Chapter XI: Part 11

The earliest serious attempt to refine copper industrially was made by G. R. Elkington, whose first patent is dated 1865. He cast crude copper, as obtained from the ore, into plates which were used as anodes, sheets of electro-deposited copper forming the cathodes. Six anodes were suspended, alternately with four cathodes, in a saturated solution of copper sulphate in a cylindrical fire-clay trough, all the anodes being connected in one parallel group, and all the cathodes in another. A hundred or more jars were coupled in series, the cathodes of one to the anodes of the next, and were so arranged that with the aid of side-pipes with leaden connexions and india-rubber joints the electrolyte could, once daily, be made to circulate through them all from the top of one jar to the bottom of the next. The current from a Wilde's dynamo was passed, apparently with a current density of 5 or 6 amperes per sq. ft., until the anodes were too crippled for further use. The cathodes, when thick enough, were either cast and rolled or sent into the market direct. Silver and other insoluble impurities collected at the bottom of the trough up to the level of the lower side-tube, and were then run off through a plug in the bottom into settling tanks, from which they were removed for metallurgical treatment. The electrolyte was used until the accumulation of iron in it was too great, but was mixed from time to time with a little water acidulated by sulphuric acid. This process is of historic interest, and in principle it is identical with that now used. The modifications introduced have been chiefly in details, in order to economize materials and labour, to ensure purity of product, and to increase the rate of deposition.

The chemistry of the process has been studied by Martin Kiliani
(_Berg- und Huttenmannische Zeitung_, 1885, p. 249), who found that,
using the (low) current-density of 1.8 ampere per sq. ft. of cathode,
and an electrolyte containing 1-1/2 lb. of copper sulphate and 1/2 lb.
of sulphuric acid per gallon, all the gold, platinum and silver
present in the crude copper anode remain as metals, undissolved, in
the anode slime or mud, and all the lead remains there as sulphate,
formed by the action of the sulphuric acid (or SO4 ions); he found
also that arsenic forms arsenious oxide, which dissolves until the
solution is saturated, and then remains in the slime, from which on
long standing it gradually dissolves, after conversion by secondary
reactions into arsenic oxide; antimony forms a basic sulphate which in
part dissolves; bismuth partly dissolves and partly remains, but the
dissolved portion tends slowly to separate out as a basic salt which
becomes added to the slime; cuprous oxide, sulphide and selenides
remain in the slime, and very slowly pass into solution by simple
chemical action; tin partly dissolves (but in part separates again as
basic salt) and partly remains as basic sulphate and stannic oxide;
zinc, iron, nickel and cobalt pass into solution--more readily indeed
than does the copper. Of the metals which dissolve, none (except
bismuth, which is rarely present in any quantity) deposits at the
anode so long as the solution retains its proper proportion of copper
and acid, and the current-density is not too great. Neutral solutions
are to be avoided because in them silver dissolves from the anode and,
being more electro-negative than copper, is deposited at the cathode,
while antimony and arsenic are also deposited, imparting a dark colour
to the copper. Electrolytic copper should contain at least 99.92% of
metallic copper, the balance consisting mainly of oxygen with not more
than 0.01% in all of lead, arsenic, antimony, bismuth and silver. Such
a degree of purity is, however, unattainable unless the conditions of
electrolysis are rigidly adhered to. It should be observed that the
free acid is gradually neutralized, partly by chemical action on
certain constituents of the slime, partly by local action between
different metals of the anode, both of which effect solution
independently of the current, and partly by the peroxidation (or
aeration) of ferrous sulphate formed from the iron in the anode. At
the same time there is a gradual substitution of other metals for
copper in the solution, because although copper _plus_ other (more
electro-positive) metals are constantly dissolving at the anode, only
copper is deposited at the cathode. Hence the composition and acidity
of the solution, on which so much depends, must be constantly watched.

The dependence of the mechanical qualities of the copper upon the
current-density employed is well known. A very weak current gives a
pale and brittle deposit, but as the current-density is increased up
to a certain point, the properties of the metal improve; beyond this
point they deteriorate, the colour becoming darker and the deposit
less coherent, until at last it is dark brown and spongy or
pulverulent. The presence of even a small proportion of hydrochloric
acid imparts a brown tint to the deposit. Baron H. v. Hubl (_Mittheil.
des k. k. militar-geograph. Inst._, 1886, vol. vi. p. 51) has found
that with neutral solutions a 5% solution of copper sulphate gave no
good result, while with a 20% solution the best deposit was obtained
with a current-density of 28 amperes per sq. ft.; with solutions
containing 2% of sulphuric acid, the 5% solution gave good deposits
with current-densities of 4 to 7.5 amperes, and the 20% solution with
11.5 to 37 amperes, per sq. ft. The maximum current-densities for a
_pure_ acid solution at rest were: for 15% pure copper sulphate
solutions 14 to 21 amperes, and for 20% solutions 18.5 to 28 amperes,
per sq. ft.; but when the solutions were kept in gentle motion these
maxima could be increased to 21-28 and 28-37 amperes per sq. ft.
respectively. The necessity for adjusting the current-density to the
composition and treatment of the electrolyte is thus apparent. The
advantage of keeping the solution in motion is due partly to the
renewal of solution thus effected in the neighbourhood of the
electrodes, and partly to the neutralization of the tendency of
liquids undergoing electrolysis to separate into layers, due to the
different specific gravities of the solutions flowing from the
opposing electrodes. Such an irregular distribution of the bath, with
strong copper sulphate solution from the anode at the bottom and acid
solution from the cathode at the top, not only alters the conductivity
in different strata and so causes irregular current-distribution, but
may lead to the current-density in the upper layers being too great
for the proportion of copper there present. Irregular and defective
deposits are therefore obtained. Provision for circulation of solution
is made in the systems of copper-refining now in use. Henry Wilde, in
1875, in depositing copper on iron printing-rollers, recognized this
principle and rotated the rollers during electrolysis, thereby
renewing the surfaces of metal and liquid in mutual contact, and
imparting sufficient motion to the solution to prevent stratification;
as an alternative he imparted motion to the electrolyte by means of
propeller blades. Other workers have followed more or less on the same
lines; reference may be made to the patents of F. E. and A. S. Elmore,
who sought to improve the character of the deposit by burnishing
during electrolysis, of E. Dumoulin, and Sherard Cowper-Coles
(_Engineering Review_, 1905, vol. xiii. p. 392), who prefers to rotate
the cathode at a speed that maintains a peripheral velocity of at
least 1000 ft. per minute. Certain other inventors have applied the
same principle in a different way. H. Thofehrn in America and J. C.
Graham in England have patented processes by which jets of the
electrolyte are caused to impinge with considerable force upon the
surface of the cathode, so that the renewal of the liquid at this
point takes place very rapidly, and current-densities per sq. ft. of
50 to 100 amperes are recommended by the former, and of 300 amperes by
the latter. Graham has described experiments in this direction, using
a jet of electrolyte forced (beneath the surface of the bath) through
a hole in the anode upon the surface of the cathode. Whilst the jet
was playing, a good deposit was formed with so high a current-density
as 280 amperes per sq. ft., but if the jet was checked, the deposit
(now in a still liquid) was instantaneously ruined. When two or more
jets were used side by side the deposit was good opposite the centre
of each, but bad at the point where two currents met, because the rate
of flow was reduced. By introducing perforated shields of ebonite
between the electrodes, so that the full current-density was only
attained at the centres of the jets, these ill effects could be
prevented. One of the chief troubles met with was the formation of
arborescent growths around the edges of the cathode, due to the
greater current-density in this region; this, however, was also
obviated by the use of screens. By means of a very brisk rotation of
cathode, combined with a rapid current of electrolyte, J. W. Swan has
succeeded in depositing excellent copper at current-densities
exceeding 1000 amperes per sq. ft. The methods by which such results
are to be obtained cannot, however, as yet be practised economically
on a working scale; one great difficulty in applying them to the
refining of metals is that the jets of liquid would be liable to carry
with them articles of anode mud, and Swan has shown that the presence
of solid particles in the electrolyte is one of the most fruitful
causes of the well-known nodular growths on electro-deposited copper.
Experiments on a working scale with one of the jet processes in
America have, it is reported, been given up after a full trial.

In copper-refining practice, the current-density commonly ranges from
7.5 to 12 or 15, and occasionally to 18, amperes per sq. ft. The
electrical pressure required to force a current of this intensity
through the solution, and to overcome a certain opposing electromotive
force arising from the more electro-negative impurities of the anode,
depends upon the composition of the bath and of the anodes, the
distance between the electrodes, and the temperature, but under the
usual working conditions averages 0.3 volt for every pair of
electrodes in series. In nearly all the processes now used, the
solution contains about 1-1/2 to 2 lb. of copper sulphate and from 5
to 10 oz. of sulphuric acid per gallon of water, and the space between
the electrodes is from 1-1/2 to 2 in., whilst the total area of
cathode surface in each tank may be 200 sq. ft., more or less. The
anodes are usually cast copper plates about (say) 3 ft. by 2 ft. by
3/4 or 1 in. The cathodes are frequently of electro-deposited copper,
deposited to a thickness of about 1/32 in. on black-leaded copper
plates, from which they are stripped before use. The tanks are
commonly constructed of wood lined with lead, or tarred inside, and
are placed in terrace fashion each a little higher than the next in
series, to facilitate the flow of solution through them all from a
cistern at one end to a well at the other. Gangways are left between
adjoining rows of tanks, and an overhead travelling-crane facilitates
the removal of the electrodes. The arrangement of the tanks depends
largely upon the voltage available from the electric generator
selected; commonly they are divided into groups, all the baths in each
group being in series. In the huge Anaconda plant, for example, in
which 150 tons of refined copper can be produced daily by the Thofehrn
multiple system (not the jet system alluded to above), there are 600
tanks about 8-1/4 ft. by 4-1/2 ft. by 3-1/4 ft. deep, arranged in
three groups of 200 tanks in series. The connexions are made by copper
rods, each of which, in length, is twice the width of the tank, with a
bayonet-bend in the middle, and serves to support the cathodes in the
one and the anodes in the next tank. Self-registering voltmeters
indicate at any moment the potential difference in every tank, and
therefore give notice of short circuits occurring at any part of the
installation. The chief differences between the commercial systems of
refining lie in the arrangement of the baths, in the disposition and
manner of supporting the electrodes in each, in the method of
circulating the solution, and in the current-density employed. The
various systems are often classed in two groups, known respectively as
the _Multiple_ and _Series_ systems, depending upon the arrangement of
the electrodes in each tank. Under the multiple system anodes and
cathodes are placed alternately, all the anodes in one tank being
connected to one rod, and all the cathodes to another, and the
potential difference between the terminals of each tank is that
between a single pair of plates. Under the series system only the
first anode and the last cathode are connected to the conductors;
between these are suspended, isolated from one another, a number of
intermediate bi-polar electrode plates of raw copper, each of these
plates acting on one side as a cathode, receiving a deposit of copper,
and on the other as an anode, passing into solution; the voltage
between the terminals of the tank will be as many times as great as
that between a single pair of plates as there are spaces between
electrodes in the tank. In time the original impure copper of the
plates becomes replaced by refined copper, but if the plates are
initially very impure and dissolve irregularly, it may happen that
much residual scrap may have to be remelted, or that some of the metal
may be twice refined, thus involving a waste of energy. Moreover, the
high potential difference between the terminals of the series tank
introduces a greater danger of short-circuiting through scraps of
metal at the bottom of the bath; for this reason, also, lead-lined
vats are inadmissible, and tarred slate tanks are often used instead.
A valuable comparison of the multiple and series systems has been
published by E. Keller (see _The Mineral Industry_, New York, 1899,
vol. vii. p. 229). G. Kroupa has calculated that the cost of refining
is 8s. per ton of copper higher under the series than it is under the
multiple system; but against this, it must be remembered that the new
works of the Baltimore Copper Smelting and Rolling Company, which are
as large as those of the Anaconda Copper Mining Company, are using the
Hayden process, which is the chief representative of the several
series systems. In this system rolled copper anodes are used; these,
being purer than many cast anodes, having flat surfaces, and being
held in place by guides, dissolve with great regularity and require a
space of only 5/8 in. between the electrodes, so that the potential
difference between each pair of plates may be reduced to 0.15-0.2
volt.

J. A. W. Borchers, in Germany, and A. E. Schneider and O. Szontagh, in
America, have introduced a method of circulating the solution in each
vat by forcing air into a vertical pipe communicating between the
bottom and top of a tank, with the result that the bubbling of the air
upward aspirates solution through the vertical pipe from below, at the
same time aerating it, and causing it to overflow into the top of the
tank. Obviously this slow circulation has but little effect on the
rate at which the copper may be deposited. The electrolyte, when too
impure for further use, is commonly recrystallized, or electrolysed
with insoluble anodes to recover the copper.

The yield of copper per ampere (in round numbers, 1 oz. of copper per
ampere per diem) by Faraday's law is never attained in practice; and
although 98% may with care be obtained, from 94 to 96% represents the
more usual current-efficiency. With 100% current-efficiency and a
potential difference of 0.3 volt between the electrodes, 1 lb. of
copper should require about 0.154 electrical horse-power hours as the
amount of energy to be expended in the tank for its production. In
practice the expenditure is somewhat greater than this; in large works
the gross horse-power required for the refining itself and for power
and lighting in the factory may not exceed 0.19 to 0.2 (or in smaller
works 0.25) horse-power hours per pound of copper refined.

Many attempts have been made to use crude sulphide of copper or matte
as an anode, and recover the copper at the cathode, the sulphur and
other insoluble constituents being left at the anode. The best known
of these is the Marchese process, which was tested on a working scale
at Genoa and Stolberg in Rhenish Prussia. As the operation proceeded,
it was found that the voltage had to be raised until it became
prohibitive, while the anodes rapidly became honeycombed through and,
crumbling away, filled up the space at the bottom of the vat. The
process was abandoned, but in a modified form appears to be now in use
in Nijni-Novgorod in Russia. Siemens and Halske introduced a combined
process in which the ore, after being part-roasted, is leached by
solutions from a previous electrolytic operation, and the resulting
copper solution electrolysed. In this process the anode solution had
to be kept separate from the cathode solution, and the membrane which
had in consequence to be used, was liable to become torn, and so to
cause trouble by permitting the two solutions to mix. Modifications of
the process have therefore been tried.

Modern methods in copper smelting and refining have effected enormous economy in time, space, and labour, and have consequently increased the world's output. With pyritic smelting a sulphuretted copper ore, fed into a cupola in the morning, can be passed directly to the converter, blown up to metal, and shipped as 99% bars by evening--an operation which formerly, with heap roasting of the ore and repeated roasting of the mattes in stalls, would have occupied not less than four months. A large furnace and a Bessemer converter, the pair capable of making a million pounds of copper a month from a low-grade sulphuretted ore, will not occupy a space of more than 25ft. by 100ft.; and whereas, in making metallic copper out of a low-grade sulphuretted ore, one day's labour used to be expended on every ton of ore treated, to-day one day's labour will carry at least four tons of ore through the different mechanical and metallurgical processes necessary to reduce them to metal. About 70% of the world's annual copper output is refined electrolytically, and from the 461,583 tons refined in the United States in 1907, there were recovered 13,995,436 oz. of silver and 272,150 oz. of gold. The recovery of these valuable metals has contributed in no small degree to the expansion of electrolytic refining.

_Production._--The sources of copper, its applications and its metallurgy, have undergone great changes. Chile was the largest producer in 1869 with 54,867 tons; but in 1899 her production had fallen off to 25,000 tons. Great Britain, though she had made half the world's copper in 1830, held second place in 1860, making from native ores 15,968 tons; in 1900 her production was 777 tons, and in 1907, 711 tons. The United States made only 572 tons in 1850, and 12,600 tons in 1870; but she to-day makes more than 60% of the world's total. In 1879, Spain was the largest producer, but now ranks third.

The estimated total production for each decade of the 19th century in metric tons is here shown:--

1801-1810 91,000
1811-1820 96,000
1821-1830 135,000
1831-1840 218,400
1841-1850 291,000
1851-1860 506,999
1861-1870 900,000
1871-1880 1,189,000
1881-1890 2,373,398
1891-1900 3,708,901

The following table gives the output of various countries and the world's production for the years 1895, 1900, 1905, 1907:--

+---------------------+----------+----------+----------+---------+
| Country. | 1895. | 1900. | 1905. | 1907. |
+---------------------+----------+----------+----------+---------+
| United States | 175,294 | 274,933 | 397,003 | 398,736 |
| Spain and Portugal | 55,755 | 53,718 | 45,527 | 50,470 |
| Japan | 18,725 | 28,285 | 36,485 | 49,718 |
| Chile | 22,428 | 26,016 | 29,632 | 27,112 |
| Germany | 16,799 | 20,635 | 22,492 | 20,818 |
| Australasia | 10,160 | 23,368 | 34,483 | 41,910 |
| Mexico | 12,806 | 22,473 | 70,010 | 61,127 |
| Russia | 5,364 | 8,128 | 8,839 | 15,240 |
| +----------+----------+----------+---------+
| World's production | 339,994 | 496,819 | 699,514 | 723,807 |
+---------------------+----------+----------+----------+---------+

As the stock on hand rarely exceeds three months' demand, and is often little more than a month's supply, it is evident that consumption has kept close pace with production.

The large demand for copper to be used in sheathing ships ceased on the introduction of iron in shipbuilding because of the difficulty of coating iron with an impervious layer of copper; but the consumption in the manufacture of electric apparatus and for electric conductors has far more than compensated.

_Alloys of Copper._--Copper unites with almost all other metals, and a
large number of its alloys are of importance in the arts. The
principal alloys in which it forms a leading ingredient are brass,
bronze, and German or nickel silver; under these several heads their
respective applications and qualities will be found.

Oxides and hydroxides.

_Compounds of Copper._--Copper probably forms six oxides, viz. Cu4O,
Cu3O, Cu2O, CuO, Cu2O3 and CuO2. The most important are cuprous oxide,
Cu2O, and cupric oxide, CuO, both of which give rise to well-defined
series of salts. The other oxides do not possess this property, as is
also the case of the hydrated oxides Cu3O22H2O and Cu4O35H2O,
described by M. Siewert.

Cuprous oxide, Cu2O, occurs in nature as the mineral cuprite (q.v.).
It may be prepared artificially by heating copper wire to a white
heat, and afterwards at a red heat, by the atmospheric oxidation of
copper reduced in hydrogen, or by the slow oxidation of the metal
under water. It is obtained as a fine red crystalline precipitate by
reducing an alkaline copper solution with sugar. When finely divided
it is of a fine red colour. It fuses at red heat, and colours glass a
ruby-red. The property was known to the ancients and during the middle
ages; it was then lost for several centuries, to be rediscovered in
about 1827. Cuprous oxide is reduced by hydrogen, carbon monoxide,
charcoal, or iron, to the metal; it dissolves in hydrochloric acid
forming cuprous chloride, and in other mineral acids to form cupric
salts, with the separation of copper. It dissolves in ammonia, forming
a colourless solution which rapidly oxidizes and turns blue. A
hydrated cuprous oxide, (4Cu2O, H2O), is obtained as a bright yellow
powder, when cuprous chloride is treated with potash or soda. It
rapidly absorbs oxygen, assuming a blue colour. Cuprous oxide
corresponds to the series of cuprous salts, which are mostly white in
colour, insoluble in water, and readily oxidized to cupric salts.

Cupric oxide, CuO, occurs in nature as the mineral melaconite (q.v.),
and can be obtained as a hygroscopic black powder by the gentle
ignition of copper nitrate, carbonate or hydroxide; also by heating
the hydroxide. It oxidizes carbon compounds to carbon dioxide and
water, and therefore finds extensive application in analytical organic
chemistry. It is also employed to colour glass, to which it imparts a
light green colour. Cupric hydroxide, Cu(OH)2, is obtained as a
greenish-blue flocculent precipitate by mixing cold solutions of
potash and a cupric salt. This precipitate always contains more or
less potash, which cannot be entirely removed by washing. A purer
product is obtained by adding ammonium chloride, filtering, and
washing with hot water. Several hydrated oxides, e.g. Cu(OH)2.3CuO,
Cu(OH)2.6H2O, 6CuO.H2O, have been described. Both the oxide
and hydroxide dissolve in ammonia to form a beautiful azure-blue
solution (Schweizer's reagent), which dissolves cellulose, or perhaps,
holds it in suspension as water does starch; accordingly, the solution
rapidly perforates paper or calico. The salts derived from cupric
oxide are generally white when anhydrous, but blue or green when
hydrated.

Copper quadrantoxide, Cu4O, is an olive-green powder formed by mixing
well-cooled solutions of copper sulphate and alkaline stannous
chloride. The trientoxide, Cu3O, is obtained when cupric oxide is
heated to 1500 deg.-2000 deg. C. It forms yellowish-red crystals,
which scratch glass, and are unaffected by all acids except
hydrofluoric; it also dissolves in molten potash. Copper dioxide,
CuO2H2O, is obtained as a yellowish-brown powder, by treating cupric
hydrate with hydrogen peroxide. When moist, it decomposes at about 6
deg. C., but the dry substance must be heated to about 180 deg.,
before decomposition sets in (see L. Moser, _Abst. J.C.S._, 1907, ii.
p. 549).

Cuprous hydride, (CuH)n, was first obtained by Wurtz in 1844, who
treated a solution of copper sulphate with hypophosphorous acid, at a
temperature not exceeding 70 deg. C. According to E. J. Bartlett and
W. H. Merrill, it decomposes when heated, and gives cupric hydride,
CuH2, as a reddish-brown spongy mass, which turns to a chocolate
colour on exposure. It is a strong reducing agent.

Cuprous fluoride, CuF, is a ruby-red crystalline mass, formed by
heating cuprous chloride in an atmosphere of hydrofluoric acid at 1100
deg.-1200 deg. C. It is soluble in boiling hydrochloric acid, but it
is not reprecipitated by water, as is the case with cuprous chloride.
Cupric fluoride, CuF2, is obtained by dissolving cupric oxide in
hydrofluoric acid. The hydrated form, (CuF2, 2H2O, 5HF), is obtained
as blue crystals, sparingly soluble in cold water; when heated to 100
deg. C. it gives the compound CuF(OH), which, when heated with
ammonium fluoride in a current of carbon dioxide, gives anhydrous
copper fluoride as a white powder.

Cuprous chloride, CuCl or Cu2Cl2, was obtained by Robert Boyle by
heating copper with mercuric chloride. It is also obtained by burning
the metal in chlorine, by heating copper and cupric oxide with
hydrochloric acid, or copper and cupric chloride with hydrochloric
acid. It dissolves in the excess of acid, and is precipitated as a
white crystalline powder on the addition of water. It melts at below
red heat to a brown mass, and its vapour density at both red and white
heat corresponds to the formula Cu2Cl2. It turns dirty violet on
exposure to air and light; in moist air it absorbs oxygen and forms an
oxychloride. Its solution in hydrochloric acid readily absorbs carbon
monoxide and acetylene; hence it finds application in gas analysis.
Its solution in ammonia is at first colourless, but rapidly turns
blue, owing to oxidation. This solution absorbs acetylene with the
precipitation of red cuprous acetylide, Cu2C2, a very explosive
compound. Cupric chloride, CuCl2, is obtained by burning copper in an
excess of chlorine, or by heating the hydrated chloride, obtained by
dissolving the metal or cupric oxide in an excess of hydrochloric
acid. It is a brown deliquescent powder, which rapidly forms the green
hydrated salt CuCl2, 2H2O on exposure. The oxychloride Cu3O2Cl2.4H2O
is obtained as a pale blue precipitate when potash is added to an
excess of cupric chloride. The oxychloride Cu4O3Cl2, 4H2O occurs in
nature as the mineral atacamite. It may be artificially prepared by
heating salt with ammonium copper sulphate to 100 deg.. Other
naturally occurring oxychlorides are botallackite and tallingite.
"Brunswick green," a light green pigment, is obtained from copper
sulphate and bleaching powder.

The bromides closely resemble the chlorides and fluorides.

Cuprous iodide, Cu2I2, is obtained as a white powder, which suffers
little alteration on exposure, by the direct union of its components
or by mixing solutions of cuprous chloride in hydrochloric acid and
potassium iodide; or, with liberation of iodine, by adding potassium
iodide to a cupric salt. It absorbs ammonia, forming the compound
Cu2I2, 4NH3. Cupric iodide is only known in combination, as in CuI2,
4NH3, H2O, which is obtained by exposing Cu2I2, 4NH3 to moist air.

Cuprous sulphide, Cu2S, occurs in nature as the mineral chalcocite or
copper-glance (q.v.), and may be obtained as a black brittle mass by
the direct combination of its constituents. (See above, METALLURGY.)
Cupric sulphide, CuS, occurs in nature as the mineral covellite. It
may be prepared by heating cuprous sulphide with sulphur, or
triturating cuprous sulphide with cold strong nitric acid, or as a
dark brown precipitate by treating a copper solution with sulphuretted
hydrogen. Several polysulphides, e.g. Cu2S5, Cu2S6, Cu4S6, Cu2S3, have
been described; they are all unstable, decomposing into cupric
sulphide and sulphur. Cuprous sulphite, CuSO3.H2O, is obtained as a
brownish-red crystalline powder by treating cuprous hydrate with
sulphurous acid. A cuproso-cupric sulphite, Cu2SO3, CuSO3,2H2O, is
obtained by mixing solutions of cupric sulphate and acid sodium
sulphite.

Cupric sulphate or "Blue Vitriol," CuSO4, is one of the most important
salts of copper. It occurs in cupriferous mine waters and as the
minerals chalcanthite or cyanosite, CuSO4.5H2O, and boothite,
CuSO4.7H2O. Cupric sulphate is obtained commercially by the oxidation
of sulphuretted copper ores (see above, METALLURGY; wet methods), or
by dissolving cupric oxide in sulphuric acid. It was obtained in 1644
by Van Helmont, who heated copper with sulphur and moistened the
residue, and in 1648 by Glauber, who dissolved copper in strong
sulphuric acid. (For the mechanism of this reaction see C. H. Sluiter,
_Chem. Weekblad_, 1906, 3, p. 63, and C. M. van Deventer, ibid., 1906,
3, p. 515.) It crystallizes with five molecules of water as large blue
triclinic prisms. When heated to 100 deg., it loses four molecules of
water and forms the bluish-white monohydrate, which, on further
heating to 25O deg.-260 deg., is converted into the white CuSO4. The
anhydrous salt is very hygroscopic, and hence finds application as a
desiccating agent. It also absorbs gaseous hydrochloric acid. Copper
sulphate is readily soluble in water, but insoluble in alcohol; it
dissolves in hydrochloric acid with a considerable fall in
temperature, cupric chloride being formed. The copper is readily
replaced by iron, a knife-blade placed in an aqueous solution being
covered immediately with a bright red deposit of copper. At one time
this was regarded as a transmutation of iron into copper. Several
basic salts are known, some of which occur as minerals; of these, we
may mention brochantite (q.v.), CuSO4, 3Cu(OH2), langite, CuSO4,
3Cu(OH)2, H2O, lyellite (or devilline), warringtonite; woodwardite and
enysite are hydrated copper-aluminium sulphates, connellite is a basic
copper chlorosulphate, and spangolite is a basic copper aluminium
chlorosulphate. Copper sulphate finds application in calico printing
and in the preparation of the pigment Scheele's green.

A copper nitride, Cu3N, is obtained by heating precipitated cuprous
oxide in ammonia gas (A. Guntz and H. Bassett, _Bull. Soc. Chim._,
1906, 35, p. 201). A maroon-coloured powder, of composition CuNO2, is
formed when pure dry nitrogen dioxide is passed over finely-divided
copper at 25 deg.-30 deg.. It decomposes when heated to 90 deg.; with
water it gives nitric oxide and cupric nitrate and nitrite. Cupric
nitrate, Cu(NO3)2, is obtained by dissolving the metal or oxide in
nitric acid. It forms dark blue prismatic crystals containing 3, 4, or
6 molecules of water according to the temperature of crystallization.
The trihydrate melts at 114.5 deg., and boils at 170 deg., giving off
nitric acid, and leaving the basic salt Cu(NO3)2.3Cu(OH)2. The mineral
gerhardtite is the basic nitrate Cu2(OH)3NO3.

Copper combines directly with phosphorus to form several compounds.
The phosphide obtained by heating cupric phosphate, Cu2H2P2O8, in
hydrogen, when mixed with potassium and cuprous sulphides or levigated
coke, constitutes "Abel's fuse," which is used as a primer. A
phosphide, Cu3P2, is formed by passing phosphoretted hydrogen over
heated cuprous chloride. (For other phosphides see E. Heyn and O.
Bauer, _Rep. Chem. Soc._, 1906, 3, p. 39.) Cupric phosphate,
Cu3(PO4)2, may be obtained by precipitating a copper solution with
sodium phosphate. Basic copper phosphates are of frequent occurrence
in the mineral kingdom. Of these we may notice libethenite,
Cu2(OH)PO4; chalcosiderite, a basic copper iron phosphate; torbernite,
a copper uranyl phosphate; andrewsite, a hydrated copper iron
phosphate; and henwoodite, a hydrated copper aluminium phosphate.

Copper combines directly with arsenic to form several arsenides, some
of which occur in the mineral kingdom. Of these we may mention
whitneyite, Cu9As, algodonite, Cu6As, and domeykite, Cu3As. Copper
arsenate is similar to cupric phosphate, and the resemblance is to be
observed in the naturally occurring copper arsenates, which are
generally isomorphous with the corresponding phosphates. Olivenite
corresponds to libethenite; clinoclase, euchroite, cornwallite and
tyrolite are basic arsenates; zeunerite corresponds to torbernite;
chalcophyllite (tamarite or "copper-mica") is a basic copper aluminium
sulphato-arsenate, and bayldonite is a similar compound containing
lead instead of aluminium. Copper arsenite forms the basis of a number
of once valuable, but very poisonous, pigments. Scheele's green is a
basic copper arsenite; Schweinfurt green, an aceto-arsenite; and
Casselmann's green a compound of cupric sulphate with potassium or
sodium acetate.

Normal cupric carbonate, CuCO3, has not been definitely obtained,
basic hydrated forms being formed when an alkaline carbonate is added
to a cupric salt. Copper carbonates are of wide occurrence in the
mineral kingdom, and constitute the valuable ores malachite and
azurite. Copper rust has the same composition as malachite; it results
from the action of carbon dioxide and water on the metal. Copper
carbonate is also the basis of the valuable blue to green pigments
verditer, Bremen blue and Bremen green. Mountain or mineral green is a
naturally occurring carbonate.

By the direct union of copper and silicon, cuprosilicon, consisting
mainly of Cu4Si, is obtained (Lebeau, C.R., 1906; Vigouroux,
ibid.).

Copper silicates occur in the mineral kingdom, many minerals owing
their colour to the presence of a cupriferous element. Dioptase (q.v.)
and chrysocolla (q.v.) are the most important forms.

_Detection._--Compounds of copper impart a bright green coloration to
the flame of a Bunsen burner. Ammonia gives a characteristic blue
coloration when added to a solution of a copper salt; potassium
ferrocyanide gives a brown precipitate, and, if the solution be very
dilute, a brown colour is produced. This latter reaction will detect
one part of copper in 500,000 of water. For the borax beads and the
qualitative separation of copper from other metals, see CHEMISTRY:
ANALYTICAL. For the quantitative estimation, see ASSAYING: COPPER.

_Medicine._--In medicine copper sulphate was employed as an emetic,
but its employment for this purpose is now very rare, as it is
exceedingly depressant, and if it fails to act, may seriously damage
the gastric mucous membrane. It is, however, a useful superficial
caustic and antiseptic. All copper compounds are poisonous, but not so
harmful as the copper arsenical pigments.

REFERENCES.--See generally H. J. Steven's _Copper Handbook_ (annual),
W. H. Weld, _The Copper Mines of the World_ (1907), _The Mineral
Industry_ (annual), and _Mineral Resources of the United States_
(annual). For the dry metallurgy, see E. D. Peters, _Principles of
Copper Smelting_ (New York, 1907); for pyritic smelting, see T. A.
Rickard, _Pyrite Smelting_ (1905); for wet methods, see Eissler,
_Hydrometallurgy of Copper_ (London, 1902); and for electrolytic
methods, see T. Ulke, _Die electrolytische Raffination des Kupfers_
(Halle, 1904). Reference should also be made to the articles
METALLURGY and ELECTRO-METALLURGY. For the chemistry of copper and its
compounds see the references in the article CHEMISTRY: Inorganic.
Toxicologic and hygienic aspects are treated in Tschirsch's _Das
Kupfer vom Standpunkt der gerichtlichen Chemie, Toxikologie und
Hygiene_ (Stuttgart, 1893).

COPPERAS (Fr. _couperose_; Lat. _cupri rosa_. the flower of copper), green vitriol, or ferrous sulphate, FeSO4.7H2O, having a bluish-green colour and an astringent, inky and somewhat sweetish taste. It is used in dyeing and tanning, and in the manufacture of ink and of Nordhausen sulphuric acid or fuming oil of vitriol (see IRON).

COPPER-GLANCE, a mineral consisting of cuprous sulphide, Cu2S, and crystallizing in the orthorhombic system. It is known also as chalcocite, redruthite and vitreous copper (German, _Kupferglaserz_ of G. Agricola, 1546). The crystals have the form of six-sided tables or prisms; the angle between the prism faces (lettered o in the figure) being 60 deg. 25'. When twinned on the prism planes o, as is frequently the case, the crystals simulate hexagonal symmetry still more closely, as in the minerals aragonite and chrysoberyl. Twinning also takes place according to two other laws, giving rise to interpenetrating crystals with the basal planes (s) of the two individuals inclined at angles of 69 deg. or 87 deg. 56' respectively. The mineral also occurs as compact masses of considerable extent. The colour is dark lead-grey with a metallic lustre, but this is never very bright, since the material is readily altered, becoming black and dull on exposure to light. The mineral is soft (H.=2-1/2) and sectile, and can be readily cut with a knife, like argentite; sp. gr. 5.7. Analyses agree closely with the formula Cu2S, which corresponds to 79.8% of copper; small quantities of iron and silver are sometimes present.

Next to chalcopyrite, copper-glance is the most important ore of copper. It usually occurs in the upper part of the copper-bearing lodes, and is a secondary sulphide derived from the chalcopyrite met with at greater depths; sometimes, however, the two minerals are found together in the same part of the lodes. The best crystals are from St Just, St Ives, and Redruth in Cornwall, and from Bristol in Connecticut. Small crystals of recent formation are found on Roman bronze coins in the thermal springs at Bourbonne-les-Bains.

Copper-glance readily alters to other minerals, such as malachite, covellite, melaconite and chalcopyrite. On the other hand, it is found as pseudomorphs after chalcopyrite, galena, and organic structures such as wood; copper-glance pseudomorphous after galena preserves the cleavage of the original mineral and is known as harrisite.

Isomorphous with copper-glance is the orthorhombic mineral stromeyerite, a double copper and silver sulphide, CuAgS, which occurs in abundance in the Altai Mountains. (L. J. S.)

COPPERHEADS, an American political epithet, applied by Union men during the Civil War to those men in the North who, deeming it impossible to conquer the Confederacy, were earnestly in favour of peace and therefore opposed to the war policy of the president and of Congress. Such men were not necessarily friends of the Confederate cause. The term originated in the autumn of 1862, and its use quickly spread throughout the North. In the Western states early in 1863 the terms "Copperhead" and "Democrat" had become practically synonymous. The name was adopted because of the fancied resemblance of the peace party to the venomous copperhead snake, and, though applied as a term of opprobrium, it was willingly assumed by those upon whom it was bestowed.

COPPERMINE, a river of Mackenzie district, Canada, about 475 m. long, rising in a small lake in approximately 110 deg. 20' W. and 65 deg. 50' N., and flowing south to Lake Gras and then north-westward to Coronation Gulf in the Arctic Ocean. Like Back's river, the only other large river of this part of Canada, it is unnavigable, being a succession of lakes and violent rapids. The country through which it flows is a mass of low hills and morasses. The river was discovered by Samuel Hearne in 1771, and was explored from Point Lake to the sea by Captain (afterwards Sir John) Franklin in 1821.

COPPER-PYRITES, or CHALCOPYRITE, a copper iron sulphide (CUFeS2), an important ore of copper. The name copper-pyrites is from the Ger. _Kupferkies_, which was used as far back as 1546 by G. Agricola; chalcopyrite (from [Greek: chalkos], "copper," and pyrites) was proposed by J. F. Henckel in his _Pyritologia, oder Kiess-Historie_ (1725). By the ancients copper-pyrites was included with other minerals under the term pyrites, though the copper-ore from Cyprus referred to by Aristotle as chalcites may possibly have been identical with this mineral.

Chalcopyrite crystallizes in the tetragonal system with inclined hemihedrism, but the form is so nearly cubic that it was not recognized as tetragonal until accurate measurements were made in 1822. Crystals are usually tetrahedral in aspect, owing to the large development of the sphenoid P {111}. The faces of this form are dull and striated, whilst the smaller faces of the complementary sphenoid P' {111} (fig. 1) are bright and smooth. The combination of these two forms produces a figure resembling an octahedron, the angle between P and P' being 70 deg. 7-1/2', corresponding to the angle 70 deg. 32' of the regular octahedron. The other faces shown in fig. 1 are the basal pinacoid, a {001}, and two square pyramids, b {101} and c {201}. Crystals are usually twinned, and are often complex and difficult to decipher. There are three twin-laws, the twin-planes being (111), (101) and (110) respectively. Twinning according to the first law is effected by rotation about an axis normal to the sphenoidal face (111), the resulting form resembling the twins of blende and spinel. Twinning according to the second law can only be explained by reflection across the plane (101), not by rotation about an axis; chalcopyrite affords an excellent example of this comparatively rare type of symmetric twinning. Interpenetration twins (fig. 2) with (110) as twin-plane are of very rare occurrence.

Crystals have imperfect cleavages parallel to the eight faces of the pyramid c {201}. The fracture is conchoidal, and the material is brittle. Hardness 4; specific gravity 4.2. The colour is brass-yellow, and the lustre metallic; the streak, or colour of the powder, is greenish-black. The mineral is especially liable to surface alteration, tarnishing with beautiful iridescent colours; a blue colour usually predominates, owing probably to the alteration of the chalcopyrite to covellite (CuS). The massive and compact mineral frequently exhibits this iridescent tarnish, and is consequently known to miners as "peacock ore" or "peacock copper." The massive mineral sometimes occurs in mammillary and botryoidal forms with a smooth brassy surface, and is then known to Cornish miners as "blister-copper-ore."

Chalcopyrite or copper-pyrites may be readily distinguished from iron-pyrites (or pyrites), which it somewhat resembles in appearance, by its deeper colour and lower degree of hardness: the former is easily scratched by a knife, whilst the latter can only be scratched with difficulty or not at all. Chalcopyrite is decomposed by nitric acid with separation of sulphur and formation of a green solution; ammonia added in excess to this solution changes the green colour to deep blue and precipitates red ferric hydroxide.

The chemical formula CuFeS2 corresponds with the percentage composition Cu=34.5, Fe=30.5, S=35.0. Analyses usually, however, show the presence of more iron, owing to the intimate admixture of iron-pyrites. Traces of gold, silver, selenium or thallium are sometimes present, and the mineral is sometimes worked as an ore of gold or silver.

Chalcopyrite is of wide distribution and is the commonest of the ores of copper. It occurs in metalliferous veins, often in association with iron-pyrites, chalybite, blende, &c., and in Cornwall and Devon, where it is abundant, with cassiterite. The large deposits at Falun in Sweden occur with serpentine in gneiss, and those at Montecatini, near Volterra in the province of Pisa, serpentine and gabbro. At Rammelsberg in the Harz it forms a bed in argillaceous schist, and at Mansfield in Thuringia it occurs in the Kupferschiefer with ores of nickel and cobalt. Extensive deposits are mined in the United States, particularly at Butte in Montana, and in Namaqualand, South Africa. Well-crystallized specimens are met with at many localities; for example, formerly at Wheal Towan (hence the name towanite, which has been applied to the species) in the St Agnes district of Cornwall, at Freiberg in Saxony, and Joplin, Missouri. (L. J. S.)

COPPICE, or COPSE (from an O. Fr. _copeis_ or _coupeis_, from Late Lat. _colpare_, to cut with a blow; _colpas_, the Late Lat. for "blow," is a shortened form of _colapus_ or _colaphus_, adapted from the Gr. [Greek: kolaphos]), a small plantation or thicket of planted or self-sown trees, which are cut periodically for use or sale, before the trees grow into large timber. Whether naturally or artificially grown the produce is looked on by the English law as _fructus industrialis_. The tenant for life or years may appropriate this produce (see _Dashwood_ v. _Magniac_, 1891, 3 Ch. 306).

COPRA (a Spanish and Portuguese adaptation of the Malay _kopperah_, and Hindustani _khopra_, the coco-nut), the dried broken kernel of the coco-nut from which coco-nut oil is extracted by boiling and pressing. Copra is the form in which the product of the coco-nut is exported for commercial purposes (see COCONUT PALM).

COPROLITES (from Gr. [Greek: kopros], dung, and [Greek: lithos], stone), the fossilized excrements of extinct animals. The discovery of their true nature was made by Dr William Buckland, who observed that certain convoluted bodies occurring in the Lias of Gloucestershire had the form which would have been produced by their passage in the soft state through the intestines of reptiles or fishes. These bodies had long been known as "fossil fir cones" and "bezoar stones." Buckland's conjecture that they were of faecal origin, and similar to the _album grecum_ or excrement of hyaenas, was confirmed by Dr W. Prout, who on analysis found they consisted essentially of calcium phosphate and carbonate, and not infrequently contained fragments of unaltered bone. The name "coprolites" was accordingly given to them by Buckland, who subsequently expressed his belief that they might be found useful in agriculture on account of the calcium phosphate they contained. The Liassic coprolites are described by Buckland as resembling oblong pebbles, or kidney-potatoes; they are mostly 2 to 4 in. long, and from 1 to 2 in. in diameter, but those of the larger ichthyosauri are of much greater dimensions. In colour they vary from ash-grey to black, and their fracture is conchoidal. Internally they are found to consist of a lamina twisted upon itself, and externally they generally exhibit a tortuous structure, produced, before the cloaca was reached, by the spiral valve of a compressed small intestine (as in skates, sharks and dog-fishes); the surface shows also vascular impressions and corrugations due to the same cause. Often the bones, teeth and scales of fishes are to be found dispersed through the coprolites, and sometimes the bones of small ichthyosauri, which were apparently a prey to the larger marine saurians. Coprolites have been found at Lyme Regis, enclosed by the ribs of ichthyosauri, and in the remains of several species of fish; also in the abdominal cavities of a species of fossil fish, _Macropoma Mantelli_, from the chalk of Lewes. Professor T. Jager has described coprolites from the alum-slate of Gaildorf in Wurttemberg; the fish-coprolites of Burdiehouse and of Newcastle-under-Lyme are of Carboniferous age. The so-called "beetle-stones" of the coal-formation of Newhaven, near Leith, which have mostly a coprolite nucleus, have been applied to various ornamental purposes by lapidaries. The name "cololites" (from the Greek [Greek: kolon], the large intestine, [Greek: lithos], stone) was given by Agassiz to fossil wormlike bodies, found in the lithographic slate of Solenhofen, which he determined to be either the petrified intestines or contents of the intestines of fishes. The bone-bed of Axmouth in Devonshire and Westbury and Aust in Gloucestershire, in the Penarth or Rhaetic series of strata, contains the scales, teeth and bones of saurians and fishes, together with abundance of coprolites; but neither there nor at Lyme Regis is there a sufficient quantity of phosphatic material to render the working of it for agricultural purposes remunerative.

The term coprolites has been made to include all kinds of phosphatic nodules employed as manures, such, for example, as those obtained from the Coralline and the Red Crag of Suffolk. At the base of the Red Crag in that county is a bed, 3 to 18 in. thick, containing rolled fossil bones, cetacean and fish teeth, and shells of the Crag period, with nodules or pebbles of phosphatic matter derived from the London Clay, and often investing fossils from that formation. These are distinguishable from the grey Chalk coprolites by their brownish ferruginous colour and smooth appearance. When ground they give a yellowish-red powder. These nodules were at first taken by Professor J. S. Henslow for coprolites; they were afterwards termed by Buckland "pseudo-coprolites." "The nodules, having been imbued with phosphatic matter from their matrix in the London Clay, were dislodged," says Buckland, "by the waters of the seas of the first period, and accumulated by myriads at the bottom of those shallow seas where is now the coast of Suffolk. Here they were long rolled together with the bones of large mammalia, fishes, and with the shells of molluscous creatures that lived in shells. From the bottom of this sea they have been raised to form the dry lands along the shores of Suffolk, whence they are now extracted as articles of commercial value, being ground to powder in the mills of Mr [afterwards Sir John] Lawes, at Deptford, to supply our farms with a valuable substitute for guano, under the accepted name of coprolite manure." The phosphatic nodules occurring throughout the Red Crag of Suffolk are regarded as derived from the Coralline Crag. The Suffolk beds have been worked since 1846; and immense quantities of coprolite have also been obtained from Essex, Norfolk and Cambridgeshire. The Cambridgeshire coprolites are believed to be derived from deposits of Gault age; they are obtained by washing from a stratum about a foot thick, resting on the Gault, at the base of the Chalk Marl, and probably homotaxeous with the Chloritic Marl. An acre used to yield on an average 300 tons of phosphatic nodules, value L750. About L140 per acre was paid for the lease of the land, which after two years was restored to its owners re-soiled and levelled. Plicatulae have been found attached to these coprolites, showing that they were already hard bodies when lying at the bottom of the Chalk ocean. The Cambridgeshire coprolites are either amorphous or finger-shaped; the coprolites from the Greensand are of a black or dark-brown colour; while those from the Gault are greenish-white on the surface, brownish-black internally. Samples of Cambridgeshire and Suffolk coprolite have been found by A. Voelcker to give on analysis phosphoric acid equivalent to about 55 and 52.5% of tribasic calcium phosphate respectively (_Journ. R. Agric. Soc. Eng._, 1860, xxi. 358). The following analysis of a saurio-coprolite from Lyme Regis is given by T. J. Herapath (ibid. xii. 91):--

Water 3.976
Organic matter 2.001
Calcium sulphate 2.026
Calcium carbonate 28.121
Calcium fluoride not determined
Calcium and magnesium phosphate 53.996
Magnesium carbonate 0.423
Aluminic phosphate 1.276
Ferric phosphate 6.182
Silica 0.773
------
98.734

An ichthyo-coprolite from Tenby was found to contain 15.4% of phosphoric anhydride. The pseudo-coprolites of the Suffolk Crag have been estimated by Herapath to be as rich in phosphates as the true ichthyo-coprolites and saurio-coprolites of other formations, the proportion of P2O5 contained varying between 12.5 and 37.25%, the average proportion, however, being 32 or 33%.

Coprolite is reduced to powder by powerful mills of peculiar construction, furnished with granite and buhrstones, before being treated with concentrated sulphuric acid. The acid renders it available as a manure by converting the calcium phosphate, Ca3P2O8, that it contains into the soluble monocalcium salt, CaH4P2O8, or "superphosphate." The phosphate thus produced forms an efficacious turnip manure, and is quite equal in value to that produced from any other source. The Chloritic Marl in the Wealden district furnishes much phosphatic material, which has been extensively worked at Froyle. In the vicinity of Farnham it contains a bed of "coprolites" of considerable extent and 2 to 15 ft. in thickness. Specimens of these from the Dippen Hall pits, analysed by Messrs J. M. Paine and J. T. Way, showed the presence of phosphates equivalent to 55.96 of bone-earth (_Journ. R. Agric. Soc. Eng._ ix. 56). Phosphatic nodules occur also in the Chloritic Marl of the Isle of Wight and Dorsetshire, and at Wroughton, near Swindon. They are found in the Lower Greensand, or Upper Neocomian series, in the Atherfield Clay at Stopham, near Pulborough; occasionally at the junction of the Hythe and Sandgate beds; and in the Folkeston beds, at Farnham. At Woburn, Leighton, Ampthill, Sandy, Upware, Wicken and Potton, near the base of Upper Neocomian iron-sands, there is a band between 6 in. and 2 ft. in thickness containing "coprolites"; these consist of phosphatized wood, bones, casts of shells, and shapeless lumps. The coprolitic stratum of the Speeton Clay, on the coast to the north of Flamborough Head, is included by Professor Judd with the Portland beds of that formation. In 1864 two phosphatic deposits, a limestone 3 ft. thick, with beds of calcium phosphate, and a shale of half that thickness, were discovered by Hope Jones in the neighbourhood of Cwmgynen, about 16 m. from Oswestry. They are at a depth of about 12 ft., in slaty shale containing Llandeilo fossils and contemporaneous felspathic ash and scoriae. A specimen of the phosphatic limestone analysed by A. Voelcker yielded 34.92% tricalcium phosphate, a specimen of the shale 52.15% (_Report of Brit. Assoc._, 1865). Phosphatic beds, supposed to have had a coprolitic origin, are found in the Lower Silurian rocks of Canada.

See T. J. Herapath, _Chem. Gaz._, 1849, p. 449; W. Buckland, _Geology
and Mineralogy_ (4th ed., 1869); O. Fisher, _Quart. Journ. Geol.
Soc._, 1873, p. 52; J. J. H. Teall, _On the Potton and Wicken
Phosphatic Deposits_ (Sedgwick Prize Essay for 1873) (1875) and "The
Natural History of Phosphatic Deposits," _Proc. Geol. Assoc._ xvi.
(1900); L. W. Collet, _Proc. Roy. Soc. Edin._ xxv. pt. 10, p. 862; T.
G. Bonney, _Cambridgeshire Geology_ (1875); L. Gruner, _Bull. soc.
geol. franc._ xxviii. (2nd series), p. 62; J. Martin, ibid. iii. (3rd
series), p. 273.