Chapter XXII: , Note 35) respecting the combination of CuSO{4} (3)

As yet the general chemical characteristics of gold and its
compounds have not been fully investigated. This is partly due to
the fact that very few researches have been undertaken on the
compounds of this metal, owing to its inaccessibility for working
in large quantities. As the atomic weight of gold is high (Au =
197), the preparation of its compounds requires that it should be
taken in large quantities, which forms an obstacle to its being
fully studied. Hence the facts concerning the history of this
metal are rarely distinguished by that exactitude with which many
facts have been established concerning other elements more
accessible, and long known in use.

_In nature_ gold occurs in the primary and chiefly in quartzose rocks, and especially in quartz veins, as in the Urals (at Berezoffsk), in Australia, and in California. The native gold is extracted from these rocks by subjecting them to a mechanical treatment consisting of crushing and washing.[29 bis] Nature has already accomplished a similar disintegration of the hard rocky matter containing gold.[30] These disintegrated rocks, washed by rain and other water, have formed gold-bearing deposits, which are known as _alluvial gold deposits_. Gold-bearing soil is sometimes met with on the surface and sometimes under the upper soil, but more frequently along the banks of dried-up water-courses and running streams. The sand of many rivers contains, however, a very small amount of gold, which it is not profitable to work; for example, that of the Alpine rivers contains 5 parts of gold in 10,000,000 parts of sand. The richest gold deposits are those of Siberia, especially in the southern parts of the Government of Yeniseisk, the South Urals, Mexico, California, South Africa, and Australia, and then the comparatively poorer alluvial deposits of many countries (Hungary, the Alps, and Spain in Europe). The extraction of the gold from alluvial deposits is based on the principle of levigation; the earth is washed, while constantly agitated, by a stream of water, which carries away the lighter portion of the earth, and leaves the coarser particles of the rock and heavier particles of the gold, together with certain substances which accompany it, in the washing apparatus. The extraction of this _washed_ gold only necessitates mechanical appliances,[31] and it is not therefore surprising that gold was known to savages and in the most remote period of history. It sometimes occurs in crystals belonging to the regular system, but in the majority of cases in nuggets or grains of greater or less magnitude. It always contains silver (from very small quantities up to 30 p.c., when it is called 'electrum') and certain other metals, among which lead and rhodium are sometimes found.

[29 bis] Sonstadt (1872) showed that sea water, besides silver, always
contains gold. Munster (1892) showed that the water of the
Norwegian fiords contains about 5 milligrams of gold per ton (or 5
milliardths)--_i.e._ a quantity deserving practical attention, and
I think it may be already said that, considering the immeasurable
amount of sea water, in time means will be discovered for
profitably extracting gold from sea water by bringing it into
contact with substances capable of depositing gold upon their
surface. The first efforts might be made upon the extraction of
salt from sea water, and as the total amount of sea water may be
taken as about 2,000,000,000,000,000,000 tons, it follows that it
contains about 10,000 million tons of gold. The yearly production
of gold is about 200 tons for the whole world, of which about one
quarter is extracted in Russia. It is supposed that gold is
dissolved in sea water owing to the presence of iodides, which,
under the action of animal organisms, yield free iodine. It is
thought (as Professor Konovaloff mentions in his work upon 'The
Industries of the United States,' 1894) that iodine facilitates
the solution of the gold, and the organic matter its
precipitation. These facts and considerations to a certain extent
explain the distribution of gold in veins or rock fissures,
chiefly filled with quartz, because there is sufficient reason for
supposing that these rocks once formed the ocean bottom. R.
Dentrie, and subsequently Wilkinson, showed that organic
matter--for instance, cork--and pyrites are able to precipitate
gold from its solutions in that metallic form and state in which
it occurs in quartz veins, where (especially in the deeper parts
of vein deposits) gold is frequently found on the surface of
pyrites, chiefly arsenical pyrites. Kazantseff (in Ekaterinburg,
1891) even supposes, from the distribution of the gold in these
pyrites, that it occurred in solution as a compound of sulphide of
gold and sulphide of arsenic when it penetrated into the veins. It
is from such considerations that the origin of vein and pyritic
gold is, at the present time, attributed to the reaction of
solutions of this metal, the remains of which are seen in the gold
still present in sea water.

[30] However, in recent times, especially since about 1870, when
chlorine (either as a solution of the gas or as bleaching powder)
and bromine began to be applied to the extraction of
finely-divided gold from poor ores (previously roasted in order to
drive off arsenic and sulphur, and oxidise the iron), the
extraction of gold from quartz and pyrites, by the wet method,
increases from year to year, and begins to equal the amount
extracted from alluvial deposits. Since the nineties the _cyanide
process_ (Chapter XIII., Note 13 bis) has taken an important place
among the wet methods for extracting gold from its ores. It
consists in pouring a dilute solution of cyanide of potassium
(about 500 parts of water and 1 to 4 parts of cyanide of potassium
per 1,000 parts of ore, the amount of cyanide depending upon the
richness of the ore) and a mixture of it with NaCN, (_see_ Chapter
XIII., Note 12) over the crushed ore (which need not be roasted,
whilst roasting is indispensable in the chlorination process, as
otherwise the chlorine is used up in oxidising the sulphur,
arsenic, &c.) The gold is dissolved very rapidly even from
pyrites, where it generally occurs on the surface in such fine and
adherent particles that it either cannot be mechanically washed
away, or, more frequently is carried away by the stream of water,
and cannot be caught by mechanical means or by the mercury used
for catching the gold in the sluices. Chlorination had already
given the possibility of extracting the finest particles of gold;
but the cyanide process enables such pyrites to be treated as
could be scarcely worked by other means. The treatment of the
crushed ore by the KCN is carried on in simple wooden vats (coated
with paraffin or tar) with the greatest possible rapidity (in
order that the KCN solution should not have time to change) by a
method of systematic lixiviation, and is completed in 10 to 12
hours. The resultant solution of gold, containing AuK(CN)_{2}, is
decomposed either with freshly-made zinc filings (but when the
gold settles on the Zn, the cyanide solution reacts upon the Zn
with the evolution of H_{2} and formation of ZnH_{2}O_{2}) or by
sodium amalgam prepared at the moment of reaction by the action of
an electric current upon a solution of NaHO poured into a vessel
partially immersed in mercury (the NaCN is renewed continually by
this means). The silver in the ore passes into solution, together
with the gold, as in amalgamation.

[31] But the particles of gold are sometimes so small that a large
amount is lost during the washing. It is then profitable to have
recourse to the extraction by chlorine and KCN (Note 30).

In speaking of the extraction of gold the following remarks may
not be out of place:

In California advantage is taken of water supplied from high
altitudes in order to have a powerful head of water, with which
the rocks are directly washed away, thus avoiding the greater
portion of the mechanical labour required for the exploitation of
these deposits.

The last residues of gold are sometimes extracted from sand by
washing them with mercury, which dissolves the gold. The sand
mixed with water is caused to come into contact with mercury
during the washing. The mercury is then distilled.

Many sulphurous ores, even pyrites, contain a small amount of
gold. Compounds of gold with bismuth, BiAu_{2}, tellurium,
AuTe_{2} (calverite), &c., have been found, although rarely.

Among the minerals which accompany gold, and from which the
presence of gold may be expected, we may mention white quartz,
titanic and magnetic iron ores, and also the following, which are
of rarer occurrence: zircon, topaz, garnet, and such like. The
concentrated gold washings first undergo a mechanical treatment,
and the impure gold obtained is treated for pure gold by various
methods. If the gold contain a considerable amount of foreign
metals, especially lead and copper, it is sometimes cupelled, like
silver, so that the oxidisable metals may be absorbed by the cupel
in the form of oxides, but in every case the gold is obtained
together with silver, because the latter metal also is not
oxidised. Sometimes the gold is extracted by means of mercury,
that is, by amalgamation (and the mercury subsequently driven off
by distillation), or by smelting it with lead (which is afterwards
removed by oxidation) and processes like those employed for the
extraction of silver, because gold, like silver, does not oxidise,
is dissolved by lead and mercury, and is non-volatile. If copper
or any other metal contain gold and it be employed as an anode,
pure copper will be deposited upon the cathode, while all the gold
will remain at the anode as a slime. This method often amply
repays the whole cost of the process, since it gives, besides the
gold, a pure electrolytic copper.

_The separation of the silver_ from gold is generally carried on with great precision, as the presence of the silver in the gold does not increase its value for exchange, and it can be substituted by other less valuable metals, so that the extraction of the silver, as a precious metal, from its alloy with gold, is a profitable operation. This separation is conducted by different methods. Sometimes the argentiferous gold is melted in crucibles, together with a mixture of common salt and powdered bricks. The greater portion of the silver is thus converted into the chloride, which fuses and is absorbed by the slags, from which it may be extracted by the usual methods. The silver is also extracted from gold by treating it with boiling sulphuric acid, which does not act on the gold but dissolves the silver. But if the alloy does not contain a large proportion of silver it cannot be extracted by this method or at all events the separation will be imperfect, and therefore a fresh amount of silver is added (by fusion) to the gold, in such quantity that the alloy contains twice as much silver as gold. The silver which is added is preferably such as contains gold, which is very frequently the case. The alloy thus formed is poured in a thin stream into water, by which means it is obtained in a granulated form; it is then boiled with strong sulphuric acid, three parts of acid being used to one part of alloy. The sulphuric acid extracts all the silver without acting on the gold. It is best, however, to pour off the first portion of the acid, which has dissolved the silver, and then treat the residue of still imperfectly pure gold with a fresh quantity of sulphuric acid. The gold is thus obtained in the form of powder, which is washed with water until it is quite free from silver. The silver is precipitated from the solution by means of copper, so that cupric sulphate and metallic silver are obtained. This process is carried out in many countries, as in Russia, at the Government mints.

Gold is generally used alloyed with copper; since pure gold, like pure silver, is very soft, and therefore soon worn away. In assaying or determining the amount of pure gold in such an alloy it is usual to add silver to the gold in order to make up an alloy containing three parts of silver to one of gold (this is known as quartation because the alloy contains 1/4 of gold), and the resultant alloy is treated with nitric acid. If the silver be not in excess over the gold, it is not all dissolved by the nitric acid, and this is the reason for the quartation. The amount of pure gold (assay) is determined by weighing the gold which remains after this treatment. English gold (= 22 carats) coinage is composed of an alloy containing 91·66 p.c. of gold, but for many articles gold is frequently used containing a larger amount of foreign metals.

_Pure gold_ may be obtained from gold alloys by dissolving in aqua regia, and then adding ferrous sulphate to the solution or heating it with a solution of oxalic acid. These deoxidising agents reduce the gold, but not the other metals. The chlorine combined with the gold then acts like free chlorine. The gold, thus reduced, is precipitated as an exceedingly fine brown powder.[31 bis] It is then washed with water, and fused with nitre or borax. Pure gold reflects a yellow light, and in the form of very thin sheets (gold leaf), into which it can be hammered and rolled,[31 tri] it transmits a bluish-green light. The specific gravity of gold is about 19·5, the sp. gr. of gold coin is about 17·1. It fuses at 1090°--at a higher temperature than silver--and can be drawn into exceedingly fine wires or hammered into thin sheets. With its softness and ductility, gold is distinguished for its tenacity, and a gold wire two millimetres thick breaks only under a load of 68 kilograms. Gold vaporises even at a furnace heat, and imparts a greenish colour to a flame passing over it in a furnace. Gold alloys with copper almost without changing its volume.[32] In its chemical aspect, gold presents, as is already seen from its general characteristics given above, an example of the so-called noble metals--_i.e._ it is incapable of being oxidised at any temperature, and its oxide is decomposed when calcined. Only chlorine and bromine combine directly with it at the ordinary temperature, but many other metals and non-metals combine with it at a red heat--for example, sulphur, phosphorus, and arsenic. Mercury dissolves it with great ease. It dissolves in potassium cyanide in the presence of air; a mixture of sulphuric acid with nitric acid dissolves it with the aid of heat, although in small quantity. It is also soluble in aqua regia and in selenic acid. Sulphuric, hydrochloric, nitric, and hydrofluoric acids and the caustic alkalis do not act on gold, but a mixture of hydrochloric acid with such oxidising agents as evolve chlorine naturally dissolves it like aqua regia.[32 bis]

[31 bis] Schottländer (1893) obtained gold in a soluble colloid form
(the solution is violet) by the action of a mixture of solutions
of cerium acetate and NaHO upon a solution of AuCl_{3}. The gold
separates out from such a solution in exactly the same manner as
Ag does from the solution of colloid silver mentioned above. There
always remains a certain amount of a higher oxide of cerium,
CeO_{2}, in the solution--_i.e._ the gold is reduced by converting
the cerium into a higher grade of oxidation. Besides which Krüss
and Hofmann showed that sulphide of gold precipitated by the
action of H_{2}S upon a solution of AuKCy_{2} mixed with HCl
easily passes into a colloid solution after being properly washed
(like As_{2}S_{3}, CuS, &c., Chapter I., Note 57).

[31 tri] Gold-leaf is used for gilding wood (leather, cardboard, and
suchlike, upon which it is glued by means of varnish, &c.), and is
about 0·003 millimetre thick. It is obtained from thin sheets
(weighing at first about 1/4 grm. to a square inch), rolled
between gold rollers, by gradually hammering them (in packets of a
number at once) between sheets of moist (but not wet) parchment,
and then, after cutting them into four pieces, between a specially
prepared membrane, which, when at the right degree of moisture,
does not tear or stick together under the blows of the hammer.

[32] The formation of the alloys Cu + Zn, Cu + Sn, Cu + Bi, Cu + Sb,
Pb + Sb, Ag + Pb, Ag + Sn, Au + Zn, Au + Sn, &c., is accompanied
by a contraction (and evolution of heat). The formation of the
alloys Fe + Sb, Fe + Pb, Cu + Pb, Pb + Sn, Pb + Sb, Zn + Sb, Ag +
Cu, Au + Cu, Au + Pb, takes place with a certain increase in
volume. With regard to the alloys of gold, it may be mentioned
that gold is only slightly dissolved by mercury (about 0·06 p.c.,
Dudley, 1890); the remaining portion forms a granular alloy, whose
composition has not been definitely determined. Aluminium (and
silicon) also have the capacity of forming alloys with gold. The
presence of a small amount of aluminium lowers the melting point
of gold considerably (Roberts-Austen, 1892); thus the addition of
4 p.c. of aluminium lowers it by 14°·28, the addition of 10 p.c.
Al by 41°·7. The latter alloy is white. The alloy AuAl_{2} has a
characteristic purple colour, and its melting point is 32°·5 above
that of gold, which shows it to be a definite compound of the two
metals. The melting points of alloys richer in Al gradually fall
to 660°--that is, below that of aluminium (665°).

Heycock and Neville (1892), in studying the triple alloys of Au,
Cd, and Sn, observed a tendency in the gold to give compounds with
Cd, and by sealing a mixture of Au and Cd in a tube, from which
the air had been exhausted, and heating it, they obtained a grey
crystalline brittle definite alloy AuCd.

[32 bis] Calderon (1892), at the request of some jewellers,
investigated the cause of a peculiar alteration sometimes found on
the surface of dead-gold articles, there appearing brownish and
blackish spots, which widen and alter their form in course of
time. He came to the conclusion that these spots are due to the
appearance and development of peculiar micro-organisms
(Aspergillus niger and Micrococcus cimbareus) on the gold, spores
of which were found in abundance on the cotton-wool in which the
gold articles had been kept.

As regards the compounds of gold, they belong, as was said above, to the types AuX_{3} and AuX. _Auric chloride_ or _gold trichloride_, AuCl_{3}, which is formed when gold is dissolved in aqua regia, belongs to the former and higher of these types. The solution of this substance in water has a yellow colour, and it may be obtained pure by evaporating the solution in aqua regia to dryness, but not to the point of decomposition. If the evaporation proceed to the point of crystallisation, a compound of gold chloride and hydrochloric acid, AuHCl_{4}, is obtained, like the allied compounds of platinum; but it easily parts with the acid and leaves auric chloride, which fuses into a red-brown liquid, and then solidifies to a crystalline mass. If dry chlorine be passed over gold in powder it forms a mixture of aurous and auric chlorides, but the aurous chloride is also decomposed by water into gold and auric chloride. Auric chloride crystallises from its solutions as AuCl_{3},2H_{2}O, which easily loses water, and the dry chloride loses two-thirds of its chlorine at 185°, forming aurous chloride, whilst above 300° the latter chloride also loses its chlorine and leaves metallic gold. Auric chloride is the usual form in which gold occurs in solutions, and in which its salts are used in the arts and for chemical purposes. It is soluble in water, alcohol, and ether. Light has a reducing action on these solutions, and after a time metallic gold is deposited upon the sides of vessels containing the solution. Hydrogen when nascent, and even in a gaseous form, reduces gold from this solution to a metallic state. The reduction is more conveniently and usually effected by ferrous sulphate, and in general by the action of ferrous salts.[33]

[33] Stannous chloride as a reducing agent also acts on auric chloride,
and gives a red precipitate known as _purple of Cassius_. This
substance, which probably contains a mixture or compound of aurous
oxide and tin oxide, is used as a red pigment for china and glass.
Oxalic acid, on heating, reduces metallic gold from its salts, and
this property may be taken advantage of for separating it from its
solutions. The oxidation which then takes place in the presence of
water may be expressed by the following equation: 2AuCl_{3} +
3C_{2}H_{2}O_{4} = 2Au + 6HCl + 6CO_{2}. Nearly all organic
substances have a reducing action on gold, and solutions of gold
leave a violet stain on the skin.

Auric chloride, like platinic chloride, is distinguished for its
clearly-developed property of forming double salts. These double
salts, as a rule, belong to the type AuMCl_{4}. The compound of
auric chloride with hydrochloric acid mentioned above evidently
belongs to the same type. The compounds 2KAuCl_{4},5H_{2}O,
NaAuCl_{4},2H_{2}O, AuNH_{4}Cl_{4},H_{2}O,
Mg(AuCl_{4})_{2},2H_{2}O, and the like are easily crystallised in
well-formed crystals. Wells, Wheeler, and Penfield (1892) obtained
RbAuCl_{4} (reddish yellow) and CsAuCl_{4} (golden yellow), and
corresponding bromides (dark coloured). AuBr_{3} is extremely like
the chloride. Auric cyanide is obtained easily in the form of a
double salt of potassium, KAu(CN)_{4} by mixing saturated and hot
solutions of potassium cyanide with auric chloride and then
cooling.

If a solution of potassium hydroxide be added to a solution of auric chloride, a precipitate is first formed, which re-dissolves in an excess of the alkali. On being evaporated under the receiver of an air-pump, this solution yields yellow crystals, which present the same composition as the double salts AuMCl_{4}, with the substitution of the chlorine by oxygen--that is to say, _potassium aurate_, AuKO_{2}, is formed in crystals containing 3H_{2}O. The solution has a distinctly alkaline reaction. _Auric oxide_, Au_{2}O_{3}, separates when this alkaline solution is boiled with an excess of sulphuric acid. But it then still retains some alkali; however, it may be obtained in a pure state as a brown powder by dissolving in nitric acid and diluting with water. The brown powder decomposes below 250° into gold and oxygen. It is insoluble in water and in many acids, but it dissolves in alkalis, which shows the acid character of this oxide. An hydroxide, Au(OH)_{3} may be obtained as a brown powder by adding magnesium oxide to a solution of auric chloride and treating the resultant precipitate of magnesium aurate with nitric acid. This hydroxide loses water at 100°, and gives auric oxide.[34]

[34] If ammonia be added to a solution of auric chloride, it forms a
yellow precipitate of the so-called fulminating gold, which
contains gold, chlorine, hydrogen, nitrogen, and oxygen, but its
formula is not known with certainty. It is probably a sort of
ammonio-metallic compound, Au_{2}O_{3},4NH_{3}, or amide (like the
mercury compound). This precipitate explodes at 140°, but when
left in the presence of solutions containing ammonia it loses all
its chlorine and becomes non-explosive. In this form the
composition Au_{2}O_{3},2NH_{3},H_{2}O is ascribed to it, but this
is uncertain. Auric sulphide, Au_{2}S_{3}, is obtained by the
action of hydrogen sulphide on a solution of auric chloride, and
also directly by fusing sulphur with gold. It has an acid
character, and therefore dissolves in sodium and ammonium
sulphides.

The starting-point of the compounds of the type AuX[35] is _gold monochloride_ or _aurous chloride_, AuCl, which is formed, as mentioned above, by heating auric chloride at 185°. Aurous chloride forms a yellowish-white powder; this, when heated with water, is decomposed into metallic gold and auric chloride, which passes into solution: 3AuCl = AuCl_{3} + 2Au. This decomposition is accelerated by the action of light. Hence it is obvious that the compounds corresponding with aurous oxide are comparatively unstable. But this only refers to the simple compounds AuX; some of the complex compounds, on the contrary, form the most stable compounds of gold. Such, for example, is the cyanide of gold and potassium, AuK(CN)_{2}. It is formed, for instance, when finely-divided gold dissolves in the presence of air in a solution of potassium cyanide: 4KCN + 2Au + H_{2}O + O = 2KAu(CN)_{2} + 2KHO (this reaction also proceeds with solid pieces of gold, although very slowly). The same compound is formed in solution when many compounds of gold are mixed with potassium cyanide, because if a higher compound of gold be taken, it is reduced by the potassium cyanide into aurous oxide, which dissolves in potassium cyanide and forms KAu(CN)_{2}. This substance is soluble in water, and gives a colourless solution, which can be kept for a long time, and is employed in electro-gilding--that is, for coating other metallic objects with a layer of gold, which is deposited if the object be connected with the negative pole of a battery and the positive pole consist of a gold plate. When an electric current is passed between them, the gold from the latter will dissolve, whilst a coating of gold from the solution will be deposited on the object.

[35] Many double salts of suboxide of gold belong to the type AuX--for
instance, the cyanide corresponding to the type AuKX_{2}, like
PtK_{2}X_{4}, with which we became acquainted in the last chapter.
We will enumerate several of the representatives of this class of
compounds. If auric chloride, AuCl_{3}, be mixed with a solution
of sodium thiosulphate, the gold passes into a colourless
solution, which deposits colourless crystals, containing a double
thiosulphate of gold and sodium, which are easily soluble in water
but are precipitated by alcohol. The composition of this salt is
Na_{3}Au(S_{2}O_{3})_{2},2H_{2}O. If the sodium thiosulphate be
represented as NaS_{2}O_{3}Na, the double salt in question will be
AuNa(S_{2}O_{3}Na)_{2},2H_{2}O, according to the type AuNaX_{2}.
The solution of this colourless and easily crystallisable salt has
a sweet taste, and the gold is not separated from it either by
ferrous sulphate or oxalic acid. This salt, which is known as
_Fordos and Gelis's salt_, is used in medicine and photography. In
general, aurous oxide exhibits a distinct inclination to the
formation of similar double salts, as we saw also with
PtX_{2}--for example, it forms similar salts with sulphurous acid.
Thus if a solution of sodium sulphite be gradually added to a
solution of oxide of gold in sodium hydroxide, the precipitate at
first formed re-dissolves to a colourless solution, which contains
the double salt Na_{3}Au(SO_{3})_{2} = AuNa(SO_{3}Na)_{2}. The
solution of this salt, when mixed with barium chloride, first
forms a precipitate of barium sulphite, and then a red barium
double salt which corresponds with the above sodium salt.

The oxygen compound of the type AuX, _aurous oxide_, Au_{2}O, is
obtained as a greenish violet powder on mixing aurous chloride
with potassium chloride in the cold. With hydrochloric acid this
oxide gives gold and auric chloride, and when heated it easily
splits up into oxygen and metallic gold.