describing Johannesburg in 1891, says that it has much of the appearance
of an English manufacturing town, but without noise, smoke, or dirt.
“The streets are crowded with a busy, bustling, active, keen,
intelligent-looking throng. There are gathered together human beings
from every quarter of the globe, the English possessing an immense
predominance. The buildings and general architecture of the town attain
an excellent standard, style having been consulted and sought after,
stone and bricks the materials, corrugated iron being confined to the
roofs, solidity, permanence, and progress being the general
characteristics.”
The Randt mines having drawn into the South African Republic great
numbers of enterprising workers, who have acquired wealth and built
cities, it would have been expected that they would have been permitted
to acquire the ordinary rights of citizenship. The Boers’ character,
however, has been manifested by their refusal of such rights, and by
their exacting grievous imposts from these _Uitlanders_ (Out-landers, or
strangers), who, being for the most part of English race, are finding
the injustice too hard to be borne, and greatly strained relations
between the Transvaal and Great Britain have again supervened. Indeed,
the situation has become so serious that it is feared actual war may
result, and that is why, in almost the last year of our century, people
are looking anxiously at the position of affairs in South Africa.
The geological conditions of the Randt are these: the upper series of
beds in the _Karoo formation_, which extends over the greater part of
South Africa, consist of quartzose strata, and in the district in
question these are much broken, faulted, and variously inclined. They
are interstratified with beds of sandstone and with the layers of
gold-bearing conglomerate, of which last there are several parallel to
one another and not far separated, ranging in their several thicknesses
from 6 inches to 6 feet, the thickest being known as the main reef.
These reefs form an oval basin,—that is, they dip with varying angle
towards a centre, and crop out at their up-turned edges. Johannesburg is
situated nearly 6,000 feet above the sea-level, on an elevated ridge,
along which for 30 miles east, and nearly the same westwards, the
northern outcrop extends, curving towards the south, while the southern
edge of the basin appears in the Orange Free State, where it has been
traced for a distance of 130 miles. There a shaft, sunk to the great
depth of 2,400 feet, found the main reef with undiminished richness. The
outcrop of the reefs stretches east and west for 130 miles, and the
distance between north and south is 30 miles. From such data it has been
inferred that the reefs contain altogether not less than 450 million
pounds worth of gold. The conglomerate of these reefs consists of
rounded quartz pebbles (which contain no gold), and pieces of sandstone
and of argillaceous material, the whole cemented together into a very
hard mass by iron pyrites. This last is the matrix in which the gold
exists, in the form, for the most part, of minute scarcely visible
crystals. To a depth of from 50 to 150 feet, air and moisture have acted
on the pyritic matter, and the material of the reef becoming in
consequence easily disintegrated, has yielded by mere mechanical
treatment most of its gold, whereas by the same operations on the
underlying hard, tough conglomerate, only about half its gold could be
obtained. Hence, after breaking up the ore, the pyritic matter is sorted
out and transported to the stamp battery, reduced to powder, from which
about five-eighths of the contained gold is removed by quicksilver. The
residue is concentrated by washing in a special machine called the “Frue
vanner,” and the _concentrates_, after roasting in order to oxidize base
metals, are subjected to the action of chlorine gas, by which the gold
is converted into a soluble chloride, from the solution of which it is
precipitated by ferrous sulphate. The _tailings_, _slimes_, and other
residues are further acted on by a solution of potassium cyanide, which
dissolves the minute remaining particles of gold, and from the solution
the metal is obtained by electrolysis. By these supplementary chemical
processes the total of the gold recovered from the ore is raised to 90
per cent. or more of all that chemical analysis shows to exist.
When it is said that the reefs are arranged in a basin-like form, it
must be understood that this applies to their general disposition, for
the regularity of geometrical shape does not belong to geological
basins. There are considerable variations in the inclinations of the
reefs: at some places they are nearly vertical, but generally they dip
towards the centre at various angles, a slope between 25° and 45° being
quite usual; and the inclination becomes less and less the deeper they
go, so that it is presumed that the beds are level towards the centre of
the basin. In the Randt the vertical shaft is rather the exception, the
entrance to the mine usually following the inclination of the reefs, and
the trucks of ore are drawn up sloping rails. From the inclined adits
horizontal galleries are excavated right and left at various depths by
which the main reef is worked, and there are cross cuts by which the
reefs to the north and south may be reached. The most active district of
the Randt is that which extends eastward of Johannesburg, where a long
succession of tall chimneys and winding headgears together with the
other appurtenances are visible. But there is nothing of a picturesque
character about a gold mine, more than is presented by the aspect of an
ordinary colliery.
The importance of the Randt gold-fields does not consist in the actual
richness of the crude material, which indeed in places here and there
cannot be profitably worked,—in mining parlance, it is not “pay ore.” It
is rather the great ascertained extent of these gold-bearing beds and
the general persistence of their character throughout that give to the
Randt its unique character amongst metalliferous workings. This
contrasts with the comparative uncertainty attending the exploitation of
auriferous quartz veins, which occur in detached unconnected patches,
that often end suddenly where least expected. There are in the Randt
nearly one hundred companies working mines, and of these there are many
that pay very handsome dividends on their original capital. A few pay
100 per cent., while a considerable number distribute 25 per cent. and
upwards; so that some of these Gold Companies are amongst the richest
and most influential financial houses in the world. The Randt is second
only to the United States in the quantity it adds annually to the
world’s production.
_DIAMONDS._
In ancient times, and down to a comparatively late period, the only
region from which were derived all the diamonds that found their way to
Europe, was India, where Golconda was long celebrated for the productive
mines in its neighbourhood, and for the high estimation in which fine
specimens of their yield were held. In the seventeenth century these
mines employed 60,000 persons, it is said; and in other districts of
India diamond-seeking has also been carried on from time immemorial. A
gradual decrease in the finds of Indian diamonds has long been observed,
and the supremacy the East had so long enjoyed as the purveyor of gems
was in the earlier part of the eighteenth century transferred to another
hemisphere. In 1727 the diamond was first discovered in Brazil; or
rather, we might say, was then first discerned there. For the
gold-seekers in washing the sands of certain Brazilian rivers had found
numberless specimens which they either threw aside as worthless, or,
seeing them prettier pebbles than the rest, used them as counters in
their card games; their true nature was not recognized, because the
rough diamond has by no means the attractive appearance of the cut and
polished brilliant flashing with refractive radiance. It must have been
these last, and not diamonds in their natural state, that presented
themselves to the imagination of the poet when he penned the line—
Or deep with diamonds in the _flaming_ mine.
The announcement of some diamonds having been found in America had no
effect on the prices in the Indian market, but the exports that soon
after came from Brazil in great abundance quite changed the conditions
of the trade, for in the first fifty years their value was estimated at
no less than £12,000,000 sterling. As already stated, the presence of
diamonds in Brazil was not recognized until 1727, and then by the
accident of one Lobo, an inhabitant of the gold district of Minas
Geräes, who had been in India and had seen rough diamonds there,
observing the resemblance; he took some of the Brazilian stones to
Lisbon, where their identity with the products of the Indian mines was
established. But the European dealers, alarmed lest this discovery
should depreciate the value of their stocks of Indian gems, spread a
report that the so-called diamonds from Brazil were but the refuse of
the Indian mines that had been sent to Brazil. This had the effect of
stopping for a time the sale of the Brazilian diamonds; but the traders
in these were not above taking a hint from their rivals—_fas est et ab
hoste doceri_—for they carried their diamonds to Bengal, and there sold
them as Indian stones at Indian prices. For nearly one hundred and forty
years after this Brazil was by far the most productive diamond region in
the whole world, and especially after 1754, when diamond-seekers
congregated by thousands in the very rich fields of Bahia, a district of
Brazil. Nor have the places above mentioned been by any means the only
localities in Brazil where diamond-finders have been at work; but the
production has decreased and has lost its relative importance by the
South African discoveries that about 1870 caused an entire change in the
diamond industry, and the high prices of the Brazilian gems no longer
capable of being maintained, the fall in value has rendered the workings
less remunerative than formerly. We may now pass over with mere mention,
discoveries of diamondiferous districts in North America, Australia, and
elsewhere.
While rejecting as entirely inapplicable and inexcusable by any stretch
of poetic licence the epithet _flaming_ for the diamond mine, we must
question whether the word _mine_, that as the customary word we have
continued to use, does not convey an equally false notion of the nature
of the workings to which hitherto reference has been made. For these in
most cases are nothing more than holes, very much like gravel pits in
the side of a hill. The diamonds which have so far been in question are
usually found among alluvial sands or gravels, the water-worn fragments
of disintegrated rocks. These are in many cases carried down by rivers,
and the diamonds under such circumstances are very frequently
accompanied by gold; indeed, it is the search for gold that has in many
cases led to their discovery. In the dry season of the year, which
extends from April to October, the lessened currents of certain of the
Brazilian streams are diverted from their course into canals, so as to
leave dry the bed of the stream, and here the mud is dug out to the
depth of six or eight feet or more, and transported near the washing
huts, these operations being continued throughout the dry season. When
this is over the digging is necessarily interrupted by great volumes of
water that fill the rivers and streams, and the diamond-seekers devote
their attention to washing the mud that has been collected. About one
cwt. of this is placed in a long trough, and water is made to flow in,
while the negro labourer stirs up the mass with his hands, until the
water runs off clear, all the particles of mud having been washed away.
The residual gravel is then very carefully examined, stone by stone, and
any diamonds found are handed to the overseer, who watches all
proceedings from an elevated seat. These Brazilian diamonds are mostly
of a small size: occasionally, but very rarely, stones of quite
exceptional value are found, but perhaps not one in 10,000. Formerly
when in the Brazilian fields a negro slave found one of 18 carats, or
more (18 carats = 72 grains), he not only obtained his freedom, but was
rewarded with gifts, and for the finding of smaller stones commensurate
rewards were given. The value of a diamond of the larger sizes depends
upon so many adventitious circumstances that it would not be easy for
any one to state the money’s worth of an 18–carat stone, but,
considering too that the price increases in a more rapid ratio than the
weight, we may to some extent draw an inference from the published
values in 1867 of smaller Brazilian brilliants, perfectly white, pure,
and flawless, when one of 5 carats (20 grains) in weight was priced at
£350. As the rough diamond gives a brilliant of only half its weight, we
may from the above assume an 18–carat stone to be worth in its finished
state at least £1,000.
It may well be asked what are the qualities possessed by the diamond
which have caused it to be so highly valued as an adornment all the
world over; and here it will be proper to invite the reader’s attention
to the chemical as well as to the physical character of the diamond. The
most obvious and attractive quality of the cut brilliant is its
unsurpassable lustre, which is due to its high refractive power. In a
section of our article on light the subject of refraction has been dealt
with, and an explanation given of the _index of refraction_. That of the
diamond is the highest known, being 2·50 to 2·75; other precious stones
have indices ranging from 1·58 to 1·78; those of glass and of quartz are
between 1·50 and 1·57. It follows from the known laws of refraction that
the _limiting_ or critical angle is less for diamond than for other
substances, as, for example, glass, for the posterior surface of a
diamond will totally reflect all the light that falls upon it at any
angle with the normal greater than 24°; glass will totally reflect only
when the incidence is greater than about 42°: hence the diamond reflects
from its farther surface about 64 per cent. of rays that glass similarly
situated would allow to pass outwards without reflection.
Another property in which the diamond excels all other substances is
hardness. It is the hardest substance in nature; for a diamond will
_scratch_ every other, but by none can it be _scratched_, except by
another diamond. Not but that by the application of a file the edges of
a diamond or brilliant may be notched and broken; but this would be
through sheer mechanical force tearing the substance, and would be a
test of _brittleness_, not of _hardness_. These two properties have not
unfrequently been confounded, as when it was foolishly prescribed as a
test for the genuineness of a diamond, that it should be placed on an
anvil and struck with a hammer. No doubt many good and valuable stones
have been sacrificed by this ignorant treatment. The hardness of the
diamond does not prevent its being reducible to powder when so required.
Again, diamonds are sometimes in such a condition of internal strain
that very slight shocks are sufficient to cause them to separate into
fragments. We read of diamonds that are suspected to be in this
condition being packed for transmission within raw potatoes. The extreme
hardness of the diamond secures it from all those accidental abrasions
and injuries to which softer materials are liable, so that it does not
deteriorate by age or use. It is unaffected also by any chemical
substances.
In chemical composition the diamond is pure carbon, one of the most
commonly diffused of the elementary bodies, as it enters into the
constitution of the atmosphere, of all organic bodies, and of a vast
number of mineral substances. Carbon in a less pure form also occurs
naturally as _graphite_, _plumbago_ or _black lead_, and in other
conditions comes into ordinary use as already explained in our article
on Iron. It was only towards the end of the eighteenth century that the
composition of the diamond was demonstrated by the celebrated French
chemist, Lavoisier, who actually burnt a diamond in oxygen gas, and
found the resulting product to be carbonic acid gas, identical with that
obtained by similarly burning a piece of charcoal. Soon afterwards
another French chemist, Clouet, confirmed Lavoisier’s conclusion by
producing _steel_ from pure iron and diamond heated together, an
experiment of much significance when considered in the light of the
remarkable relation between these substances, which is one of the latest
discoveries of our century. It should be observed that Clouet’s result
implies a fusion of the diamond as well as of the iron in the act of
entering into chemical combination.
Like nearly every solid substance of definite chemical composition, this
pure carbon takes the crystalline form. The phenomena of crystallization
are of the highest interest and beauty, for in them we see shapeless
matter fashioning itself into definite and often perfect geometrical
solids, as if it had been wrought by the hand of some mathematical
artist. Every substance forms crystals of some one shape when the
conditions are identical, and one essential condition for any
crystallization is that the particles should be capable of free movement
in arranging themselves, and this condition can occur only when the
substance is in the state of liquid or of gas. Crystals are commonly
deposited from solutions when the solvent evaporates or is cooled down;
or they are formed when a fused substance solidifies. In either case the
crystals are the larger and more perfect as they are allowed the greater
time to form. Now, carbon in any of its conditions has been found to be
absolutely infusible and insoluble, and therefore the origin of the
diamond has long been a puzzle to scientific men, very diverse surmises
having been propounded on this subject. Some have thought it was
separated from carbonic acid by the action of heat, or of electricity;
others, that the carbon had been gasified by subterranean heat; others,
among whom were Newton and the German chemist, Liebig, believed that
heat had nothing to do with it, but that the crystals slowly separated
from vegetable matters (hydro-carbons) in the process of decomposition
under some unknown conditions; others, that the diamond crystallized out
from liquid carbonic acid, holding under pressure some unknown form of
carbon in solution; others, that carbon was ejected by volcanic action
in a fused state; and so on. We hope to show that the problem has at
length been solved, and how.
The shapes of the natural crystals of the diamond must not be confounded
with those of the cut brilliants. The most frequently met with of the
former is the octahedron, or eight-sided figure, such as would result
from two square pyramids joined base to base, the triangles forming the
sides of the pyramids being of such a height that the three pairs of
opposite points are equidistant one from another, so that the octahedron
enclosed in a cube would have an apex in the middle of each surface of
the cube. There are other shapes of diamond crystals, but they are all
related to the cube, that is, they are all obtainable from the cube by
successively slicing off edges and angles. The natural diamond sometimes
has as many as 48 faces formed by such a process. This will easily be
understood by the reader if he will take a cube of common soap and
perform on it these operations _gradually_ with a sharp knife, taking
care always to make the new faces he produces equally inclined to the
adjoining ones. He may begin by cutting off a tiny piece from one corner
of the cube, forming a small equilateral triangle; then let him do the
same at two opposite corners, and again at all the eight corners. Then
he should make the cuts larger and larger, always producing equal sized
equilateral triangles so long as these can be formed. In every case he
will have shaped out such forms as belong to diamond crystals. Instead
of this, he may pare off one or more edges of the cube, or he may in
various ways combine the two operations, and he will probably be
surprised at the variety of forms producible in this manner, all derived
from the original cube and all representing possible forms of natural
diamonds, and indeed those of any substance that crystallizes in the
_cubical system_. A model of the diamond octahedron can be readily made
from the description already given, and the whole series of operations
will constitute an elementary but very instructive lesson in the science
of crystallography.
Diamonds are liable to occur with every imaginable distortion, so as to
be scarcely recognizable by their external form. A very pure smooth
uncut diamond, belonging to the Rajah of Mattam in Borneo, is shaped
exactly like a pear, two inches in length. By the way, battles have been
fought for the possession of this gem, and it is said that,£200,000 was
vainly offered for it. The diamond, notwithstanding its hardness, splits
with comparative ease in certain planes, and by such cleavage (a
property common to all crystals) the octahedral form commonly emerges.
It was not until the middle of the fifteenth century that the art of
cutting the diamond into regular facets was practised, and this can be
done only by the aid of diamond powder, prepared by crushing fragments
and faulty stones in a hard steel mortar. The first operation is to
split the stone by its natural cleavage, and the rough facets so
produced of two diamonds are ground together until they are quite
smooth. The grinding of other facets and the polishing are effected on
horizontal discs of steel making 2,000 revolutions per minute, and
overspread with diamond powder mixed with olive oil.
The external surface of the diamond in its natural state is often very
rough, the stone being always coated with a more or less opaque crust,
so that its translucent interior is concealed or veiled; but when the
reflection from its inner surfaces pierces this veil it glows as if
lighted from within, giving that peculiar appearance which is called its
“fire.” The surfaces of the diamond crystals are very often curved
instead of being flat, and the dodecahedral shape, when this is the
case, takes on an almost globular appearance. Diamonds of all colours
are found, as well as the highly esteemed colourless stones. Yellow ones
of various tints are frequent,—orange, brown, and pink are not very
rare; but red, green, blue, and black are almost unique, at least in a
condition to form large and perfect gems, and are accordingly much
prized. The black diamonds found in Borneo are so hard that ordinary
diamond powder has no effect whatever upon them; they have to be
manipulated with their own dust. The nature of the substances that
impart these colours to the diamond has never been made out; they must
be excessively small in quantity. When a diamond is burnt in air or in
oxygen gas by aid of a large burning glass or otherwise, an extremely
minute quantity of ash remains, and this often retains the shape of the
stone, in the form of a most delicate network; and of the composition of
the ash, this much has been made out: it contains silica and _iron_. We
shall find that the presence of the last named element, although but in
the merest trace, is not without significance.
The purely utilitarian uses of the diamond are few, but of importance.
The most familiar is in the glazier’s tool for cutting glass, and in
connection with this we may mention a fact not generally known, namely,
that though any point of a diamond will _scratch_ glass, it is only by a
natural point of the crystal, and that point of a certain shape, that
glass can be _cut_. Another kind of diamond, valueless as a gem, has
been turned to good account in Major Beaumont’s invention, described in
our section on Rock Drilling Machines, to which the reader is referred.
Minute diamonds are employed for writing on glass, for very fine
engraving, etc.