouring parts of the glass from
which it had been wiped off. It must be remembered that
such a preparation is a layer of separate particles ; that these
particles are not like those of starch or of crystals, for they
have no action whilst in a plane perpendicular to the polarized
ray ; nor have they a better action for being in a thick layer,
as in the central parts of the deposit. The particles seem to
form the equivalent of a continuous plate of transparent sub-
stance ; and as in such a plate it is the two surfaces which act,
so there appears to be the equivalent of these two surfaces

1857.] of Gold (and other Metals) to Light. 4*33

here ; which would seem to imply that the particles are so
small and so near, that two or more can act at once upon the
individual atoms of the vibrating ether. Their association is
such as to present as it were an optical continuity.

The gold films by phosphorus were then submitted to ex-
periment, and gave exactly the same result. All of them de-
polarized, and required direct rotation of the analyser to ar-
rive at a minimum, or to pass from the red to the blue tints.
Graduated films, of which I should judge from the depth of
tint that one place was at least twenty times as thick as an-
other, gave the effect as well in the thinnest as the thicker or
any intermediate part ; indicating that thickness of the plate,
and therefore any quality equivalent to crystalline force of the
particles, had nothing to do with the matter. A glass beaker,
which had been employed to contain ruby fluid, had a film of
gold deposited on its inner surface so thin, as to be scarcely
perceptible either by reflexion or otherwise, except by a ruby
tint which appeared upon it in certain positions ; but being ex-
amined by a polarized ray, it gave an effect as strong and as
perfect as gold-leaf, showing how thin a film of gold was suffi-
cient for the purpose. This thin film appeared to be almost
perfect in its continuity, for when the red image was brought
in, direct rotation of the analyser reduced it to a minimum
which was quite dark ; after which, further rotation brought
in a good blue image. The least touch of the finger removed
the film of gold and all these effects with it. These films,
though they are certainly porous to gas, and to water in some
form, for it can evaporate from beneath them through its body,
have evidently optical continuity.

In order to submit the gold fluids to experiments, cells were
made of two glass plates, separated by the thickness of a card,
and fastened at the edges by varnish internally and gum ex-
ternally. These being filled with dense ruby or blue fluid,
gave no indication of action on the ray, showing that the
diffused particles were inoperative. The same fluids, dried
on plates of glass so as to leave films, did act just as the gold
deflagrations had done ; for though the particles were very
irregularly spread, parts of the general deposit, and these not
the thickest, could be selected, which produced the effect
excellently well.

440 On the Experimental Relations [1857.

When the coloured jellies are laid upon glass plates and
allowed to dry, the plates introduced obliquely into the sul-
phide of carbon affect the ray, but not as gold films ; the light
image becomes visible, but the plane of polarization is not
changed ; the light is coloured by the ruby or blue tint of
the gold present, but a film of jelly without gold makes it
visible to the same extent. In this case the gold is not in one
plane, but diffused through the dry jelly, and the effect is the
same as if it were diffused through water, being negative.

Such are the effects with the various preparations of divided
gold. I will hastily notice what occurs with some other metals.
Platinum deflagrated in hydrogen : it depolarized the ray,
required direct rotation of the analyser to attain a minimum,
therefore rotated the plane of polarization ; but did not pre-
sent sensible colour on either side of the minimum of light.
Palladium deflagrated in hydrogen : it depolarized, producing
a red image ; direct rotation of the analyser lessened the light
to a minimum, and then brought in a blue image. The films
of palladium obtained by phosphorus acted well in the same
manner. These films appear to be exceedingly continuous,
and it could be observed in them, that though the thickest
were not the best, yet films could be obtained so thin as to be
distinctly inferior to other parts a little thicker ; also that where
the brilliancy of reflexion which indicates perfect smoothness
passed in any degree into dulness, the action of the film was
injured : the perfect condition of the surfaces of the films
seems to be essential to their good action. Rhodium films by
phosphorus gave good actions, like those produced by gold.
Silver deflagrations, either in air or hydrogen, gave depo-
larizing results like those with gold. Silver films also gave
excellent results of the like kind. A thin pale brown film was
much better than a thicker one. Copper deflagrated in hy-
drogen : depolarized, bringing in a red image, which by di-
rect rotation of the analyser was lowered a little, and then con-
verted to blue. The copper films obtained from oil acted in
the same manner ; the red and blue images appeared in their
order ; but very little direct rotation of the analyser was re-
quired to produce the minimum of light. Tin deflagrated in
hydrogen : depolarized and rotated the ray, as with gold ; the
images were only feeble in colour. Lead deflagrated in hy-

1857.] of Gold (and other Metals] to Light. 441

drogen : acted as tin. Iron deflagrated in hydrogen : acted
as tin. Zinc deflagrated in hydrogen : acted as tin. Aluminium
deflagrated in hydrogen : had like action with the rest ; the
image brought in by it was red, which direct revolution of the
analyser reduced at a little distance to a minimum, and then
converted to blue. A film of mercury produced by sublima-
tion, a film of arsenic produced in like manner, and a film of
smoke from a candle, though all of them sufficiently pervious
to light, did not produce any result of depolarization. Films
of the smoke of burning sine, of antimony, or of oxide of iron
produced no effect.

I placed some metallic solutions in a weak atmosphere of
sulphuretted hydrogen. Gold and platinum gave no films ;
silver so poor a film as to be of no use ; and lead one so brittle
as to be unserviceable. That obtained with palladium I be-
lieve to be the metal itself. The films of sulphuret of mercury,
sulphuret of antimony which was orange, and sulphuret of
copper which was pale brown, all acted on the light, and de-
polarized it. The sulphuret of copper presented a difference
from the metals generally, worth recording ; it depolarized the
light, producing an image, which, if not blue at once, was
rendered blue by a little direct rotation of the analyser ; after
which the same motion brought in a minimum and then pro-
duced an orange or red tint, i. e. with the sulphuret of copper
the warm and cold tints appear on opposite sides of the
minimum to those where they occur when films of the metals
are employed, though the minimum in both cases is in the
same direction.

Many of the results obtained in the sulphide of carbon were
produced also in camphine, the analyser being in each case
adjusted to the minimum of light before the metallic plate or
film was introduced. I pass, however, to a very brief account
of some polarizations effected by the metals themselves in the
sulphide of carbon, in which case the polarizing Nicol prism
was dispensed with. The results show that all the dry forms
of gold accord in giving the same manifestation of action on
light, whatever the state of their division, provided they be
disposed in a thin regular layer, equivalent to a continuous
film. It was first ascertained that a plate of crown-glass in an
inclined position in sulphide of carbon gave no signs of polarity

442 On the Experimental Relations of Gold to Light. [1857.

to a ray of light passing through it. When fine gold-leaf was
on the glass and inclined to the ray, it polarized the light, and
exactly in the same manner and direction as a bundle of glass
plates in the same position in the air. More light passed than
when the gold-leaf was in air, but it could not be so completely
polarized ; the minimum light was of a pale bluish colour. A
thinned gold-leaf produced the same effect, but let more com-
mon light through. I think the difference between gold-leaf and
sulphide of carbon is sensibly less than that between the
metal and air. The depositions of deflagrated gold, the films of
gold obtained by phosphorus, and even the heated deflagrated
gold, produced polarizing effects, which, though not large,
were easily recognized and distinguished from the non-action
of the glass. Gold-leaf and gold films on glass produced a
like effect in a camphine-bath, the results being easily di-
stinguished from those of the glass and camphine only, in
places where the glass had been cleared from gold.

Films of palladium, rhodium, silver, a plate with deposited
gold particles, and a layer of deflagrated silver particles gave
a like result, the effect varying in degree. The sulphuret of
copper before spoken of as in contrast with the metals, gave
only doubtful result, if any.

Before concluding, I may briefly describe the following
negative results with the preparations of gold. I prepared a
powerful electro-magnet, sent a polarized ray across the mag-
netic field, parallel to the magnetic axis, and then placed por-
tions of the ruby and violet fluids, also of their deposits wet
and dry, also portions of the gold films, of gold-leaf, the results
of deflagrations, &c., in the course of the ray ; but on exciting
the magnet, could not obtain any effect beyond that due to the
water or glass, which in any case accompanied the substance
into the magnetic field. In some cases very dense preparations
of the ruby and blue deposits were employed, the intense
electric lamplight being required to penetrate them.

I passed the coloured rays of the solar beam through the
various gold fluids and films that have been described. For
this purpose a beam of sunlight entering a dark room through an
aperture ^th of an inch in width, was sent through two of Bon-
temps's flint-glass prisms, and its rays were either separated, or
at once thrown on to a pure white screen ; the different objects

1857.] On the Conservation of Force. 443

were then interposed in the course of the ray, but I could not
perceive when any portion of a ray passed (and that was gene-
rally the case) that it differed sensibly in colour or quality from
the ray passing into the preparation. In like manner, the ob-
jects were put into the differently coloured rays and observed
by the reflected light, a lens being sometimes employed to con-
centrate the light ; but I could not find any marked difference
between the colour or character of the ray reflected and the
impinging ray, except in quantity.

On the Conservation of Force*.

VARIOUS circumstances induce me at the present moment to
put forth a consideration regarding the conservation of force.
I do not suppose that I can utter any truth respecting it that
has not already presented itself to the high and piercing in-
tellects which move within the exalted regions of science ; but
the course of my own investigations and views makes me think
that the consideration may be of service to those persevering
labourers (amongst whom I endeavour to class myself), who,
occupied in the comparison of physical ideas with fundamental
principles, and continually sustaining and aiding themselves by
experiment and observation, delight to labour for the advance
of natural knowledge, and strive to follow it into undiscovered

There is no question which lies closer to the root of all phy-
sical knowledge, than that which inquires whether force can be
destroyed or not. The progress of the strict science of modern
times has tended more and more to produce the conviction that
" force can neither be created nor destroyed," and to render
daily more manifest the value of the knowledge of that truth in
experimental research. To admit, indeed, that force may be
destructible or can altogether disappear, would be to admit that
matter could be uncreated ; for we know matter only by its
forces : and though one of these is most commonly referred to,
namely gravity, to prove its presence, it is not because gravity
has any pretension, or any exemption amongst the forms offeree,
as regards the principle of conservation ; but simply that being,
as far as we perceive, inconvertible in its nature and un-
* Proceedings of the Royal Institution, Feb. 27, 1857, vol. ii. p. 352.

444 On the Conservation of Force. [1857.

changeable in its manifestation, it offers an unchanging test of
the matter which we recognize by it.

Agreeing with those who admit the conservation of force to
be a principle in physics as large and sure as that of the inde-
structibility of matter, or the invariability of gravity, I think
that no particular idea of force has a right to unlimited or un-
qualified acceptance, that does not include assent to it ; and
also, to definite amount and definite disposition of the force,
either in one effect or another, for these are necessary conse-
quences : therefore, I urge, that the conservation of force ought
to be admitted as a physical principle in all our hypotheses,
whether partial or general, regarding the actions of matter. I
have had doubts in my own mind whether the considerations I
am about to advance are not rather metaphysical than physical.
I am unable to define what is metaphysical in physical science ;
and am exceedingly adverse to the easy and unconsidered ad-
mission of one supposition upon another, suggested as they often
are by very imperfect induction from a small number of facts,
or by a very imperfect observation of the facts themselves :
but, on the other hand, I think the philosopher may be bold in
his application of principles which have been developed by

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