Chapter XXIV: Copper, Silver, and Gold

That degree of analogy and difference which exists between iron, cobalt, and nickel repeats itself in the corresponding triad ruthenium, rhodium, and palladium, and also in the heavy platinum metals, osmium, iridium, and platinum. These nine metals form Group VIII. of the elements in the periodic system, being the intermediate group between the even elements of the large periods and the uneven, among which we know zinc, cadmium, and mercury in Group II. Copper, silver, and gold complete[1] this transition, because their properties place them in proximity to nickel, palladium, and platinum on the one hand, and to zinc, cadmium, and mercury on the other. Just as Zn, Cd, and Hg; Fe, Ru, and Os; Co, Rh, and Ir; Ni, Pd, and Pt, resemble each other in many respects, so also do Cu, Ag, and Au. Thus, for example, the atomic weight of copper Cu = 63, and in all its properties it stands between Ni = 59 and Zn = 65. But as the transition from Group VIII. to Group II., where zinc is situated, cannot be otherwise than through Group I., so in copper there are certain properties of the elements of Group I. Thus it gives a suboxide, Cu_{2}O, and salts, CuX, like the elements of Group I., although at the same time it forms an oxide, CuO, and salts CuX_{2}, like nickel and zinc. In the state of the oxide, CuO, and the salts, CuX_{2}, copper is analogous to zinc, judging from the insolubility of the carbonates, phosphates, and similar salts, and by the isomorphism, and other characters.[2] In the cuprous salts there is undoubtedly a great resemblance to the silver salts--thus, for example, silver chloride, AgCl, is characterised by its insolubility and capacity of combining with ammonia, and in this respect cuprous chloride closely resembles it, for it is also insoluble in water, and combines with ammonia and dissolves in it, &c. Its composition is also RCl, the same as AgCl, NaCl, KCl, &c., and silver in many compounds resembles, and is even isomorphous with, sodium, so that this again justifies their being brought together. Silver chloride, cuprous chloride, and sodium chloride crystallise in the regular system. Besides which, the specific heats of copper and silver require that they should have the atomic weights ascribed to them. To the oxides Cu_{2}O and Ag_{2}O there are corresponding sulphides Ag_{2}S and Cu_{2}S. They both occur in nature in crystals of the rhombic system, and, what is most important, copper glance contains an isomorphous mixture of them both, and retains the form of copper glance with various proportions of copper and silver, and therefore has the composition R_{2}S where R = Cu, Ag.

[1] The perfectly unique position held by copper, silver, and gold in
the periodic system of the elements, and the degree of affinity
which is found between them, is all the more remarkable, as nature
and practice have long isolated these metals from all others by
having employed them--for example, for coinage--and determined
their relative importance and value in conformity with the order
(silver between copper and gold) of their atomic weights, &c.

[2] Cupric sulphate contains 5 molecules of water, CuSO_{4},5H_{2}O,
and the isomorphous mixtures with ZnSO_{4},7H_{2}O contain either 5
or 7 equivalents, according to whether copper or zinc predominates
(Vol. II. p. 6). If there be a large proportion of copper, and if
the mixture contain 5H_{2}O, the form of the isomorphous mixture
(triclinic) will be isomorphous with cupric sulphate,
CuSO_{4},5H_{2}O, but if a large amount of zinc (or magnesium,
iron, nickel, or cobalt) be present the form (rhombic or
monoclinic) will be nearly the same as that of zinc sulphate,
ZnSO_{4},7H_{2}O. Supersaturated solutions of each of these salts
crystallise in that form and with that amount of water which is
contained in a crystal of one or other of the salts brought in
contact with the solution (Chapter XIV., Note 27).

Notwithstanding the resemblance in the atomic composition of the cuprous compounds, CuX, and silver compounds, AgX, with the compounds of the alkali metals KX, NaX, there is a considerable degree of difference between these two series of elements. This difference is clearly seen in the fact that the alkali metals belong to those elements which combine with extreme facility with oxygen, decompose water, and form the most alkaline bases; whilst silver and copper are oxidised with difficulty, form less energetic oxides, and do not decompose water, even at a rather high temperature. Moreover, they only displace hydrogen from very few acids. The difference between them is also seen in the dissimilarity of the properties of many of the corresponding compounds. Thus cuprous oxide, Cu_{2}O, and silver oxide, Ag_{2}O, are insoluble in water: the cuprous and silver carbonates, chlorides, and sulphates are also sparingly soluble in water. The oxides of silver and copper are also easily reduced to metal. This difference in properties is in intimate relation with that difference in the density of the metals which exists in this case. The alkali metals belong to the lightest, and copper and silver to the heaviest, and therefore the distance between the molecules in these metals is very dissimilar--it is greater for the former than the latter (tables in Chapter XV.). From the point of view of the periodic law, this difference between copper and silver and such elements of Group I. as potassium and rubidium, is clearly seen from the fact that copper and silver stand in the middle of those large periods (for example, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br) which start with the true metals of the alkalis--that is to say, the analogy and difference between potassium and copper are of the same nature as that between chromium and selenium, or vanadium and arsenic.

_Copper_ is one of the few metals which have long been known in a metallic form. The Greeks and Romans imported copper chiefly from the island of Cyprus--whence its Latin name, _cuprum_. It was known to the ancients before iron, and was used, especially when alloyed with other metals, for arms and domestic utensils. That copper was known to the ancients will be understood from the fact that it occurs, although rarely, in a _native state_, and is easily extracted from its other natural compounds. Among the latter are the oxygen compounds of copper. When ignited with charcoal, they easily give up their oxygen to it, and yield metallic copper; hydrogen also easily takes up the oxygen from copper oxide when heated. Copper occurs in a native state, sometimes in association with other ores, in many parts of the Urals and in Sweden, and in considerable masses in America, especially in the neighbourhood of the great American lakes; and also in Chili, Japan, and China. The oxygen compounds of copper are also of somewhat common occurrence in certain localities; in this respect certain deposits of the Urals are especially famous. The geological period of the Urals (Permian) is characterised by a considerable distribution of copper ores. Copper is met with in the form of _cuprous oxide_, or _suboxide of copper_, Cu_{2}O, and is then known as _red copper ore_, because it forms red masses which not unfrequently are crystallised in the regular system. It is found much more rarely in the state of _cupric oxide_, CuO, and is then called _black copper ore_. The most common of the oxygenised compounds of copper are the _basic carbonates_ corresponding with the oxides. That these compounds are undoubtedly of aqueous origin is apparent, not only from the fact that specimens are frequently found of a gradual transition from the metallic, sulphuretted, and oxidised copper into its various carbonates, but also from the presence of water in their composition, and from the laminar, reniform structure which many of them present. In this respect _malachite_ is particularly well known; it is used as a green paint and also for ornaments, owing to the diversity of the shades of colour presented by the different layers of deposited malachite. The composition of malachite corresponds with the basic carbonate containing one molecule of cupric carbonate to one of hydroxide: CuCO_{3},CuH_{2}O_{2}. In this form the copper frequently occurs in admixture with various sedimentary rocks, forming large strata, which confirms the aqueous origin of these compounds. There are many such localities in the Perm and other Governments bounding the Urals. _Blue carbonate of copper_, or _azurite_, is also often met with in the same localities; it contains the same ingredients as malachite, but in a different proportion, its composition being CuH_{2}O_{2},2CuCO_{3}. Both these substances may be obtained artificially by the action of the alkali carbonates on solutions of cupric salts at various temperatures. These native carbonates are often used for the extraction of copper, all the more as they very readily give metallic copper, evolving water and carbonic anhydride when ignited, and leaving the easily-reducible cupric oxide. Copper is, however, still more often met with in the form of the sulphides. The sulphides of copper generally occur in chemical combination with the sulphides of iron.[3] These copper-sulphur compounds (copper pyrites CuFeS_{2}, variegated copper ore Cu_{3}FeS_{3}, &c.) generally occur in veins in a rock gangue.

[3] Iron pyrites, FeS_{2}, very often contain a small quantity of
copper sulphide (_see_ Chapter XXII., Note 2 bis), and on burning
the iron pyrites for sulphurous anhydride the copper oxide remains
in the residue, from which the copper is often extracted with
profit. For this purpose the whole of the sulphur is not burnt off
from the iron pyrites, but a portion is left behind in the ore,
which is then slowly ignited (roasted) with access of air. Cupric
sulphate is then formed, and is extracted by water; or what is
better and more frequently done, the residue from the roasting of
the pyrites is roasted with common salt, and the solution of cupric
chloride obtained by lixiviating is precipitated with iron. A far
greater amount of copper is obtained from other sulphuretted ores.
Among these _copper glance_, Cu_{2}S, is more rarely met with. It
has a metallic lustre, is grey, generally crystalline, and is
obtained in admixture with organic matter; so that there is no
doubt that its origin is due to the reducing action of the latter
on solutions of cupric sulphate. _Variegated copper ore_, which
crystallises in octahedra, not infrequently forms an admixture in
copper glance; it has a metallic lustre, and is reddish-brown; it
has a superficial play of colours, due to oxidation proceeding on
its surface. Its composition is Cu_{3}FeS_{3}. But the most common
and widely-distributed copper ore is _copper pyrites_, which
crystallises in regular octahedra; it has a metallic lustre, a sp.
gr. of 4·0, and yellow colour. Its composition is CuFeS_{2}. It
must be remarked that the sulphurous ores of copper are oxidised in
the presence of water containing oxygen in solution, and form
cupric sulphate, blue vitriol, which is easily soluble in water. If
this water contains calcium carbonate, gypsum and cupric carbonate
are formed by double decomposition: CuSO_{4} + CaCO_{3} = CuCO_{3}
+ CaSO_{4}. Hence copper sulphide in the form of different ores
must be considered as the primary product, and the many other
copper ores as secondary products, formed by water. This is
confirmed by the fact that at the present time the water extracted
from many copper mines contains cupric sulphate in solution. From
this liquid it is easy to extract cupric oxide by the action of
organic matter and various impurities of water. Hence metallic
copper is sometimes found in natural products of the modification
of copper sulphide and is probably deposited by the action of
organic matter present in the water.

_The extraction of copper from its oxide ores_ does not present any difficulty, because the copper, when ignited with charcoal and melted, is reduced from the impurities which accompany it. This mode of smelting copper ores is carried on in cupola or cylindrical furnaces, fluxes forming a slag being added to the mixture of ore and charcoal. The smelted copper still contains sulphur, iron, and other metallic impurities, from which it is freed by fusion in reverberatory furnaces, with access of air to the surface of the molten metal, as the iron and sulphur are more easily oxidised than the copper. The iron then separates as oxides, which collect in the slag.[4]

[4] Copper ores rich in oxygen are very rare; the sulphur ores are of
more common occurrence, but the extraction of the copper from them
is much more difficult. The problem here not only consists in the
removal of the sulphur, but also in the removal of the iron
combined with the sulphur and copper. This is attained by a whole
series of operations, after which there still sometimes remains the
extraction of the metallic silver which generally accompanies the
copper, although in but small quantity. These processes commence
with the roasting--_i.e._ calcination--of the ore with access of
air, by which means the sulphur is converted into sulphurous
anhydride. It should here be remarked that iron sulphide is more
easily oxidised than copper sulphide, and therefore the greater
part of the iron in the residue from roasting is no longer in the
form of sulphide but of oxide of iron. The roasted ore is mixed
with charcoal, and siliceous fluxes, and smelted in a cupola
furnace. The iron then passes into the slag, because its oxide
gives an easily-fusible mass with the silica, whilst the copper, in
the form of sulphide, fuses and collects under the slag. The
greater part of the iron is removed from the mass by this smelting.
The resultant _coarse metal_ is again roasted in order to remove
the greater part of the sulphur from the copper sulphide, and to
convert the metal into oxide, after which the mass is again
smelted. These processes are repeated several times, according to
the richness of the ore. During these smeltings a portion of the
copper is already obtained in a metallic form, because copper
sulphide gives metallic copper with the oxide (CuS + 2CuO = 3Cu +
SO_{2}). We will not here describe the furnaces used or the details
of this process, but the above remarks include the explanation of
those chemical processes which are accomplished in the various
technical operations which are made use of in the process (for
details _see_ works on metallurgy).

Besides the smelting of copper there also exist methods for its
extraction from solutions in the wet way, as it is called. Recourse
is generally had to these methods for poor copper ores. The copper
is brought into solution, from which it is separated by means of
metallic iron or by other methods (by the action of an electric
current). The sulphides are roasted in such a manner that the
greater part of the copper is oxidised into cupric sulphate, whilst
at the same time the corresponding iron salts are as far as
possible decomposed. This process is based on the fact that the
copper sulphides absorb oxygen when they are calcined in the
presence of air, forming cupric sulphate. The roasted ore is
treated with water, to which acid is sometimes added, and after
lixiviation the resultant solution containing copper is treated
either with metallic iron or with milk of lime, which precipitates
cupric hydroxide from the solution. Copper oxide ores poor in metal
may be treated with dilute acids in order to obtain the copper
oxides in solution, from which the copper is then easily
precipitated either by iron or as hydroxide by lime. According to
Hunt and Douglas's method, the copper in the ore is converted by
calcination into the cupric oxide, which is brought into solution
by the action of a mixture of solutions of ferrous sulphate and
sodium chloride; the oxide converts the ferrous chloride into
ferric oxide, forming copper chlorides, according to the equation
3CuO + 2FeCl_{2} = CuCl_{2} + 2CuCl + Fe_{2}O_{3}. The cupric
chloride is soluble in water, whilst the cuprous chloride is
dissolved in the solution of sodium chloride, and therefore all the
copper passes into solution, from which it is precipitated by iron.

The same American metallurgists give the following wet method for
extracting the Ag and Au occurring in many copper ores, especially
in sulphurous ores: (1) The Cu_{2}S is first converted into oxide
by roasting in a calciner; (2) the CuO is extracted by the dilute
sulphuric acid obtained in the fourth process, the Cu then passes
into solution, while the Ag, Au and oxides of iron remain behind in
the residue (from which the noble metals may be extracted); (3) a
portion of the copper in solution is converted into CuCl_{2} (and
CaSO_{4} precipitated) by means of the CaCl_{2} obtained in the
fifth process; (4) the mixture of solutions of CuSO_{4} and
CuCl_{2} is converted into the insoluble CuCl (salt of the
suboxide) by the action of the SO_{2} obtained by roasting the ore
(in the first operation), sulphuric acid is then formed in the
solution, according to the equation: CuSO_{4} + CuCl_{2} + SO_{2} +
2H_{2}O = 2H_{2}SO_{4} + 2CuCl; (5) the precipitated CuCl is
treated with lime and water, and gives CuCl_{2} in solution and CuO
in the residue; and lastly (6) the Cu_{2}O is reduced to metallic
Cu by carbon in a furnace. According to Crooke's method the impure
copper regulus obtained by roasting and smelting the ore is broken
up and immersed repeatedly in molten lead, which extracts the Ag
and Au occurring in the regulus. The regulus is then heated in a
reverberatory furnace to run off the lead, and is then smelted for
Cu.

The copper brought into the market often contains small quantities
of various impurities. Among these there are generally present
iron, lead, silver, arsenic, and sometimes small quantities of
oxides of copper. As copper, when mixed with a small amount of
foreign substances, loses its tenacity to a certain degree, the
manufacture of very thin sheet copper requires the use of Chili
copper, which is distinguished for its great softness, and
therefore when it is desired to have pure copper, it is best to
take thin sheet copper, like that which is used in the manufacture
of cartridges. But the purest copper is electrolytic copper--that
is, that which is deposited from a solution by the action of an
electric current.

If the copper contains silver, as is often the case, it is used in
gold refineries for the precipitation of silver from its solutions
in sulphuric acid. Iron and zinc reduce copper salts, but copper
reduces mercury and silver salts. The precipitate contains not only
the silver which was previously in solution, but also all that
which was in the copper. The silver solutions in sulphuric acid are
obtained in the separation of silver from gold by treating their
alloys with sulphuric acid, which only dissolves the silver.

Copper is characterised by its red colour, which distinguishes it from all other metals. Pure copper is soft, and may be beaten out by a hammer at the ordinary temperature, and when hot may be rolled into very thin sheets. Extremely thin leaves of copper transmit a green light. The tenacity of copper is also considerable, and next to iron it is one of the most durable metals in this respect. Copper wire of 1 sq. millimetre in section only breaks under a weight of 45 kilograms. The specific gravity of copper is 8·8, unless it contains cavities due to the fact that molten copper absorbs oxygen from the air, which is disengaged on cooling, and therefore gives a porous mass whose density is much less. Rolled copper, and also that which is deposited by the electric current, has a comparatively high density. Copper melts at a bright red heat, about 1050°, although below the temperature at which many kinds of cast iron melt. At a high temperature it is converted into vapour, which communicates a green colour to the flame. Both native copper and that cooled from a molten state crystallise in regular octahedra. Copper is not oxidised in dry air at the ordinary temperature, but when calcined it becomes coated with a layer of oxide, and it does not burn even at the highest temperature. Copper, when calcined in air, forms either the red cuprous oxide or the black cupric oxide, according to the temperature and quantity of air supplied. In air at the ordinary temperature, copper--as everyone knows--becomes coated with a brown layer of oxides or a green coating of basic salts, due to the action of the damp air containing carbonic acid. If this action continue for a prolonged time, the copper is covered with a thick coating of basic carbonate, or the so-called verdigris (the _ærugo nobilis_ of ancient statues). This is due to the fact that copper, although scarcely capable of oxidising by itself,[5] _in the presence of water and acids_--even very feeble acids, like carbonic acid--_absorbs oxygen from the air and forms salts_, which is a very characteristic property of it (and of lead).[6] _Copper does not decompose water_, and therefore does not disengage hydrogen from it either at the ordinary or at high temperatures. Nor does copper liberate hydrogen from the oxygen acids; these act on it in two ways: they either give up a portion of their oxygen, forming lower grades of oxidation, or else only react in the presence of air. Thus, when nitric acid acts on copper it evolves nitric oxide, the copper being oxidised at the expense of the nitric acid. In the same way copper converts sulphuric acid into the lower grade of oxidation--into sulphurous anhydride, SO_{2}. In these cases the copper is oxidised to copper oxide, which combines with the excess of acid taken, and therefore forms a cupric salt, CuX_{2}. Dilute nitric acid does not act on copper at the ordinary temperature, but when heated it reacts with great ease; dilute sulphuric acid does not act on copper except in presence of air.

[5] Schützenberger showed that when the basic carbonate of copper is
decomposed by an electric current it gives, besides the ordinary
copper, an allotropic form which grows on the negative platinum
electrode, if its surface be smaller than that of the positive
copper electrode, in the form of brittle crystalline growths of sp.
gr. 8·1. It differs from ordinary copper by giving not nitric oxide
but nitrous oxide when treated with nitric acid, and in being very
easily oxidised in air, and coated with red shades of colour. It is
possible that this is copper hydride, or copper which has occluded
hydrogen. Spring (1892) observed that copper reduced from the oxide
by hydrogen at the lowest possible temperature was pulverulent,
while that reduced from CuCl_{2} at a somewhat high temperature
appeared in bright crystals. The same difference occurs with many
other metals, and is probably partly due to the volatility of the
metallic chlorides.

[6] This is taken advantage of in practice; for instance, by pouring
dilute acids over copper turnings on revolving tables in the
preparation of copper salts, such as verdigris, or the basic
acetate 2C_{4}H_{6}CuO_{4},CuH_{2}O_{2},5H_{2}O, which is so much
used as an oil paint (_i.e._ with boiled oil). The capacity of
copper for absorbing oxygen in the presence of acids is so great
that it is possible by this means (by taking, for example, thin
copper shavings moistened with sulphuric acid) to take up all the
oxygen from a given volume of air, and this is even employed for
the analysis of air.

The combination of copper with oxygen is not only aided by acids
but also by alkalis, although cupric oxide does not appear to have
an acid character. Alkalis do not act on copper except in the
presence of air, when they produce cupric oxide, which does not
appear to combine with such alkalis as caustic potash or soda. But
the _action of ammonia_ is particularly distinct (Chapter V., Note
2). In the action of a solution of ammonia not only is oxygen
absorbed by the copper, but it also acts on the ammonia, and a
definite quantity of ammonia is always acted on simultaneously with
the passage of the copper into solution. The ammonia is then
converted into nitrous acid, according to the reaction: NH_{3} +
O_{3} = NHO_{2} + H_{2}O, and the nitrous acid thus formed passes
into the state of ammonium nitrite, NH_{4}NO_{2}. In this manner
three equivalents of oxygen are expended on the oxidation of the
ammonia, and six equivalents of oxygen pass over to the copper,
forming six atoms of cupric oxide. The latter does not remain in
the state of oxide, but combines with the ammonia.

A strong solution of common salt does not act on copper, but a
dilute solution of the salt corrodes copper, converting it into
oxychloride--that is, in the presence of air. This action of salt
water is evident in those cases where the bottoms of ships are
coated with sheet copper. From what has been said above it will be
evident that copper vessels should not be employed in the
preparation of food, because this contains salts and acids which
act on copper in the presence of air, and give copper salts, which
are poisonous, and therefore the food prepared in untinned copper
vessels may be poisonous. Hence tinned vessels are employed for
this purpose--that is, copper vessels coated with a thin layer of
tin, on which acid and saline solutions do not act.

Both the oxides of copper, Cu_{2}O and CuO, are unacted on by air, and, as already mentioned, they both occur in nature.[6 bis] However, in the majority of cases copper is obtained in the form of cupric oxide and its salts--and the copper compounds used industrially generally belong to this type. This is due to the fact that the _cuprous compounds absorb oxygen_ from the air and pass into cupric compounds. The cupric compounds may serve as the source for the preparation of cuprous oxide, because many reducing agents are capable of deoxidising the oxide into the suboxide. Organic substances are most generally employed for this purpose, and especially saccharine substances, which are able, in the presence of alkalis, to undergo oxidation at the expense of the oxygen of the cupric oxide, and to give acids which combine with the alkali: 2CuO - O = Cu_{2}O. In this case the deoxidation of the copper may be carried further and metallic copper obtained, if only the reaction be aided by heat. Thus, for example, a fine powder of metallic copper may be obtained by heating an ammoniacal solution of cupric oxide with caustic potash and grape sugar. But if the reducing action of the saccharine substance proceed in the presence of a sufficient quantity of alkali in solution, and at not too high a temperature, cuprous oxide is obtained. To see this reaction clearly, it is not sufficient to take any cupric salt, because the alkali necessary for the reaction might precipitate cupric oxide--it is necessary to add previously some substance which will prevent this precipitation. Among such substances, tartaric acid, C_{4}H_{6}O_{6}, is one of the best. In the presence of a sufficient quantity of tartaric acid, any amount of alkali may be added to a solution of cupric salt without producing a precipitate, because a soluble double salt of cupric oxide and alkali is then formed. If glucose (for instance, honey or molasses) be added to such an alkaline tartaric solution, and the temperature be slightly raised, it first gives a yellow precipitate (this is cuprous hydroxide, CuHO), and then, on boiling, a red precipitate of (anhydrous) cuprous oxide. If such a mixture be left for a long time at the ordinary temperature, it deposits well-formed crystals of anhydrous cuprous oxide belonging to the regular system.[7]

[6 bis] Copper, besides the cuprous oxide, Cu_{2}O, and cupric oxide,
CuO, gives two known higher forms of oxidation, but they have
scarcely been investigated, and even their composition is not well
known. _Copper dioxide_ (CuO_{2}, or CuO_{2},H_{2}O, perhaps
CuOH_{2}O_{2}) is obtained by the action of hydrogen peroxide on
cupric hydroxide, when the green colour of the latter is changed to
yellow. It is very unstable, and is decomposed even by boiling
water, with the evolution of oxygen, whilst the action of acids
gives cupric salts, oxygen being also disengaged. A still higher
_copper peroxide_ is formed by heating a mixture of caustic potash,
nitre, and metallic copper to a red heat, and by dissolving cupric
hydroxide in solutions of the hypochlorites of the alkali metals. A
slight heating of the soluble salt formed is enough for it to be
decomposed into oxygen and copper dioxide, which is precipitated.
Judging from Frémy's researches, the composition of the
copper-potassic compound should be K_{2}CuO_{4}. Perhaps this is a
compound of the peroxides of potassium, K_{2}O_{2}, and of copper,
CuO_{2}.

[7] Colourless solutions of cuprous salts may also be obtained by the
action of sulphurous or phosphorous acid and similar lower grades
of oxidation on the blue solutions of the cupric salts. This is
very clearly and easily effected by means of sodium thiosulphate,
Na_{2}S_{2}O_{3}, which is oxidised in the process. Cuprous oxide
can not only be obtained by the deoxidation of cupric oxide, but
also directly from metallic copper itself, because the latter, in
oxidising at a red heat in air, first gives cuprous oxide. It is
prepared in this manner on a large scale by heating sheet copper
rolled into spirals in reverberatory furnaces. Care must be taken
that the air is not in great excess, and that the coating of red
cuprous oxide formed does not begin to pass into the black cupric
oxide. If the oxidised spiral sheet is then unbent, the brittle
cuprous oxide falls away from the soft metal. The suboxide obtained
in this manner fuses with ease. It is necessary to prevent the
access of air during the fusion, and if the mass contains cupric
oxide it must be mixed with charcoal, which reduces the latter.
Cuprous chloride, CuCl, corresponding with cuprous oxide (as sodium
chloride corresponds with sodium oxide), when calcined with sodium
carbonate, gives sodium chloride and cuprous oxide, carbonic
anhydride being evolved, because it does not combine with the
cuprous oxide under these conditions. The reaction can be expressed
by the following equation: 2CuCl + Na_{2}CO_{3} = Cu_{2}O + 2NaCl +
CO_{2}. The cupric oxide itself, when calcined with finely-divided
copper, this copper powder may be obtained by many methods--for
instance, by immersing zinc in a solution of a copper salt, or by
igniting cupric oxide in hydrogen), gives the fusible cuprous
oxide: Cu + CuO = Cu_{2}O. Both the native and artificial cuprous
oxide have a sp. gr. of 5·6. It is insoluble in water, and is not
acted on by (dry) air. When heated with acids the suboxide forms a
solution of a cupric salt and metallic copper--for example, Cu_{2}O
+ H_{2}SO_{4} = Cu + CuSO_{4} + H_{2}O. However, strong
hydrochloric acid does not separate metallic copper on dissolving
cuprous oxide, which is due to the fact that the cuprous chloride
formed is soluble in strong hydrochloric acid. Cuprous oxide also
dissolves in a solution of ammonia, and in the absence of air gives
a colourless solution, which turns blue in the air, absorbing
oxygen, owing to the conversion of the cuprous oxide into cupric
oxide. The blue solution thus formed may be again reconverted into
a colourless cuprous solution by immersing a copper strip in it,
because the metallic copper then deoxidises the cupric oxide in the
solution into cuprous oxide. Cuprous oxide is characterised by the
fact that it gives red glasses when fused with glass or with salts
forming vitreous alloys. Glass tinted with cuprous oxide is used
for ornaments. The access of air must be avoided during its
preparation, because the colour then becomes green, owing to the
formation of cupric oxide, which colours glass blue. This may even
be taken advantage of in testing for copper under the blow-pipe by
heating the copper compound with borax in the flame of a blow-pipe;
a red glass is obtained in the reducing flame, and a blue glass in
the oxidising flame, owing to the conversion of the cuprous into
cupric oxide.

Étard (1882), by passing sulphurous anhydride into a solution of
cupric acetate, obtained a white precipitate of cuprous sulphite,
Cu_{2}SO_{3},H_{2}O, whilst he obtained the same salt, of a red
colour, from the double salt of sodium and copper; but there are
not any convincing proofs of isomerism in this case.

Cupric chloride, CuCl_{2}, when ignited, gives _cuprous chloride_, CuCl--_i.e._ the salt corresponding with suboxide of copper--and therefore cuprous chloride is always formed when copper enters into reaction with chlorine at a high temperature. Thus, for example, when copper is calcined with mercuric chloride, it forms cuprous chloride and vapours of mercury. The same substance is obtained on heating metallic copper in hydrochloric acid, hydrogen being disengaged; but this reaction only proceeds with finely-divided copper, as hydrochloric acid acts very feebly on compact masses of copper, and, in the presence of air, gives cupric chloride. The green solution of cupric chloride is decolorised by metallic copper, cuprous chloride being formed; but this reaction is only accomplished with ease when the solution is very concentrated and in the presence of an excess of hydrochloric acid to dissolve the cuprous chloride. The addition of water to the solution precipitates the cuprous chloride, because it is less soluble in dilute than in strong hydrochloric acid. Many reducing agents which are able to take up half the oxygen from cupric oxide are able, in the presence of hydrochloric acid, to form cuprous chloride. Stannous salts, sulphurous anhydride, alkali sulphites, phosphorous and hypophosphorous acids, and many similar reducing agents, act in this manner. The usual method of preparing cuprous chloride consists in passing sulphurous anhydride into a very strong solution of cupric chloride: 2CuCl_{2} + SO_{2} + 2H_{2}O = 2CuCl + 2HCl + H_{2}SO_{4}. Cuprous chloride forms colourless cubic crystals which are insoluble in water. It is easily fusible, and even volatile. Under the action of oxidising agents, it passes into the cupric salt, and it absorbs oxygen from moist air, forming cupric oxychloride, Cu_{2}Cl_{2}O. _Aqueous ammonia_ easily _dissolves_ cuprous chloride as well as cuprous oxide; the solution also turns blue on exposure to the air. Thus an ammoniacal solution of cuprous chloride serves as an excellent absorbent for oxygen; but this solution absorbs not only oxygen, but also certain other gases--for example, carbonic oxide and acetylene.[8]

[8] The solubility of cuprous chloride in ammonia is due to the
formation of compounds between the ammonia and the chloride. In a
warm solution the compound NH_{3},2CuCl is formed, and at the
ordinary temperature CuCl,NH_{3}. This salt is soluble in
hydrochloric acid, and then forms a corresponding double salt of
cuprous chloride and ammonium chloride. By the action of a certain
excess of ammonia on a hydrochloric acid solution of cuprous
chloride, very well formed crystals, having the composition
CuCl,NH_{3},H_{2}O, are obtained. Cuprous chloride is not only
soluble in ammonia and hydrochloric acid, but it also dissolves in
solutions of certain other salts--for example, in sodium chloride,
potassium chloride, sodium thiosulphate, and certain others. All
the solutions of cuprous chloride act in many cases as very
powerful deoxidising substances; for example, it is easy, by means
of these solutions, to precipitate gold from its solutions in a
metallic form, according to the equation AuCl_{3} + 3CuCl = Au +
3CuCl_{2}.

Among the other compounds corresponding with cuprous oxide,
_cuprous iodide_, CuI, is worthy of remark. It is a colourless
substance which is insoluble in water and sparingly soluble in
ammonia (like silver iodide), but capable of absorbing it, and in
this respect it resembles cuprous chloride. It is remarkable from
the fact that it is exceedingly easily formed from the
corresponding cupric compound CuI_{2}. A solution of cupric iodide
easily decomposes into iodine and cuprous iodide, even at the
ordinary temperature, whilst cupric chloride only suffers a similar
change on ignition. If a solution of a cupric salt be mixed with a
solution of potassium iodide the cupric iodide formed immediately
decomposes into free iodine and cuprous iodide, which separates out
as a precipitate. In this case the cupric salt acts in an oxidising
manner, like, for example, nitrous acid, ozone, and other
substances which liberate iodine from iodides, but with this
difference, that it only liberates half, whilst they set free the
whole of the iodine from potassium iodide: 2KI + CuCl_{2} = 2KCl +
CuI + I.

It must also be remarked that cuprous oxide, when treated with
hydrofluoric acid, gives an insoluble cuprous fluoride, CuF.
Cuprous cyanide is also insoluble in water, and is obtained by the
addition of hydrocyanic acid to a solution of cupric chloride
saturated with sulphurous anhydride. This cuprous cyanide, like
silver cyanide, gives a double soluble salt with potassium cyanide.
The double cyanide of copper and potassium is tolerably stable in
the air, and enters into double decompositions with various other
salts, like those double cyanides of iron with which we are already
acquainted.

_Copper hydride_, CuH, also belongs to the number of the cuprous
compounds. It was obtained by Würtz by mixing a hot (70°) solution
of cupric sulphate with a solution of hypophosphorous acid,
H_{3}PO_{2}. The addition of the reducing hypophosphorous acid must
be stopped when a brown precipitate makes its appearance, and when
gas begins to be evolved. The brown precipitate is the hydrated
cuprous hydride. When gently heated it disengages hydrogen; it
gives cuprous oxide when exposed to the air, burns in a stream of
chlorine, and liberates hydrogen with hydrochloric acid: CuH + HCl
= CuCl + H_{2}. Zinc, silver, mercury, lead, and many other heavy
metals do not form such a compound with hydrogen, neither under
these circumstances nor under the action of hydrogen at the moment
of the decomposition of salts by a galvanic current. The greatest
resemblance is seen between cuprous hydride and the hydrogen
compounds of potassium, sodium, Pd, Ca, and Ba.

When copper is oxidised with a considerable quantity of oxygen at a high temperature, or at the ordinary temperature in the presence of acids, and also when it decomposes acids, converting them into lower grades of oxidation (for example, when submitted to the action of nitric and sulphuric acids), it forms _cupric oxide_, CuO, or, in the presence of acids, cupric salts. Copper rust, or that black mass which forms on the surface of copper when it is calcined, consists of cupric oxide. The coating of the oxidised copper is very easily separated from the metallic copper, because it is brittle and very easily peels off, when it is struck or immersed in water. Many copper salts (for instance, the nitrite and carbonate) leave oxide of copper[8 bis] in the form of friable black powder, after being ignited. If the ignition be carried further, Cu_{2}O may be formed from the CuO.[8 tri] Anhydrous cupric oxide is very easily dissolved in acids, forming cupric salts, CuX_{2}. They are analogous to the salts MgX_{2}, ZnX_{2}, NiX_{2}, FeX_{2}, in many respects. On adding potassium or ammonium hydroxide to a solution of a cupric salt, it forms a gelatinous blue precipitate of the hydrated oxide of copper, CuH_{2}O_{2}, insoluble in water. The resultant precipitate _is redissolved by an excess of ammonia_, and gives a very beautiful azure blue solution, of so intense a colour that the presence of small traces of cupric salts may be discovered by this means.[9] An excess of potassium or sodium hydroxide does not dissolve cupric hydroxide. A hot solution gives a black precipitate of the anhydrous oxide instead of the blue precipitate, and the precipitate of the hydroxide of copper becomes granular, and turns black when the solution is heated. This is due to the fact that the blue hydroxide is exceedingly unstable, and when slightly heated it loses the elements of water and gives the black anhydrous cupric oxide: CuH_{2}O_{2} = CuO + H_{2}O.

[8 bis] The oxide of copper obtained by igniting the nitrate is
frequently used for organic analyses. It is hygroscopic and retains
nitrogen (1·5 c.c. per gram) when the nitrate is heated in vacuo
(Richards and Rogers, 1893).

[8 tri] Oxide of copper is also capable of dissociating when heated.
Debray and Joannis showed that it then disengages oxygen, whose
maximum tension is constant for a given temperature, providing that
fusion does not take place (the CuO then dissolves in the molten
Cu_{2}O); that this loss of oxygen is followed by the formation of
suboxide, and that on cooling, the oxygen is again absorbed,
forming CuO.

[9] Cupric oxide and many of its salts are able to give definite,
although unstable, _compounds with ammonia_. This faculty already
shows itself in the fact that cupric oxide, as well as the salts of
copper, dissolves in aqueous ammonia, and also in the fact that
salts of copper absorb ammonia gas. If ammonia be added to a
solution of any cupric salt, it first forms a precipitate of cupric
hydroxide, which then dissolves in an excess of ammonia. The
solution thus formed, when evaporated or on the addition of
alcohol, frequently deposits crystals of salts containing both the
elements of the salt of copper taken and of ammonia. Several such
compounds are generally formed. Thus cupric chloride, CuCl_{2},
according to Deherain, forms four compounds with ammonia--namely,
with one, two, four, and six molecules of ammonia. Thus, for
example, if ammonia gas be passed into a boiling saturated solution
of cupric chloride, on cooling, small octahedral crystals of a blue
colour separate out, containing CuCl_{2},2NH_{3},H_{2}O. At 150°
this substance loses half the ammonia and all the water contained
in it, leaving the compound CuCl_{2},NH_{3}. Nitrate of copper
forms the compound Cu(NO_{3})_{2},2NH_{3}· This compound remains
unchanged on evaporation. Dry cupric sulphate absorbs ammonia gas,
and gives a compound containing five molecules of ammonia to one of
sulphate (Vol. I., p. 257, and Chapter XXII., Note 35). If this
compound is dissolved in aqueous ammonia, on evaporation it
deposits a crystalline substance containing
CuSO_{4},4NH_{3},H_{2}O. At 150° this substance loses the molecule
of water and one-fourth of its ammonia. On ignition all these
compounds part with the remaining ammonia in the form of an
ammoniacal salt, so that the residue consists of cupric oxide. Both
the hydrated and anhydrous cupric oxide are soluble in aqueous
ammonia.

The solution obtained by the action of aqueous ammonia and air on
copper turnings (Note 6) is remarkable for its faculty of
_dissolving cellulose_, which is insoluble in water, dilute acids,
and alkalis. Paper soaked in such a solution acquires the property
of not rotting, of being difficultly combustible, and waterproof,
&c. It has therefore been applied, especially in England, to many
practical purposes--for example, to the construction of temporary
buildings, for covering roofs, &c. The composition of the substance
held in solution is Cu(HO)_{2},4NH_{3}.

If dry ammonia gas be passed over cupric oxide heated to 265°, a
portion of the oxide of copper remains unaltered, whilst the other
portion gives _copper nitride_, the oxygen of the copper oxide
combining with the hydrogen and forming water. The oxide of copper
which remains unchanged is easily removed by washing the resultant
product with aqueous ammonia. Copper nitride is very stable, and is
insoluble; it has the composition Cu_{3}N (_i.e._ the copper is
monatomic here as in Cu_{2}O), and is an amorphous green powder,
which is decomposed when strongly ignited, and gives cuprous
chloride and ammonium chloride when treated with hydrochloric acid.
Like the other nitrides, copper nitride, Cu_{3}N, has scarcely been
investigated. Granger (1892), by heating copper in the vapour of
phosphorus, obtained hexagonal prisms of Cu_{5}P, which passed into
Cu_{6}P (previously obtained by Abel) when heated in nitrogen.
Arsenic is easily absorbed by copper, and its presence (like P),
even in small quantities, has a great influence upon the properties
of copper--for instance, pure copper wire 1 sq. mm. in section
breaks under a load of 35 kilos, while the presence of O·22 p.c. of
arsenic raises the breaking load to 42 kilos.

Cupric oxide fuses at a strong heat, and on cooling forms a heavy crystalline mass, which is black, opaque, and somewhat tenacious. It is a feebly energetic base, so that not only do the oxides of the metals of the alkalis and alkaline earths displace it from its compounds, but even such oxides as those of lead and silver precipitate it from solutions, which is partially due to these oxides being soluble, although but slightly so, in water. However, cupric oxide, and especially the hydroxide, easily combines with even the least energetic acids, and does not give any compounds with bases; but, on the other hand, _it easily forms basic salts_,[9 bis] and in this respect outstrips magnesium and recalls the oxides of lead or mercury. Hence the hydroxide of copper dissolves in solutions of neutral cupric salts. The cupric salts are generally blue or green, because cupric hydroxide itself is coloured. But some of the salts in the anhydrous state are colourless.[10]

[9 bis] As a comparatively feeble base, oxide of copper easily forms
both basic and double salts. As an instance we may mention the
double salts composed of the dichloride CuCl_{2},2H_{2}O and
potassium chloride. The double salt CuK_{2}Cl_{4},2H_{2}O
crystallises from solutions in _blue_ plates, but when heated alone
or with substances taking up water easily gives _brown_ needles
CuKCl_{3} and at the same time KCl, and this reaction is reversible
at 92° as Meyerhoffer (1889) showed (_i.e._ above 92° the simpler
double salt is formed and below 92° the more complex salt). With an
excess of the copper salt, KCl gives another double salt,
Cu_{2}KCl_{5},4H_{2}O, the transition temperature of which is 55°.
The instances of equilibria which are encountered in such complex
relations (_see_ Chapter XIV., Note 25, astrakhanite, and Chapter
XXII., Note 23) are embraced by the _law of phases_ given by Gibbs
(Transactions of the Connecticut Academy of Sciences, 1875-1878, in
J. Willard Gibbs' memoir 'On the equilibrium of heterogeneous
substances:' and in a clearer and more accessible form in H. W.
Bakhuis Roozeboom's papers, Rec. trav. chim., Vol. VI., and in W.
Meyerhoffer's memoir _Die Phasenregel und ihre Anwendungen_, 1893,
to which sources we refer those desiring fuller information
respecting this law). Gibbs calls '_bodies_' substances (simple or
compound) capable of forming homogeneous complexes (for instance,
solutions or intercombinations) of a varied composition; a
_phase_--a mechanically separable portion of such bodies or of
their homogeneous complexes (for instance, a vapour, liquid or
precipitated solid), _perfect equilibrium_--such a state of bodies
and of their complexes as is characterised by a constant pressure
at a constant temperature even under a change in the amount of one
of the component parts (for instance, of a salt in a saturated
solution), while an _imperfect equilibrium_ is such a one for which
such a change corresponds with a change of pressure (for instance,
an unsaturated solution). The law of phases consists in the fact
that: _n bodies only give a perfect equilibrium when n + 1 phases
participate in that equilibrium_--for example, in the equilibrium
of a salt in its saturated solution in water there are two bodies
(the salt and water) and three phases, namely, the salt, solution,
and vapour, which can be mechanically separated from each other,
and to this equilibrium there corresponds a definite tension. At
the same time, _n bodies may occur in n + 2 phases, but only at one
definite temperature and one pressure_; a change of one of these
may bring about another state (perfect or not--equilibrium stable
or unstable). Thus water when liquid at the ordinary temperature
offers two phases (liquid and vapour) and is in perfect equilibrium
(as also is ice below 0°), but water, ice, and vapour (three phases
and only one body) can only be in equilibrium at 0°, and at the
ordinary pressure; with a change of _t_ there will remain either
only ice and vapour or only liquid water and vapour; whilst with a
rise of pressure not only will the vapour pass into the liquid
(there again only remain two phases) but also the temperature of
the formation of ice will fall (by about 7° per 1000 atmospheres).
The same laws of phases are applicable to the consideration of the
formation of simple or double salts from saturated solutions and to
a number of other purely chemical relations. Thus, for example, in
the above-mentioned instance, when the bodies are KCl, CuCl_{2},
and H_{2}O, perfect equilibrium (which here has reference to the
solubility) consisting of four phases, corresponds to the following
seven cases, considering only the phases (above 0°) A =
CuCl_{2},2KCl,2H_{2}O; B = CuCl_{2}KCl; C = CuCl_{2},2H_{2}O,KCl,
solution and vapour: (1) A + B + solution + vapour; (2) A + C +
solution + vapour; (3) A + KCl + solution + vapour; (4) A + B + C +
vapour (it follows that B + KCl + solution gives A); (5) A + C +
KCl + vapour; (6) B + C + solution + vapour; and (7) B + KCl +
solution + vapour. Thus above 92° A gives B + KCl. The law of
phases by bringing complex instances of chemical reaction under
simple physical schemes, facilitates their study in detail and
gives the means of seeking the simplest chemical relations dealing
with solutions, dissociation, double decompositions and similar
cases, and therefore deserves consideration, but a detailed
exposition of this subject must be looked for in works on physical
chemistry.

[10] The normal _cupric nitrate_, CuN_{2}O_{6},3H_{2}O, is obtained as
a deliquescent salt of a blue colour (soluble in water and in
alcohol) by dissolving copper or cupric oxide in nitric acid. It
is so easily decomposed by the action of heat that it is
impossible to drive off the water of crystallisation from it
before it begins to decompose. During the ignition of the normal
salt the cupric oxide formed enters into combination with the
remaining undecomposed normal salt, and gives a basic salt,
CuN_{2}O_{6},2CuH_{2}O_{2}. The same basic salt is obtained if a
certain quantity of alkali or cupric hydroxide or carbonate be
added to the solution of the normal salt, which is even decomposed
when boiled with metallic copper, and forms the basic salt as a
green powder, which easily decomposes under the action of heat and
leaves a residue of cupric oxide. The basic salt, having the
composition CuN_{2}O_{6},3CuH_{2}O_{2}, is nearly insoluble in
water.

The normal _carbonate of copper_, CuCO_{3}, occurs in nature,
although extremely rarely. If solutions of cupric salts be mixed
with solutions of alkali carbonates, then, as in the case of
magnesium, carbonic anhydride is evolved and basic salts are
formed, which vary in composition according to the temperature and
conditions of the reaction. By mixing cold solutions, a voluminous
blue precipitate is formed, containing an equivalent proportion of
cupric hydroxide and carbonate (after standing or heating, its
composition is the same as malachite, sp. gr. 3·51: 2CuSO_{4} +
2Na_{2}CO_{3} + H_{2}O = CuCO_{3},CuH_{2}O_{2} + 2Na_{2}SO_{4} +
CO_{2}. If the resultant blue precipitate be heated in the liquid,
it loses water and is transformed into a granular green mass of
the composition Cu_{2}CO_{4}--_i.e._ into a compound of the normal
salt with anhydrous cupric oxide. This salt of the oxide
corresponds with orthocarbonic acid, C(OH)_{4} = CH_{4}O_{4} where
4H is replaced by 2Cu. On further boiling this salt loses a
portion of the carbonic acid, forming black cupric oxide, so
unstable is the compound of copper with carbonic anhydride.
Another basic salt which occurs in nature, 2CuCO_{3},CuH_{2}O_{2},
is known as azurite, or blue carbonate of copper; it also loses
carbonic acid when boiled with water. On mixing a solution of
cupric sulphate with sodium sesquicarbonate no precipitate is at
first obtained, but after boiling a precipitate is formed having
the composition of malachite. Debray obtained artificial azurite
by heating cupric nitrate with chalk.

The commonest normal salt is _blue vitriol_--_i.e._ the normal cupric sulphate. It generally contains five molecules of water of crystallisation, CuSO_{4},5H_{2}O. It forms the product of the action of strong sulphuric acid on copper, sulphurous anhydride being evolved. The same salt is obtained in practice by carefully roasting sulphuretted ores of copper, and also by the action of water holding oxygen in solution on them: CuS + O_{4} = CuSO_{4}. This salt forms a by-product, obtained in gold refineries, when the silver is precipitated from the sulphuric acid solution by means of copper. It is also obtained by pouring dilute sulphuric acid over sheet copper in the presence of air, or by heating cupric oxide or carbonate in sulphuric acid. The crystals of this salt belong to the triclinic system, have a specific gravity of 2·19, are of a beautiful blue colour, and give a solution of the same colour. 100 parts of water at 0° dissolve 15, at 25° 23, and at 100° about 45 parts of cupric sulphate, CuSO_{4}.[10 bis] At 100° this salt loses a portion of its water of crystallisation, which it only parts with entirely at a high temperature (220°) and then gives a white powder of the anhydrous sulphate; and the latter, on further calcination, loses the elements of sulphuric anhydride, leaving cupric oxide, like all the cupric salts. The anhydrous (colourless) cupric sulphate is sometimes used for absorbing water; it turns blue in the process. It offers the advantage that it retains both hydrochloric acid and water, but not carbonic anhydride.[11] Cupric sulphate is used for steeping seed corn; this is said to prevent the growth of certain parasites on the plants. In the arts a considerable quantity of cupric sulphate is also used in the preparation of other copper salts--for instance, of certain pigments[11 bis]--and a particularly large quantity is used _in the galvanoplastic process_, which consists in the deposition of copper from a solution of cupric sulphate by the action of a galvanic current, when the metallic copper is deposited on the negative pole and takes the shape of the latter. The description of the processes of galvanoplastic art introduced by Jacobi in St. Petersburg forms a part of applied physics, and will not be touched on here, and we will only mention that, although first introduced for small articles, it is now used for such articles as type moulds (_clichés_), for maps, prints, &c., and also for large statues, and for the deposition of iron, zinc, nickel, gold, silver, &c. on other metals and materials. The beginning of the application of the galvanic current to the practical extraction of metals from solutions has also been established, especially since the dynamo-electric machines of Gramme, Siemens, and others have rendered it possible to cheaply convert the mechanical motion of the steam engine into an electric current. It is to be expected that the application of the electric current, which has long since given such important results in chemistry, will, in the near future, play an important part in technical processes, the example being shown by electric lighting.

[10 bis] Although sulphate of copper usually crystallises with 5H_{2}O,
that is, differently to the sulphates of Mg, Fe, and Mn, it is
nevertheless perfectly isomorphous with them, as is seen not only
in the fact that it gives isomorphous mixtures with them,
containing a similar amount of water of crystallisation, but also
in the ease with which it forms, like all bases analogous to MgO,
double salts, R_{2}Cu(SO_{4})_{2},6H_{2}O, where R = K, Rb, Cs, of
the monoclinic system.

Salts of this kind, like CuCl_{2},2KCl,2H_{2}O,PtK_{2}Cy_{4}, &c.,
present a composition CuX_{2} if the representation of double
salts given in Chapter XXIII., Note 11, be admitted, because they,
like Cu(HO)_{2}, contain Cu(X_{2}K)_{2}, where X_{2} = SO_{4},
_i.e._ the residue of sulphuric acid, which combines with H_{2},
and is therefore able to replace the H_{2} by X_{2} or O. A
detailed study of the crystalline forms of these salts, made by
Tutton (1893) (_see_ Chapter XIII., Note 1), showed: (1) that 22
investigated salts of the composition R_{2}M(SO_{4}),6H_{2}O,
where R = K, Rb, Cs, and M = Mg, Zn, Cd, Mn, Fe, Co, Ni, Cu,
present a complete crystallographic resemblance; (2) that in all
respects the Rb salts present a transition between the K and Cs
salts; (3) that the Cs salts form crystals most easily, and the K
salts the most difficultly, and that for the K salts of Cd and Mn
it was even impossible to obtain well-formed crystals; (4) that
notwithstanding the closeness of their angles, the general
appearance (habit) of the potassium compound differs very clearly
from the Cs salts, while the Rb salts present a distinct
transition in this respect; (5) that the angle of the inclination
of one of the axes to the plane of the two other axes showed that
in the K salts (angle from 75° to 75° 38´) the inclination is
least, in the Cs salts (from 72° 52´ to 73° 50´) greatest, and in
the Rb salts (from 73° 57´ to 74° 42´) intermediate between the
two; the replacement of Mg ... Cu produces but a very small change
in this angle; (6) that the other angles and the ratio of the axes
of the crystals exhibit a similar variation; and (7) that thus the
variation of the form is chiefly determined by the atomic weight
of the alkaline metal. As an example we cite the magnitude of the
inclination of the axes of R_{2}M(SO_{4})_{2},6H_{2}O.

R = K Rb Cs
M = Mg 75° 12´ 74° 1´ 72° 54´
Zn 75° 12´ 74° 7´ 72° 59´
Cd -- 74° 7´ 72° 49´
Mn -- 73° 3´ 72° 53´
Fe 75° 28´ 74° 16´ 73° 8´
Co 75° 5´ 73° 59´ 72° 52´
Ni 75° 0´ 73° 57´ 72° 58´
Cu 75° 32´ 74° 42´ 73° 50´

This shows clearly (within the limits of possible error, which may
be as much as 30´) the almost perfect identity of the independent
crystalline forms notwithstanding the difference of the atomic
weights of the diatomic elements, M = Mg, Cu.

[11] In addition to what has been said (Chapter I., Note 65, and