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allowed to stand for months in a place of equable temperature,
the colouring particles will appear in floating clouds, and
probably the aggregation is then still going on. That the
particles of gold when they touch each other do in many cases
adhere together with facility, is shown in many experiments.
In order to test this matter mechanically, I gave much agitation
to a dense ruby fluid, but did not find it cause any sensible
change in the character. When gold particles of a much
larger size were agitated in water, they did cohere together,
and the fluid, which required a certain time for settling at the
beginning of the experiment, settled in a much shorter time at
the termination.

If these fluids be examined generally their appearances differ
not merely under different circumstances, but also under the
same circumstances, though they always consist of a colourless
liquid and diffused particles of gold. A certain fluid in a bottle
or glass, looked at from the front, i. e. the illuminated side by
general daylight, may appear hazy and amethystine, whilst in

1857.] of Gold (and other Metals) to Light. 415

bright sunlight it will appear light brown and almost opake.
From behind, the same fluid may appear of a pure blue in both
lights, whilst from the side it may appear amethystine or ruby.
These differences result from the mixture of reflected and
transmitted lights, both derived from the particles, the former
appearing in greatest abundance from the front or side, and the
latter from behind. The former is seen by common observation
in a purer state if a black background be placed behind the
fluid ; when a white background is there, much of the trans-
mitted light from that source comes to the eye, and the ap-
pearance is greatly altered. A mode of observing the former
by a strong ray of light and a lens has been already described ;
but even in that case some effects of transmitted light are
observed if the focus is thrown deep into the fluid ; and it is
only the particles near the surface, whether illuminated by the
base or the apex of the cone, which give the nearly pure effect
of reflexion. In order to observe the transmitted ray in an
unmingled state, a glass tube closed at one end was surrounded
with a tube of black paper longer than itself, and with the
black surface inwards. When a fluid (or the particles in it)
was to be examined, it was put into this tube, and a surface of
white paper illuminated by daylight or the sun, regarded
through it, other light being excluded from the eye ; or the
tube was sometimes interposed between the eye and the sky,
and sometimes the rays of the sun itself were reflected up to
the eye through it. In speaking hereafter of the tints of the
light transmitted by the particles (which will of course vary
with the proportion of different rays in the original beam of
light), a pure white original light is to be understood, but
occasionally differently-tinted papers were employed with this
tube as sources of different coloured lights.

The very oblique angle at which reflected light comes to
the eye from the diffused particles, is well seen when the lens
cone, or a direct ray of the sun, is passed into the fluid and
observed from different positions ; it is only when the eye is
behind and nearly in the line of the ray, that the unmixed
transmitted ray is observed. In the dark tube I think that no
reflected light arrives at the eye : for if half an inch in depth
of water be introduced, white light passes ; if a drop of the
washed deposit, to be hereafter described, be introduced, the

416 On the Experimental Relations [1857.

light transmitted is either blue or ruby, or of other inter-
mediate tint, according to the character of the deposit ; but
if water be then added until the column is six inches or
more in length, the quantity of light transmitted does not
sensibly alter, nor its tint; a fact, which I think excludes the
idea of any light being reflected from particle to particle, and
finally to the eye.

If a given ruby-tinted fluid, containing no gold in solution,
be allowed to stand for a few days, a deposit will fall from
which the fluid may be removed by a siphon ; being now
allowed to stand for a week, a second deposit will be produced ;
if the fluid be again removed and allowed to stand for some
months, another deposit will be obtained, and the fluid will
probably be of a bright ruby ; if it be now allowed to stand for
several months, it will still yield a deposit, looking, however,
more like a ruby fluid than a collection of fine particles at the
bottom of the fluid, whilst traces of yet finer particles of gold
in suspension may be obtained by the lens. All these deposits
may be washed with water and will settle again ; the coarser
are not much affected, but the finer are, and tend to aggregate ;
nevertheless specimens often occur, especially after boiling,
which tend to preserve their fine character after washing, if
the water be very clean and pure.

The colour of these particles whilst under, or diffused
through water, is by common reflected light brown, paler and
richer, sometimes tending to yellow, and sometimes to red.
The same difference is shown when illuminated by sunlight.
Everything tends to show that the light reflected is very
bright considering the size of the particles, and therefore of
the reflecting surfaces ; yet comparing by the cone of light a
ruby fluid when first prepared and before it has become very
sensibly turbid, with the same fluid after the evident turbidity
is produced, in both of which cases I believe the gold to be in
solid metallic particles, though of different sizes, it would seem
that more light is transmitted and absorbed and less reflected
by the finer particles than by the coarser set, the same quantity
of gold being in the same space. I believe that there may be
particles so fine as to reflect very little light indeed, that
function being almost gone. Occasionally some of the fluids
containing the very finest particles in suspension, when illu-

1857.] of Gold (and other Metals) to Light. 417

minated by the sun's rays and a lens, appeared to give a
fine green reflexion, but whether this is a true colour as com-
pared to white light, or only the effect of contrast with the
bright ruby in the other parts of the fluid, I am not prepared
to say.

When the deposits were examined in the dark tube by trans-
mitted light, being first diffused in more or less water to give
them the form of fluid, those first deposited, and therefore pre-
sumed to be the heavier and larger, transmitted a pure blue
light. The second and the third had the same character,
perhaps the fourth, if the subdivision into portions had been
numerous ; then came some which transmitted an amethystine
ray from the white of paper ; and others followed progressing
to the finest, which transmitted a rich ruby tint. It is probable
that many of these deposits were mixtures of particles having
different characters, and this is perhaps the reason that in some
cases, when the fluids were contained in round-bottomed flasks,
the lens-like deposit was ruby at the edges, though deep violet
in the middle, the former having settled last ; but as a pure
blue deposit could be obtained, and also one transmitting a
pure ruby ray, and as a comparatively pure intermediate pre-
paration transmitting a ruby violet, or amethystine ray, was
obtained, it is probable that all gradations from blue to ruby
exist; for the production of which I can see no reason to
imagine any other variation than the existence of particles of
intermediate sizes or proportions.

When light other than white was passed through the fluids,
then of course other tints were produced, yet some of these
were unexpected. A fluid of a pure blue colour, whilst in the
dark tube, would in an open glass and by reflected light appear
of a strong ruby-violet tint. Dropping some of the wet deposit
into pure water, the striae which it formed would in one part
be ruby in colour and in another violet: these effects were
referable to the light reflected from the solid particles back
through the fluid to the eye, but it seemed redder than any
which light reflected from gold was likely to produce. How-
ever, upon regarding the surface of dull gold-leaf, or the thick
wet deposit of gold, or the hand, it was found that the red rays
easily passed through the blue fluid and formed a ruby-violet
tint. Prevost showed in old times, how much the red and

2 E

418 On the Experimental Relations [1857.

warm rays are reflected by gold, in preference to the others
contained in white light.

The supernatant fluid in specimens that had stood long and
deposited, was always ruby ; yet because it showed no dissolved
gold, because it showed the illuminated cone by the lens, and
because by standing ruby clouds settled in it, there was every
reason to believe that the gold was there in separated particles,
and that such specimens afforded cases of extreme division,
which by long standing would form deposits of the finest kind.

Those fluids which on standing gave abundance of deposits,
transmitting blue light, consisted in the first instance of particles
transmitting a ruby light, and in these cases it would seem that
the particles at their first separation were always competent to
transmit this ruby light ; and if the preparation were not too
rich in gold, the ruby condition appeared to be retained, the
division being then most extreme. But purple or amethystine
fluids could be procured, which, containing no colouring par-
ticles other than suspended gold, still retained them in suspen-
sion for many months together, so that they must have been
as light or as finely divided as those in the ruby fluids. When
the phosphorous ether was employed for the reduction of the
gold, such fluids occurred ; also when the solution of the phos-
phorus in sulphide of carbon was used, provided the solution
of gold had a very little chloride of sodium contained in it.
They appear to show that the mere degree of division is not
the only circumstance which determines the aptitude to trans-
mit in preference this or that ray of light.

Considering the fluids as owing their properties to diffused
particles, it may be observed, that many of them which in small
quantities in the dark tube transmit an amethystine light, send
forward a ruby light when the quantity is increased ; and this
appears to be the general progression. 1 have not found any
which by increase in quantity tended to transmit the blue rays
in preference to the red.

Elevation of temperature had an effect upon these fluids
which is advantageous in their preparation. On boiling an
apparently clear ruby fluid for some time, its colour passed a
little towards amethystine, and on boiling a like amethystine
fluid, its tint passed towards blue. The separation of the gold
particles was also facilitated, for now they would settle in three

1857.] of Gold (and other Metals) to Light. 419

or four days from a fluid which, prior to this operation, would
not have deposited them in an equal degree for weeks. In the
case of the ruby fluids the colour often became more rosy and
luminous, and by reflected light the fluid seemed to have become
more turbid, as if the particles had gained in reflective power ;
in fact the boiling often appeared to confer a sort of permanency
on the particles in their new state. When settled, they formed
collections looking like little lenses of a deep ruby or violet
colour, at the bottom of the flasks containing the fluid ; when
all was shaken up the original fluid was reproduced, and then,
by rest, the gold re-settled. This effect could be obtained
repeatedly. The particles could fall together within a certain
limit, but many weeks did not bring them nearer or into contact;
for they remained free to be diffused by agitation. The space
they occupied in this lens-like form must have been a hundred-
fold or even a thousandfold, more than that, which they would
have filled as solid gold. Whether the particles be considered
as mutually repulsive, or else as molecules of gold with asso-
ciated envelopes of water, they evidently differ in their physical
condition, for the time, from those particles which by the
application of salt or other substances are rendered mutually
adhesive, and so fall and clot together.

In preparing some of these fluids, I made the solution of
gold hot and boiling before adding the solution of phosphorus.
The phenomena were the same in kind as before : but when the
phosphorus was dissolved in sulphide of carbon, the gold soon
fell as a dark flocculent deposit ; when it was dissolved in ether a
more permanent turbid ruby fluid was obtained, which, if it does
notgoon changing in aggregation, may give a good ruby deposit.
The particles in these fluids are remarkable for a set of
physical alterations occasioned by bodies in small quantities,
which do not act chemically on the gold, or change its intrinsic
nature ; for through all of them it seems to remain gold in a
fine state of division. They occur most readily where the
particles are finest, i. e. in the ruby fluids, and so readily that
it is difficult to avoid them ; they are often occasioned by the
contact of vessels which are supposed to be perfectly clean.
An idea of their nature may be obtained in the following manner.
Place a layer of ruby fluid in a clean white plate, dip the tip
of a glass rod in a solution of common salt and touch the ruby

420 On the Experimental Relations [1857.

fluid ; in a few moments the fluid will become blue or violet-
blue, and sometimes almost colourless : by mingling up the
neighbouring parts of the fluid, it will be seen how large a
portion of it can be affected by a small quantity of the salt.
By leaving the whole quiet, it will be found that the changed
gold tends to deposit far more readily than when in the ruby
state. If the experiment be made with a body of fluid in a
glass, twelve or twenty-four hours will suffice to separate gold,
which in the ruby state has remained suspended for six months.

The fluid changed by common salt or otherwise, when most
altered, is of a violet-blue, or deep blue. Any tint, however,
between this and the ruby may be obtained, and, as it appears
to me, in either of two ways ; for the intermediate fluid may
be a mixture of ruby and violet fluids, or, as is often the case,
all the gold in the fluid may be in the state producing the
intermediate colour ; but as the fluid may in all cases be carried
on to the final violet-blue state, I will, for brevity sake, describe
that only in a particular manner. The violet or blue fluid,
when examined by the sun's rays and a lens, always gives evi-
dence showing that the gold has not been redissolved, but is
still in solid separate particles ; and this is confirmed by the
non-action of protochloride of tin, which, in properly prepared
fluids, gives no indication of dissolved gold. When a ruby
solution is rendered blue by common salt, the separation of the
gold as a precipitate is greatly hastened ; thus when a glass
jar containing about half a pint of the ruby fluid had a few
drops of brine added and stirred into the lower part, the lower
half of the fluid became blue whilst the upper remained ruby ;
in that state the cone of sun's rays was beautifully developed
in both parts. On standing for four hours the lower part
became paler, a dark deposit of gold fell, and then the cone
was feebly luminous there, though as bright as ever in the ruby
above. In three days no cone was visible in the lower fluid ;
a fine cone appeared in the upper. After many days, the salt
diffused gradually through the whole, first turning the gold it
came in contact with blue, and then causing its precipitation.

Such results would seem to show that this blue gold is aggre-
gated gold, i. e. gold in larger particles than before, since
they precipitate through the fluid in a time which is as nothing
to that required by the particles of the ruby fluid from which

1 857.] of Gold (and other Metals) to Light. 421

they are obtained. But that the blue particles are always
merely larger particles does not seem admissible for a moment,
inasmuch as violet or blue fluids may be obtained in which the
particles will remain in suspension as long as in the ruby fluids ;
there is probably some physical change in the condition of the
particles, caused by the presence of the salt and such affecting
media, which is not a change of the gold as gold, but rather a
change of the relation of the surface of the particles to the
surrounding medium.

When salt is added in such quantity as to produce its effect
in a short time, it is seen that the gold reflexion of the particles
is quickly diminished, so that either as a general turbidness or
by the cone of rays it becomes less visible ; at last the metal
contracts into masses, which are comparatively so few and
separate, that when shaken up in the fluid, they confer little
or no colour or character, either by reflected or transmitted
light. In these cases no re-solution of the metal is effected,
for neither the salt nor hydrochloric acid, when used in like
manner, has any power to redissolve the gold. The same
aggregating effect is shown with all the fluids whatever their
colour, and also with the deposits that settle down from them.
When salt is added to the solution of gold before the phos-
phorus, and therefore before the reduction of the gold, the
fluid first produced is always ruby ; but it becomes violet, purple,
or blue, with a facility in proportion to the quantity of salt
present. If that be but small, the ruby will remain for many
days unchanged in colour, and the violet-ruby for many weeks,
before the gold will be deposited, the degree of dilution or
concentration always having its own particular effect, as before
described; the more finely divided preparations, i. e. the ruby
and amethystine, appear to be more permanent than when the
salt is added after the separation of the gold.

Many other bodies besides salt have like action on the par-
ticles of gold. A ruby fluid is changed to or towards blue by
solutions of chlorides of calcium, strontium, manganese ; sul-
phates of magnesia, manganese, lime ; nitrates of potassa, soda,
baryta, magnesia, manganese ; acetates of potassa, soda, and
lime ; these effect the change freely : the sulphate of soda,
phosphates of soda and potassa, chlorate of potassa, and acetate
of ammonia acted feebly. Sulphuric and hydrochloric acids

422 On the Experimental Relations [1857.

produce the change, but show no tendency to dissolve the gold.
Nitric acid acts in the same manner, but not so strongly : it
often causes re-solution of the gold after some time, because of
the hydrochloric acid which remains in the fluid.

Amongst the alkalies, potash produces a similar action in a
weak degree. So also does soda. Lime-water produces a
change in the same direction, but the gold quickly precipitates
associated with the lime.

Ammonia causes the ruby fluid to assume a violet tint; the
deposit is slow of formation and often ruby in colour ; the alkali
apparently retards the action of common salt.

Chlorine or nitromuriatic acid turns the ruby fluid blue or
violet-blue before they dissolve the gold.

Solution of sulphuretted hydrogen changes the ruby slowly
to purple, and finally to deep blue. Ether, alcohol, camphine,
sulphide of carbon, gum, sugar and glycerine cause little or no
change in the fluids ; but glycerine added to the dense deposits
causes serious condensation and alteration of them, so that it
could not be employed as a medium for the suspension of par-
ticles in the microscope.

All endeavours to convert the violet gold back into ruby
were either failures, or very imperfect in their results. A violet
fluid will, upon long standing, yield a deposit and a supernatant
ruby fluid, but this I believe to be a partial separation of a
mixture of violet and ruby gold, by the settlement of the blue
or violet gold from ruby gold, which remains longer in suspen-
sion. Mucus, which often forms in portions of these fluids
that have been exposed to the air, appears sometimes to render
a fluid more ruby, but this it does by gathering up the larger
violet particles ; it often becomes dark blue or even black by
the particles of gold adhering to it, many of which maybe shaken
out by agitation in water ; but I never saw it become ruby-
coloured as a filter can, and I think that in these cases it is the
gathering out of the blue or violet particles which makes the
fluid left appear more ruby in tint. I have treated blue or violet
fluid with phosphorus in various ways, but saw no appearance
of a return in any degree towards ruby. Sometimes the fluids
possess a tendency to re-solution of the gold, a condition which
may often be given by addition of a very little nitric acid, but
in these cases the gold does not become ruby before solution.

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

It would rather appear that the finer ruby particles dissolve
first, for the tint of the fluid, if ruby- violet at the commencement,
changes towards blue. One effect only seemed to show the
possibility of a reversion. Filtering-paper rendered ruby by a
ruby fluid was washed and dried ; being wetted by solution of
caustic potash, it did not change ; but being heated in a tube
with the alkali, it became of a grey-blue tint ; pouring off the
alkali, washing the paper, and then adding dilute sulphuric or
nitric acid to it, there was no change ; but on boiling the paper
in the mixed acids there was a return, and when the paper was
washed and dried it approached considerably to the original
ruby state. Again, potash added to it rendered it blue, which
by washing with water, and especially with a little nitric acid,
was much restored towards ruby. These changes may be due
to an affection of the surface, or that which may be considered
the surface of the particles.

The state of division of these particles must be extreme ;
they have not as yet been seen by any power of the microscope.
Whether those that are ruby have their colour dependent upon
a particular degree of division, or generally upon their being
under a certain size, or whether it is consequent in part upon
some other condition of the particles, is doubtful ; for judging
of their magnitude by the time occupied in their descent through
the fluid, it would appear that violet and blue fluids occur
giving violet deposits, which still consist of particles so small as
to require a time equally long with the ruby particles for their
deposition, and indeed in some specimens to remain undepo-
sited in any time which has yet occurred since their formation.
These deposits, when they occur, look like clear solutions in
the fluid, even under the highest power of the microscope.

I endeavoured to obtain an idea of the quantity of gold in a
given ruby fluid, and for this purpose selected a plate of gold
ruby glass, of good full colour, to serve as a standard, and
compared different fluids with it, varying their depth, until
the light from white paper, transmitted through them, was
apparently equal to that transmitted by the standard glass.
Then known quantities of these ruby fluids were evaporated
to dryness, the gold converted into chloride, and compared by
reduction on glass and otherwise with solutions of gold of
known strengths. A portion of chloride of gold, containing

424 On the Experimental Relations [1857.

0'7 of a grain of metal, was made up to 70 cubic inches by the
addition of distilled water and converted into ruby fluid : on
the sixth day it was compared with the ruby glass standard,
and with a depth of 1*4 inch was found equal to it ; there was
just one hundredth of a grain of geld diffused through a cubic
inch of fluid. In another comparison, some gold leaves were
dissolved and converted into ruby fluid, and compared ; the
result was a fluid, of which 1'5 inch in depth equalled the
standard, a leaf of gold being contained in 27 cubic inches of
the fluid. Hence looking through a depth of 2*7 inches, the
quantity of gold interposed between the light and the eye
would equal that contained in the thickness of a leaf of
gold. Though the leaf is green and the fluid ruby, yet it is
easy to perceive that more light is transmitted by the latter
than the former ; but inasmuch as it appears that ruby fluids
may exist containing particles of very different sizes (or that
settle at least with very different degrees of rapidity), so it is
probable that the degree of colour, and the quantity of gold

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