DIAMETER DIEDENHOFEN
DIAMOND DIEKIRCH
DIAMOND NECKLACE, THE AFFAIR OF THE DIELECTRIC
DIANA DIELMANN, FREDERICK
DIANA MONKEY DIEMEN, ANTHONY VAN
DIANE DE FRANCE DIEPENBECK, ABRAHAM VAN
DIANE DE POITIERS DIEPPE
DIAPASON DIERX, LEON
DIAPER DIES, CHRISTOPH ALBERT
DIAPHORETICS DIEST
DIAPHRAGM DIESTERWEG, FRIEDRICH ADOLF WILHELM
DIARBEKR DIET
DIARRHOEA DIETARY
DIARY DIETETICS
DIASPORE DIETRICH, CHRISTIAN WILHELM ERNST
DIASTYLE DIETRICH OF BERN
DIATOMACEAE DIEZ, FRIEDRICH CHRISTIAN
DIAULOS DIEZ
DIAVOLO, FRA DIFFERENCES, CALCULUS OF
DIAZ, NARCISSE VIRGILIO DIFFERENTIAL EQUATION
DIAZ, PORFIRIO DIFFLUGIA
DIAZ DE NOVAES, BARTHOLOMEU DIFFRACTION OF LIGHT
DIAZO COMPOUNDS DIFFUSION
DIAZOMATA DIGBY, SIR EVERARD
DIBDIN, CHARLES DIGBY, SIR KENELM
DIBDIN, THOMAS FROGNALL DIGBY, KENELM HENRY
DIBDIN, THOMAS JOHN DIGENES ACRITAS, BASILIUS
DIBRA DIGEST
DIBRUGARH DIGESTIVE ORGANS
DICAEARCHUS DIGGES, WEST
DICE DIGIT
DICETO, RALPH DE DIGITALIS
DICEY, EDWARD DIGNE
DICHOTOMY DIGOIN
DICK, ROBERT DIJON
DICK, THOMAS DIKE
DICKENS, CHARLES JOHN HUFFAM DIKKA
DICKINSON, ANNA ELIZABETH DILAPIDATION
DICKINSON, JOHN DILATATION
DICKSON, SIR ALEXANDER DILATORY
DICKSON, SIR JAMES ROBERT DILEMMA
DICOTYLEDONS DILETTANTE
DICTATOR DILIGENCE
DICTIONARY DILKE, SIR CHARLES WENTWORTH
DICTYOGENS DILL
DICTYS CRETENSIS DILLEN, JOHANN JAKOB
DICUIL DILLENBURG
DIDACHE, THE DILLENS, JULIEN
DIDACTIC POETRY DILLINGEN
DIDEROT, DENIS DILLMANN, CHRISTIAN AUGUST
DIDIUS SALVIUS JULIANUS, MARCUS DILLON, ARTHUR RICHARD
DIDO DILLON, JOHN
DIDON, HENRI DILUVIUM
DIDOT DIME
DIDRON, ADOLPHE NAPOLEON DIMENSION
DIDYMI DIMITY
DIDYMIUM DINAJPUR
DIDYMUS DINAN
DIDYMUS CHALCENTERUS DINANT
DIE (town of France) DINAPUR
DIE (datum) DINARCHUS
DIEBITSCH, HANS KARL FRIEDRICH ANTON
DIAMETER (from the Gr. [Greek: dia], through, [Greek: metron],
measure), in geometry, a line passing through the centre of a circle or
conic section and terminated by the curve; the "principal diameters" of
the ellipse and hyperbola coincide with the "axes" and are at right
angles; "conjugate diameters" are such that each bisects chords parallel
to the other. The diameter of a quadric surface is a line at the
extremities of which the tangent planes are parallel. Newton defined the
diameter of a curve of any order as the locus of the centres of the mean
distances of the points of intersection of a system of parallel chords
with the curve; this locus may be shown to be a straight line. The word
is also used as a unit of linear measurement of the magnifying power of
a lens or microscope.
In architecture, the term is used to express the measure of the lower
part of the shaft of a column. It is employed by Vitruvius (iii. 2) to
determine the height of a column, which should vary from eight to ten
diameters according to the intercolumniation: and it is generally the
custom to fix the lower diameter of the shaft by the height required and
the Order employed. Thus the diameter of the Roman Doric should be about
one-eighth of the height, that of the Ionic one-ninth, and of the
Corinthian one-tenth (see ORDER).
DIAMOND, a mineral universally recognized as chief among precious
stones; it is the hardest, the most imperishable, and also the most
brilliant of minerals.[1] These qualities alone have made it supreme as
a jewel since early times, and yet the real brilliancy of the stone is
not displayed until it has been faceted by the art of the lapidary
(q.v.); and this was scarcely developed before the year 1746. The
consummate hardness of the diamond, in spite of its high price, has made
it most useful for purposes of grinding, polishing and drilling.
Numerous attempts have been made to manufacture the diamond by
artificial means, and these attempts have a high scientific interest on
account of the mystery which surrounds the natural origin of this
remarkable mineral. Its physical and chemical properties have been the
subject of much study, and have a special interest in view of the
extraordinary difference between the physical characters of the diamond
and those of graphite (blacklead) or charcoal, with which it is
chemically identical, and into which it can be converted by the action
of heat or electricity. Again, on account of the great value of the
diamond, much of the romance of precious stones has centred round this
mineral; and the history of some of the great diamonds of historic times
has been traced through many extraordinary vicissitudes.
The name [Greek: Adamas], "the invincible," was probably applied by the
Greeks to hard metals, and thence to corundum (emery) and other hard
stones. According to Charles William King, the first undoubted
application of the name to the diamond is found in Manilius (A.D.
16),--_Sic Adamas_, _punctum lapidis_, _pretiosior auro_,--and Pliny
(A.D. 100) speaks of the rarity of the stone, "the most valuable of
gems, known only to kings." Pliny described six varieties, among which
the Indian, having six pointed angles, and also resembling two pyramids
(_turbines_, whip-tops) placed base to base, may probably be identified
as the ordinary octahedral crystal (fig. 1). The "diamond" (_Yahalom_)
in the breastplate of the high priest (Ex. xxxix. 11) was certainly some
other stone, for it bore the name of a tribe, and methods of engraving
the true diamond cannot have been known so early. The stone can hardly
have become familiar to the Romans until introduced from India, where it
was probably mined at a very early period. But one or other of the
remaining varieties mentioned by Pliny (the Macedonian, the Arabian, the
Cyprian, &c.) may be the true diamond, which was in great request for
the tool of the gem-engraver. Later Roman authors mentioned various
rivers in India as yielding the _Adamas_ among their sands. The name
_Adamas_ became corrupted into the forms _adamant_, _diamaunt_,
_diamant_, _diamond_; but the same word, owing to a medieval
misinterpretation which derived it from _adamare_ (compare the French
word _aimant_), was also applied to the lodestone.
Like all the precious stones, the diamond was credited with many
marvellous virtues; among others the power of averting insanity, and of
rendering poison harmless; and in the middle ages it was known as the
"pietra della reconciliazione," as the peacemaker between husband and
wife.
_Scientific Characters._--The majority of minerals are found most
commonly in masses which can with difficulty be recognized as aggregates
of crystalline grains, and occur comparatively seldom as distinct
crystals; but the diamond is almost always found in single crystals,
which show no signs of previous attachment to any matrix; the stones
were, until the discovery of the South African mines, almost entirely
derived from sands or gravels, but owing to the hardness of the mineral
it is rarely, if ever, water-worn, and the crystals are often very
perfect. The crystals belong to the cubic system, generally assuming the
form of the octahedron (fig. 1), but they may, in accordance with the
principles of crystallography, also occur in other forms symmetrically
derived from the octahedron,--for example, the cube, the 12-faced figure
known as the rhombic dodecahedron (fig. 2), or the 48-faced figure known
as the hexakis-octahedron (fig. 3), or in combinations of these. The
octahedron faces are usually smooth; most of the other faces are rounded
(fig. 4). The cube faces are rough with protruding points. The cube is
sometimes found in Brazil, but is very rare among the S. African stones;
and the dodecahedron is perhaps more common in Brazil than elsewhere.
There is often a furrow running along the edges of the octahedron, or
across the edges of the cube, and this indicates that the apparently
simple crystal may really consist of eight individuals meeting at the
centre; or, what comes to the same thing, of two individuals
interpenetrating and projecting through each other. If this be so the
form of the diamond is really the tetrahedron (and the various figures
derived symmetrically from it) and not the octahedron. Fig. 5 shows how
the octahedron with furrowed edge may be constructed from two
interpenetrating tetrahedra (shown in dotted lines). If the grooves be
left out of account, the large faces which have replaced each
tetrahedron corner then make up a figure which has the aspect of a
simple octahedron. Such regular interpenetrations are known in
crystallography as "twins." There are also twins of diamond in which two
octahedra (fig. 6) are united by contact along a surface parallel to an
octahedron face without interpenetration. On account of their
resemblance to the twins of the mineral spinel (which crystallizes in
octahedra) these are known as "spinel twins." They are generally
flattened along the plane of union. The crystals often display
triangular markings, either elevations or pits, upon the octahedron
faces; the latter are particularly well defined and have the form of
equilateral triangles (fig. 7). They are similar to the "etched figures"
produced by moistening an octahedron of alum, and have probably been
produced, like them, by the action of some solvent. Similar, but
somewhat different markings are produced by the combustion of diamond in
oxygen, unaccompanied by any rounding of the edges.
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
Diamond possesses a brilliant "adamantine" lustre, but this tends to be
greasy on the surface of the natural stones and gives the rounded
crystals somewhat the appearance of drops of gum. Absolutely colourless
stones are not so common as cloudy and faintly coloured specimens; the
usual tints are grey, brown, yellow or white; and as rarities, red,
green, blue and black stones have been found. The colour can sometimes
be removed or changed at a high temperature, but generally returns on
cooling. It is therefore more probably due to metallic oxides than to
hydrocarbons. Sir William Crookes has, however, changed a pale yellow
diamond to a bluish-green colour by keeping it embedded in radium
bromide for eleven weeks. The black coloration upon the surface produced
by this process, as also by the electric bombardment in a vacuum tube,
appears to be due to a conversion of the surface film into graphite.
Diamond may break with a conchoidal fracture, but the crystals always
cleave readily along planes parallel to the octahedron faces: of this
property the diamond cutters avail themselves when reducing the stone to
the most convenient form for cutting; a sawing process, has, however,
now been introduced, which is preferable to that of cleavage. It is the
hardest known substance (though tantalum, or an alloy of tantalum now
competes with it) and is chosen as 10 in the mineralogist's scale of
hardness; but the difference in hardness between diamond (10) and
corundum (9) is really greater than that between corundum (9) and talc
(1); there is a difference in the hardness of the different faces; the
Borneo stones are also said to be harder than those of Australia, and
the Australian harder than the African, but this is by no means certain.
The specific gravity ranges from 3.56 to 3.50, generally about 3.52. The
coefficient of expansion increases very rapidly above 750 deg., and
diminishes very rapidly at low temperatures; the maximum density is
attained about -42 deg. C.
The very high refractive power (index = 2.417 for sodium light) gives
the stone its extraordinary brilliancy; for light incident within a
diamond at a greater angle than 24-1/2 deg. is reflected back into the
stone instead of passing through it; the corresponding angle for glass
is 40-1/2 deg. The very high dispersion (index for red light = 2.402,
for blue light = 2.460) gives it the wonderful "fire" or display of
spectral colours. Certain absorption bands at the blue end of the
spectrum are supposed to be due to rare elements such as samarium.
Unlike other cubic crystals, diamond experiences a diminution of
refractive index with increase of temperature. It is very transparent
for Rontgen rays, whereas paste imitations are opaque. It is a good
conductor of heat, and therefore feels colder to the touch than glass
and imitation stones. The diamond has also a somewhat greasy feel. The
specific heat increases rapidly with rising temperature up to 60 deg.
C., and then more slowly. Crystals belonging to the cubic system should
not be birefringent unless strained; diamond often displays double
refraction particularly in the neighbourhood of inclusions, both liquid
and solid; this is probably due to strain, and the spontaneous explosion
of diamonds has often been observed. Diamond differs from graphite in
being a bad conductor of electricity: it becomes positively electrified
by friction. The electrical resistance is about that of ordinary glass,
and is diminished by one-half during exposure by Rontgen rays; the
dielectric constant (16) is greater than that which should correspond to
the specific gravity.
The phosphorescence produced by friction has been known since the time
of Robert Boyle (1663); the diamond becomes luminous in a dark room
after exposure to sunlight or in the presence of radium; and many stones
phosphoresce beautifully (generally with a pale green light) when
subjected to the electric discharge in a vacuum tube. Some diamonds are
more phosphorescent than others, and different faces of a crystal may
display different tints. The combustibility of the diamond was predicted
by Sir Isaac Newton on account of its high refractive power; it was
first established experimentally by the Florentine Academicians in 1694.
In oxygen or air diamond burns at about 850 deg., and only continues to do
so if maintained at a high temperature; but in the absence of oxidising
agents it may be raised to a much higher temperature. It is, however,
infusible at the temperature of the electric arc, but becomes converted
superficially into graphite. Experiments on the combustion of diamond
were made by Smithson Tennant (1797) and Sir Humphry Davy (1816), with
the object of proving that it is pure carbon; they showed that burnt in
oxygen it yields exactly the same amount of carbon dioxide as that
produced by burning the same weight of carbon. Still more convincing
experiments were made by A. Krause in 1890. Similarly Guyton de Morveau
showed that, like charcoal, diamond converts soft iron into steel.
Diamond is insoluble in acid and alkalis, but is oxidised on heating
with potassium bichromate and sulphuric acid.
Bort (or Boart) is the name given to impure crystals or fragments
useless for jewels; it is also applied to the rounded crystalline
aggregates, which generally have a grey colour, a rough surface, often a
radial structure, and are devoid of good cleavage. They are sometimes
spherical ("shot bort"). Carbonado or "black diamond," found in Bahia
(also recently in Minas Geraes), is a black material with a minutely
crystalline structure somewhat porous, opaque, resembling charcoal in
appearance, devoid of cleavage, rather harder than diamond, but of less
specific gravity; it sometimes displays a rude cubic crystalline form.
The largest specimen found (1895) weighed 3078 carats. Both bort and
carbonado seem to be really aggregates of crystallized diamond, but the
carbonado is so nearly structureless that it was till recently regarded
as an amorphous modification of carbon.
_Uses of the Diamond._--The use of the diamond for other purposes than
jewelry depends upon its extreme hardness: it has always been the only
material used for cutting or engraving the diamond itself. The
employment of powdered bort and the lapidary's wheel for faceting
diamonds was introduced by L. von Berquen of Bruges in 1476. Diamonds
are now employed not only for faceting precious stones, but also for
cutting and drilling glass, porcelain, &c,; for fine engraving such as
scales; in dentistry for drilling; as a turning tool for electric-light
carbons, hard rubber, &c.; and occasionally for finishing accurate
turning work such as the axle of a transit instrument. For these tools
the stone is actually shaped to the best form: it is now electroplated
before being set in its metal mount in order to secure a firm fastening.
It is also used for bearings in watches and electric meters. The best
glaziers' diamonds are chosen from crystals such that a natural curved
edge can be used. For rock drills, and revolving saws for stone cutting,
either diamond, bort or carbonado is employed, set in steel tubes, disks
or bands. Rock drilling is the most important industrial application;
and for this, owing to its freedom from cleavage, the carbonado is more
highly prized than diamond; it is broken into fragments about 3 carats
in weight; and in 1905 the value of carbonado was no less than from L10
to L14 a carat. It has been found that the "carbons" in drills can
safely be subjected to a pressure of over 60 kilograms per square
millimetre, and a speed of 25 metres per second. A recent application of
the diamond is for wire drawing; a hole tapering towards the centre is
drilled through a diamond, and the metal is drawn through this. No other
tool is so endurable, or gives such uniform thickness of wire.
_Distribution and Mining._--The most important localities for diamonds
have been: (1) India, where they were mined from the earliest times till
the close of the 19th century; (2) South America, where they have been
mined since the middle of the 18th century; and (3) South Africa, to
which almost the whole of the diamond-mining industry has been
transferred since 1870.
_India._--The diamond is here found in ancient sandstones and
conglomerates, and in the river gravels and sands derived from them.
The sandstones and conglomerates belong to the Vindhyan formation and
overlie the old crystalline rocks: the diamantiferous beds are well
defined, often not more than 1 ft. in thickness, and contain pebbles
of quartzite, jasper, sandstone, slate, &c. The mines fall into five
groups situated on the eastern side of the Deccan plateau about the
following places (beginning from the south), the first three being in
Madras. (1) Chennur near Cuddapah on the river Pennar. (2) Kurnool
near Baneganapalle between the rivers Pennar and Kistna. (3) Kollar
near Bezwada on the river Kistna. (4) Sambalpur on the river Mahanadi
in the Central Provinces. (5) Panna near Allahabad, in Bundelkhand.
The mining has always been carried on by natives of low caste, and by
primitive methods which do not differ much from those described by the
French merchant Jean Baptiste Tavernier (1605-1689), who paid a
prolonged visit to most of the mines between 1638 and 1665 as a
dealer in precious stones. According to his description shallow pits
were sunk, and the gravel excavated was gathered into a walled
enclosure where it was crushed and water was poured over it, and it
was finally sifted in baskets and sorted by hand. The buying and
selling was at that period conducted by young children. In more modern
times there has been the same excavation of shallow pits, and
sluicing, sifting and sorting, by hand labour, the only machinery used
being chain pumps made of earthen bowls to remove the water from the
deeper pits.
At some of the Indian localities spasmodic mining has been carried on
at different periods for centuries, at some the work which had been
long abandoned was revived in recent times, at others it has long been
abandoned altogether. Many of the large stones of antiquity were
probably found in the Kollar group, where Tavernier found 60,000
workers in 1645 (?), the mines having, according to native accounts,
been discovered about 100 years previously. Golconda was the fortress
and the market for the diamond industry at this group of mines, and so
gave its name to them. The old mines have now been completely
abandoned, but in 1891 about 1000 carats were being raised annually in
the neighbourhood of Hyderabad. The Sambalpur group appear to have
been the most ancient mines of all, but they were not worked later
than 1850. The Panna group were the most productive during the 19th
century. India was no doubt the source of all the large stones of
antiquity; a stone of 67-3/8 carats was found at Wajra Karur in the
Chennur group in 1881, and one of 210-1/2 carats at Hira Khund in 1809.
Other Indian localities besides those mentioned above are Simla, in
the N.W. Provinces, where a few stones have been found, and a district
on the Gouel and the Sunk rivers in Bengal, which V. Ball has
identified with the Soumelpour mentioned by Tavernier. The mines of
Golconda and Kurnool were described as early as 1677 in the twelfth
volume of the _Philosophical Transactions_ of the Royal Society. At
the present time very few Indian diamonds find their way out of the
country, and, so far as the world's supply is concerned, Indian mining
of diamonds may be considered extinct. The first blow to this industry
was the discovery of the Brazilian mines in Minas Geraes and Bahia.
_Brazil._---Diamonds were found about 1725 at Tejuco (now Diamantina)
in Minas Geraes, and the mining became important about 1740. The chief
districts in Minas Geraes are (1) Bagagem on the W. side of the Serra
da Mata da Corda; (2) Rio Abaete on the E. side of the same range;
these two districts being among the head waters of the Rio de San
Francisco and its tributaries; (3) Diamantina, on and about the
watershed separating the Rio de San Francisco from the Rio
Jequitinhonha; and (4) Grao Mogul, nearly 200 m. to the N.E. of
Diamantina on the latter river.
The Rio Abaete district was worked on a considerable scale between
1785 and 1807, but is now abandoned. Diamantina is at present the most
important district; it occupies a mountainous plateau, and the
diamonds are found both on the plateau and in the river valleys below
it. The mountains consist here of an ancient laminated micaceous
quartzite, which is in parts a flexible sandstone known as
itacolumite, and in parts a conglomerate; it is interbedded with
clay-slate, mica-schist, hornblende-schist and haematite-schist, and
intersected by veins of quartz. This series is overlain unconformably
by a younger quartzite of similar character, and itself rests upon the
crystalline schists. The diamond is found under three conditions: (1)
in the gravels of the present rivers, embedded in a ferruginous
clay-cemented conglomerate known as _cascalho_; (2) in terraces
(gupiarras) in a similar conglomerate occupying higher levels in the
present valleys; (3) in plateau deposits in a coarse surface
conglomerate known as _gurgulho_, the diamond and other heavy minerals
being embedded in the red clay which cements the larger blocks. Under
all these three conditions the diamond is associated with fragments of
the rocks of the country and the minerals derived from them,
especially quartz, hornstone, jasper, the polymorphous oxide of
titanium (rutile, anatase and brookite), oxides and hydrates of iron
(magnetite, ilmenite, haematite, limonite), oxide of tin, iron
pyrites, tourmaline, garnet, xenotime, monazite, kyanite, diaspore,
sphene, topaz, and several phosphates, and also gold. Since the heavy
minerals of the _cascalho_ in the river beds are more worn than those
of the terraces, it is highly probable that they have been derived by
the cutting down of the older river gravels represented by the
terraces; and since in both deposits the heavy minerals are more
abundant near the heads of the valleys in the plateau, it is also
highly probable that both have really been derived from the plateau
deposit. In the latter, especially at Sao Joao da Chapada, the
minerals accompanying the diamond are scarcely worn at all; in the
terraces and the river beds they are more worn and more abundant; the
terraces, therefore, are to be regarded as a first concentration of
the plateau material by the old rivers; and the _cascalho_ as a second
concentration by the modern rivers. The mining is carried on by
negroes under the supervision of overseers; the _cascalho_ is dug out
in the dry season and removed to a higher level, and is afterwards
washed out by hand in running water in shallow wooden basins
(_bateas_). The terraces can be worked at all seasons, and the
material is partly washed out by leading streams on to it. The washing
of the plateau material is effected in reservoirs of rain water.
It is difficult to obtain an estimate of the actual production of the
Minas Geraes mines, for no official returns have been published, but
in recent years it has certainly been rivalled by the yield in Bahia.
The diamond here occurs in river gravels and sands associated with
the same minerals as in Minas Geraes; since 1844 the richest mines
have been worked in the Serra de Cincora, where the mountains are
intersected by the river Paraguassu and its tributaries; it is said
that there were as many as 20,000 miners working here in 1845, and it
was estimated that 54,000 carats were produced in Bahia in 1858. The
earlier workings were in the Serra de Chapada to the N.W. of the mines
just mentioned. In 1901 there were about 5000 negroes employed in the
Bahia mines; methods were still primitive; the _cascalho_ was dug out
from the river beds or tunnelled out from the valley side, and washed
once a week in sluices of running water, where it was turned over with
the hoe, and finally washed in wooden basins and picked over by hand;
sometimes also the diamantiferous material is scooped out of the bed
of the shallow rivers by divers, and by men working under water in
caissons. It is almost exclusively in the mines of Bahia, and in
particular in the Cincora district, that the valuable carbonado is
found. The carbonado and the diamond have been traced to an extensive
hard conglomerate which occurs in the middle of the sandstone
formation. Diamonds are also mined at Salobro on the river Pardo not
far inland from the port of Canavieras in the S.E. corner of Bahia.
The enormous development of the South African mines, which supplied in
1906, about 90% of the world's produce, has thrown into the shade the
Brazilian production; but the _Bulletin_ for Feb. 1909 of the
International Bureau of American Republics gave a very confident
account of its future, under improved methods.
_South Africa._---The first discovery was made in 1867 by Dr W. G.
Atherstone, who identified as diamond a pebble obtained from a child
in a farm on the banks of the Orange river and brought by a trader to
Grahamstown; it was bought for L500 and displayed in the Paris
Exhibition of that year. In 1869 a stone weighing 83-1/2 carats was
found near the Orange river; this was purchased by the earl of Dudley
for L25,000 and became famous as the "Star of South Africa." A rush of
prospectors at once took place to the banks of the Orange and Vaal
rivers, and resulted in considerable discoveries, so that in 1870
there was a mining camp of no less than 10,000 persons on the "River
Diggings." In the River Diggings the mining was carried on in the
coarse river gravels, and by the methods of the Brazilian negroes and
of gold placer-miners. A diggers' committee limited the size of claims
to 30 ft. square, with free access to the river bank; the gravel and
sand were washed in cradles provided with screens of perforated metal,
and the concentrates were sorted by hand on tables by means of an iron
scraper.
But towards the close of 1870 stones were found at Jagersfontein and
at Dutoitspan, far from the Vaal river, and led to a second great rush
of prospectors, especially to Dutoitspan, and in 1871 to what is now
the Kimberley mine in the neighbourhood of the latter. At each of
these spots the diamantiferous area was a roughly circular patch of
considerable size, and in some occupied the position of one of those
depressions or "pans" so frequent in S. Africa. These "dry diggings"
were therefore at first supposed to be alluvial in origin like the
river gravels; but it was soon discovered that, below the red surface
soil and the underlying calcareous deposit, diamonds were also found
in a layer of yellowish clay about 50 ft. thick known as "yellow
ground." Below this again was a hard bluish-green serpentinous rock
which was at first supposed to be barren bed-rock; but this also
contained the precious stone, and has become famous, under the name of
"blue ground," as the matrix of the S. African diamonds. The yellow
ground is merely decomposed blue ground. In the Kimberley district
five of these round patches of blue ground were found within an area
little more than 3 m. in diameter; that at Kimberley occupying 10
acres, that at Dutoitspan 23 acres. There were soon 50,000 workers on
this field, the canvas camp was replaced by a town of brick and iron
surrounded by the wooden huts of the natives, and Kimberley became an
important centre.
It was soon found that each mine was in reality a huge vertical funnel
or crater descending to an unknown depth, and filled with
diamantiferous blue ground. At first each claim was an independent pit
31 ft. square sunk into the blue ground; the diamantiferous rock was
hoisted by bucket and windlass, and roadways were left across the pit
to provide access to the claims. But the roadways soon fell in, and
ultimately haulage from the claims could only be provided by means of
a vast system of wire ropes extending from a triple staging of
windlasses erected round the entire edge of the mine, which had by
this time become a huge open pit; the ropes from the upper windlasses
extended to the centre, and those from the lower tier to the sides of
the pit; covering the whole mass like a gigantic cobweb. (See Plate
II. fig. 12.) The buckets of blue ground were hauled up these ropes by
means of horse whims, and in 1875 steam winding engines began to be
employed. By this time also improved methods in the treatment of the
blue ground were introduced. It was carried off in carts to open
spaces, where an exposure of some weeks to the air was found to
pulverize the hard rock far more efficiently than the old method of
crushing with mallets. The placer-miner's cradle and rocking-trough
were replaced by puddling troughs stirred by a revolving comb worked
by horse power; reservoirs were constructed for the scanty
water-supply, bucket elevators were introduced to carry away the
tailings; and the natives were confined in compounds. For these
improvements co-operation was necessary; the better claims, which in
1872 had risen from L100 to more than L4000 in value, began to be
consolidated, and a Mining Board was introduced.