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present, may not be always in the same proportion. I need
hardly say that mere dilution does not alter the tint sensibly,
f. e. if a deep ruby fluid be put into a cylindrical vessel, and
the eye look through it along the axis of the vessel, dilution
of the fluid to eight or ten times its volume does not sensibly
alter the light transmitted. From these considerations, it
would appear that one volume of gold is present in the ruby
fluid in about 750,600 volumes of water ; and that whatever
the state of division to which the gold may be reduced, still
the proportion of the solid particles to the amount of space
through which they are dispersed, must be of that extreme
proportion. This accords perfectly with their invisibility in
the microscope ; with the manner of their separation from the
dissolved state ; with the length of time during which they can
remain diffused; and with their appearance when illuminated
by the cone of the sun's rays.

The deposits, when not fixed upon glass or paper, are much
changed by drying ; they cannot be again wetted to the same
degree as before, or be again diffused ; and the light reflected
or refracted is as to colour much altered, as might be expected.
Whilst diffused through water, they seem to be physical asso-
ciations of metallic centres with enveloping films of water, and



1857.] of Gold (and other Metals] to Light. 425

as they sink together will lie for months at the bottom of the
fluid without uniting or coming nearer to each other, or without
being taken up by metallic mercury put into the same vessel.
This is consistent with what we know of the manner in which
gold and platinum can be thoroughly wetted if cleaned in water,
and of the difference which occurs when they are dried and
become invested with air. I endeavoured to transfer the gold
particles unchanged into other media, for the purpose of noting
any alteration in the action on light. By decanting the water
very closely, and then carefully adding alcohol with agitation,
I could diffuse them through that fluid ; they still possessed a
blue colour when looked through in the dark tube, but seemed
much condensed or aggregated, for the fluid was obscure, not
clear, and the particles soon subsided. I could not transfer
them from alcohol to camphine ; they refused association with
the latter fluid, retaining a film of alcohol or water, and adhe-
ring by it to the glass of the vessel ; but when the camphine
was removed, a partial diffusion of them in fresh alcohol could
be effected, and gave the colour as before. All these transfers,
however, injured the particles as to their condition of division.
In one case I obtained a ruby film on a white plate ; on pour-
ing off the water and allowing parts to become dry, these be-
came violet, seen by the light going through them to the plate
and back again to the eye. I could not wet these places with
water ; a thin feebly reflecting surface remained between it and
them. Using alcohol, the parts already dry remained violet,
when wetted by it ; but wetting other parts with alcohol before
they were dry from water, they remained rosy, became bluish
when dry from the alcohol, and became rosy again when re-
wetted by it.

It will be necessary to speak briefly of the reduction of gold
into a divided state by some other chemical agents than those
already described*. If a drop of solution of protosulphate of
iron be introduced to, and instantly agitated with, a weak
neutral solution of chloride of gold in such proportion that
the latter shall be in excess, the fluid becomes of a blue-grey
colour by transmission and brown by reflexion ; and a deposit is
formed of a green colour by transmitted light, greatly resembling

* See Gmelin's ' Chemistry,' vi. p. 219, " Terchloride of gold," for nume-
rous references in relation to changes of these kinds.



On the Experimental Relations [1857.

the colour of beaten or pressed metal. It is not, however,
pure gold, but an association of it and oxide of iron. Hydro-
chloric or other acids remove the iron and reduce the gold to
a dark, dense, insoluble set of particles, in very small quantity
apparently, yet containing all that was present in the bulky
green deposit. If the solution of gold be made slightly acid
beforehand, then the change and precipitation is to appear-
ance much less ; the reflexion by the particles is feeble, but of
a pale brown colour : the general transmitted light is ame-
thystine ; in the dark tube the tint is blue ; the particles are
much condensed and settle quickly, but occasionally leave a good
ruby film on the side of the glass, which has all the characters
of the ruby films and particles before described. The loose
gold particles quickly adhere together. Hence it appears that
the green precipitate often obtained by protosulphate of iron
is not pure gold in a divided state ; and that when care is
taken to produce such pure divided gold, it presents the ap-
pearances of divided gold obtained by other means, the gold
being competent to produce the ruby, amethystine, and blue
colours by transmission. Usually the gold rapidly contracts
and becomes almost insensible, and yet the test of protochlo-
ride of tin will show that all has been separated from solution ;
it then forms a striking contrast to the depth of colour presented
by the same solution of gold precipitated by phosphorus, and
most impressively directs attention to the molecular condition
of the metal in the latter state.

A very small quantity of protochloride of tin, added to a
dilute solution of gold, gave, first the ruby fluid, showing dif-
fused particles by the cone of rays ; this gradually became
purple, and if the gold were in sufficient quantity, a precipitate
soon began to fall, being the purple of Cassius. If the chlo-
ride of tin were in larger quantity, a more bulky precipitate
fell and more quickly. Acid very much reduced this in quan-
tity, dissolving out oxide of tin, and leaving little else than
finely-divided gold, which, when diffused and examined in the
dark tube, transmitted a blue colour. I believe the purple of
Cassius to be essentially finely-divided gold, associated with
more or less of oxide of tin.

Tartaric acid being added to a weak solution of gold gra-
dually reduced it. The amethystine tint produced by diffused



1857.] of Gold (and other Metals) to Light. 427

particles first appeared, and then a blue deposit of larger
particles ; whilst the side and bottom of the glass became
covered by an adhering film of finer particles, presenting the
perfect ruby tint of gold.

Ether added to a weak solution of gold gradually reduced
it ; the fluid was brown by reflected light, fine blue by trans-
mitted light, and gave a good cone by the sun's rays and lens.
The blue colour was not deep, though all the gold had been
separated from solution ; the preparation closely resembled
that made with protosulphate of iron and a little acid.

A weak solution of gold, mingled with a little sugar, being
heated, yielded a very characteristic decomposition. The gold
was reduced into diffused particles, which rendered the fluid
of a ruby-amethystine colour, and which, upon standing for
twenty-four hours, gave signs of separation by settling as on
former occasions. A little glycerine with solution of gold re-
duces it at common temperatures, producing a fluid, brown by
reflexion, blue by transmission^ giving a fine cone of rays by
its suspended particles. Heat quickens the action, and causes
a blue deposit.

Organic tissues often reduce solutions of gold, light if pre-
sent assisting the action ; and they afford valuable evidence
in aid of the solution of the question relative to the condition
of the metal in the divided state. If the skin be touched with
a solution of gold, it soon becomes stained of a dull purple
colour. If a piece of the large gut of an ox be soaked first in
water, then in a solution of gold, and be afterwards taken out
and allowed to dry, either exposed to light or not, the inner
membrane will become so stained, that though of a dull purple
colour by common observation, a transmitted ray will show it
to be generally a very fine ruby, equal to that of ruby-coloured
glass, or the gold fluids already described, though perhaps in
places of a beautiful violet hue. The character of the parti-
cles which are here located and not allowed to diffuse and
aggregate, as in the fluids, will be resumed when dealing with
the whole question of the metallic nature of the particles of the
variously divided gold.

Chloride of gold is reducible by heat alone. If a drop of
solution of chloride of gold be evaporated in a watch-glass, or
on a plate of rock-crystal, and then heated over a spirit-lamp



428 On the Experimental Relations [1857.

until the gold is reduced, it will generally be found that the
vapour has carried a portion of gold on to the neighbouring
part of the glass, and that this part, when placed over a sheet
of white paper, has the ruby tint. With the rock-crystal both
ruby and blue parts are produced ; and when the ruby parts
are subjected to rock-crystal pressure, they become beautifully
green. In the arts also glass is oftentimes coloured ruby by
gold ; I think that glass in this state derives its colour from
diffused divided gold; and if either the ruby glass or the
watch-glass be examined by a lens and the cone of rays, it will
be seen that the colours are not due to any gold dissolved, but
to solid and diffused particles. There is nothing in any of the
appearances or characters, or in the processes resorted to to
obtain the several effects, that point at any physical difference
in the nature of the results ; and without saying that gold can-
not produce a ruby colour whilst in combination or solution, I
think that in all these cases the ruby tint is due simply to the
presence of diffused finely-divided gold.

Metallic character of the divided gold.

Hitherto it may seem that I have assumed the various pre-
parations of gold, whether ruby, green, violet, or blue in colour,
to consist of that substance in a metallic divided state. I will
now put together the reasons which caused me to draw that
conclusion. With regard to gold-leaf no question respecting
its metallic nature can arise, but it offers evidence reaching to
the other preparations. The green colour conferred by press-
ure, and the removal of this colour by heat, evidently belong
to it as a metal ; these effects are very striking and important
as regards the action on light; and where they recur with other
forms of gold, may be accepted as proof that the gold is in the
metallic state. Although I do not attach equal importance to
the fact already described, that gold-leaf frequently presents
fine parts that appear to be ruby in colour, I am not as yet
satisfied that they are not in themselves ruby ; and if they
should be so, it will be another proof by analogy of the metallic
nature of other kinds of preparations eminently ruby.

The deflagrations of gold wire by the Leyden discharge can
be nothing but divided gold. They are the same whatever the
atmosphere surrounding them at the time, or whatever the



1857.] of Gold (and other Metals) to Light. 429

substance on which they are deposited. They have all the
chemical reactions of gold, being, though so finely divided, in-
soluble in the fluids that refuse to act on the massive metal,
and soluble in those that dissolve it, producing the same result.
Heat makes these divided particles assume a ruby tint, yet
such heat is not likely to take away their metallic character,
and when heated they still act with chemical agents as gold.
Pressure then confers the green colour, which heat takes away,
and pressure reconfers. All these changes occur with parti-
cles attached to the substances which support them by the
slightest possible mechanical force, just enough indeed to pre-
vent their coalescence and to keep them apart and in place,
and yet offering no resistance to any chemical action of test
agents, as the acids, &c., not allowing any supposition of che-
mical action between them and the body supporting them.
Still this gold, unexceptionable as to metallic state, presents
different colours when viewed by transmitted light. Ruby,
green, violet, blue, &c. occur, and the mere degree of division
appears to be the determining cause of many of these colours.
The deflagrations by the voltaic battery lead to the same con-
clusion.

The gold films produced by phosphorus have every charac-
ter belonging to the metallic state. When thick, they are in
colour, lustre, weight, &c. equal to gold-leaf; but in the un-
pressed state, their transmitted colour is generally grey, or
violet-grey. The progression of their lustre and colour is
gradual from the thickest to the thinnest, and the same is
generally true, if thick films are gradually thinned and dis-
solved whilst floating on solvents ; the thick and the thin films
must both be accepted as having the same amount of evidence
for their metallic nature. When subjected to chemical agents,
both the thick and the thin films have the same relations as pure
metallic gold. These relations are not changed by the action
of heat, yet heat shows the same peculiar effect that it had
with preparations of gold obtained by beating, or by electric
deflagrations. The remarkable and characteristic effect of
pressure is here reproduced, and sometimes with extraordinary
results ; since from the favourable manner in which the par-
ticles are occasionally divided and then held in place on the
glass, the mere touch of a finger or card is enough to produce



430 On the Experimental Relations [1857.

the result. Yet with gold thus proved to he metallic, colours
including grey, grey-violet, green, purple, ruby, especially by
heat, and green again by pressure, and by thinning of grey
films, may be obtained by transmitted light, almost all of them
at pleasure.

It may be thought that the fluid preparations present more
difficulty to the admission, that they are simply cases of pure
gold in a divided state ; yet I have come to that conclusion,
and believe that the differently -coloured fluids and particles
are quite analogous to those that occur in the deflagrations
and the films. In the first place they are produced as the
films are, except that the particles are separated under the
surface and out of the contact of the air ; still, when produced
in sufficient quantity against the side of the containing vessel
to form an adhering film, that film has every character of lustre,
colour, &c. in the parts differing in thickness, that a film formed
at the surface has. Whilst the particles are diffused through
the fluid it is difficult to deal with them by tests and reagents ;
for their absolute quantity is very small, and their physical cha-
racters are very changeable, chiefly as I believe by aggregation ;
still there are some expedients which enable one to submit
even the finest of them to proof. In several cases particles
from ruby and amethystine fluids adhere to the sides of the
bottles or flasks in which the fluids had been preserved, and
the process of boiling seemed to favour such a result; the
adhesion was so strong, that when the fluid contents were re-
moved and the bottles well-washed, the glass remained tinged
of a ruby or of a violet colour. These films, in which the fine
particles were fixed mechanically apart and in place, were then
submitted to the action of various chemical agents. Drying
and access of air did not cause any marked alterations in them.
Strong nitric acid produced no change, nor hydrochloric acid,
nor sulphuric acid. Neither did a solution of chloride of so-
dium, even up to brine, cause any alteration in the colour or
any other character of the deposit. A little solution of chlo-
rine or of nitromuriatic acid dissolved them at once, producing
the ordinary solutions of gold. I can see no other mode of
accounting for these effects (which are in strong contrast with
what happens when ruby fluid is acted on by these agents),
than to suppose that the gold particles, being in a high state



1857.] of Gold (and other Metals) to Light. 431

of division, were retained in that state for the time by their
adhesion to the glass. Of course chemical change was free to
occur, but not a change dependent upon their mutual aggrega-
tion ; yet they were not held by any special chemical attraction
to, or combination with, the glass ; for a touch with a card, a
feather, or the finger, was sufficient to remove them at once ;
and if rubbed off with a point of wood, they coated it with bril-
liant metallic gold.

Again, though these particles are so finely divided that they
pass easily through ordinary filters, still a close filter catches
some ; and if a ruby fluid be passed through again and again,
the paper at last becomes of a rosy hue, because of the gold
which adheres to it; being then well-washed, and, if needful,
dried, the gold is again ready for experiment. Such gold
paper, placed across the middle of the dark tube and examined
by transmitted light, was of the same ruby tint as when looked
through in the open air. It was unaffected by salt or brine,
though these, added to the rosy fluid which had passed the
filter, instantly changed it to violet-blue. Portions of the paper
were put into separate glasses with brine, solutions of hydro-
chloric, nitric and sulphuric acids, ammonia, potassa, soda and
sulphuretted hydrogen, but no change occurred with any of
them in two days. On the other hand, a very dilute solution
of chlorine immediately turned the ruby to blue, and then
gradually dissolved the gold. A piece of the ruby paper im-
mersed in a strong solution of cyanide of potassium suffered a
very slow action, if any, and remained unaltered in colour ;
being brought out into the air, the gold very gradually dis-
solved, becoming first blue. A portion of the ruby paper was
dried and heated in oil until the oil and the paper began to
change their hue ; the gold had not altered in its colour or
character. Another portion was heated in the vapour of
alcohol and also of ether until the paper began to alter ;
the gold remained unaltered. A blue fluid being passed
oftentimes through a filter gave a blue paper, which, being
washed and tried in the same manner, was found to contain
particles unchanged by the simple acids or alkalies, or by
heat or vapours, but dissolving, as gold would do, in chlorine
or nitromuriatic acid. These tests are, I think, sufficient to
prove the metallic nature and permanence of the gold as it



432 On the Experimental Relations [1857.

exists in the ruby, amethystine, violet, and other coloured
fluids.

The production by such different agents as phosphorus,
sulphide of carbon, ether, sugar, glycerine, gelatine, tartaric
acid, protosulphate of iron and protochloride of tin, of gold
fluids all more or less red or ruby at the commencement, and all
passing through the same order of changes, is again a proof
that only gold was separated ; no single one or common com-
pound of gold, as an oxide or a phosphide, could be expected
in all these cases. Many of the processes, very different as to
the substances employed to reduce the gold, left good ruby
films adhering to the glass vessels used, presenting all the
characters of the gold described already : this was the case
with phosphorus, sugar, tartaric acid, protosulphate of iron,
and some other bodies.

Again, the high reflective power of these particles (unalter-
able by acids and salts), when illuminated by the sun's rays and
a lens, and the colour of the light reflected, is in favour of
their metallic character. So also is their aggregation, and
their refusal to return from blue, violet or amethystine to
ruby ; for the cohesive and adhering force of the gold particles
and their metallic nature and perfect cleanliness is against
such a reverse change. Particles transmitting blue light could
be obtained in such quantity as to admit of their being washed
and dried in a tube, and being so prepared they presented
every character of gold : when heated, no oxygen, water, phos-
phorus, acid of phosphorus, nor any other substance was
evolved from them : they changed a little, as the film when
heated changed, becoming more reflective and of a pale brown
colour, and contracted into aggregated porous masses of pure
ordinary gold.

Gold is reduced from its solution by organic tissues ; and
stained gut has been quoted as a case. I have a very fine
specimen which by transmitted light is as pure a ruby as gold-
stained glass, and I believe that the gold has been simply re-
duced and diffused through the tissue. The preparation
stood all the trials that had been applied to the ruby films on
glass or the gold deposit on filtering-paper. Portions of it
remained soaking in water, solution of chloride of sodium and
dilute sulphuric acid for weeks, but these caused no change



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

from ruby to blue, such as could be effected on loose ruby
particles. Strong hydrochloric acid caused no change as long
as the tissue held together ; but as that became loose the gold
flowed out into the acid in ruby-amethystine streams, finally
changing to blue. Caustic potassa caused no change for days
whilst the tissue kept together, but on mixing all up by press-
ure the loosened gold became at last blue. Strong nitric acid
caused no change of colour until, by altering the tissue, the
gold particles first flowed out in ruby and amethystine streams,
and then were gradually changed to the condition of common
aggregated gold. All these effects, and the actions on light,
accord with the idea that the stain was simply due to diffused
particles of finely-divided gold ; and I am satisfied that all
such stains upon the skin, or other organic matter, are of ex-
actly the same nature.

As to the gold in ruby glass, I think a little consideration is
sufficient to satisfy one that it is in the metallic condition.
The action of heat tends to separate gold from its state of
combination, and when so separated from the chloride, either
upon the surface of glass, rock-crystal, topaz, or other inactive
bodies, a ruby film of particles is frequently obtained. The
sunlight and lens show that in ruby glass the gold is in sepa-
rated and diffused particles. The parity of the gold glass,
with the ruby-gold deflagrations and fluids described, is very
great. These considerations, with the sufficiency of the as-
signed cause to produce the ruby tint, are strong reasons, in
the absence of any to the contrary, to induce the belief that
finely divided metallic gold is the source of the ruby colour.

When a pure, clean, stiff jelly is prepared, and mixed, whilst
warm and fluid, with a little dilute chloride of gold, as if to
prepare a ruby fluid, it gelatinizes when cold, and if left for
two or three days may become a ruby jelly ; sometimes, how-
ever, the gold in the jelly changes but little or changes to blue,
or it may happen that it is reduced on the surface as a film,
brilliant and metallic by reflected light, and blue-grey by trans-
mitted light. I have not yet ascertained the circumstances
determining one or the other state. If a trace of phosphorus
in sulphide of carbon be added to the solution of gold in a
dilute state, arid some salt be added to the warm jelly, and the
latter be then mixed gradually and with agitation with the gold



434 On the Experimental Relations [1857.

solution, a ruby jelly is generally produced. In such ruby
jelly the reduced particles of gold preserve their state and
relative place, and the tint does not pass to blue, even though
a considerable proportion of salt be present. Such jelly will
remain in the air for weeks before it decays, and has every
character, in colour and appearance, of gold ruby glass. It is
hardly possible to examine the series of ruby glass, ruby mem-
brane, ruby jelly cold and gelatinous, ruby jelly warm and
fluid, and the ruby fluids, to consider their production, and
then to conclude that the cause of their common ruby colour is
not the same in all.

When the warm ruby jelly is poured into a capsule or on to
a plate, allowed to gelatinize and then left in the air, it gra-
dually becomes dry. When dry, some of these jellies remain
ruby ; others will probably be of an amethystine violet colour,
or perhaps almost blue. When one of the latter is moistened
with water, and has absorbed that fluid, it becomes gelatinous,
and whilst in that state resumes its first ruby colour ; but on
being suffered to dry again, it returns to its amethystine or blue



Online LibraryMichael FaradayExperimental researches in chemistry and physics → online text (page 42 of 49)