PHYSICAL AND CHEMICAL PROPERTIES OF THE DIAMOND
I need scarcely say the diamond is almost pure carbon, and it is
the hardest substance in nature.
When heated in air or oxygen to a temperature varying from 760°
to 875° C., according to its hardness, the diamond burns with
production of carbonic acid. It leaves an extremely light ash,
sometimes retaining the shape of the crystal, consisting of iron,
lime, magnesia, silica, and titanium. In boart and carbonado
the amount of ash sometimes rises to 4 per cent, but in clear
crystallised diamonds it is seldom higher than 0·05 per cent. By
far the largest constituent of the ash is iron.
The following table shows the temperatures of combustion in oxygen
of different kinds of carbon:
°C.
Condensed vapour of carbon 650
Carbon from sugar, heated in an electrical furnace 660
Artificial graphites, generally 660
Graphite from ordinary cast-iron 670
Carbon from blue ground, of an ochre colour 690
Carbon from blue ground, very hard and black 710
Diamond, soft Brazilian 760
Diamond, hard Kimberley 780
Boart from Brazil 790
Boart from Kimberley 790
Boart, very hard, almost impossible to cut 900
HARDNESS
Diamonds vary considerably in hardness, and even different parts
of the same crystal differ in their resistance to cutting and
grinding.
Beautifully white diamonds have been found at Inverel, New South
Wales, and from the rich yield of the mine and the white colour of
the stones great things were expected. In the first parcel which
came to England the stones were found to be so much harder than
South African diamonds that it was at first feared they would be
useless except for rock-boring purposes. The difficulty of cutting
them disappeared with improved appliances, and they now are highly
prized.
The famous Koh-i-noor, when being cut into its present form, showed
a notable variation in hardness. In cutting one of the facets near
a yellow flaw, the crystal became harder and harder the further
it was cut, until, after working the mill for six hours at the
usual speed of 2400 revolutions a minute, little impression was
made. The speed was increased to more than 3000, when the work
slowly proceeded. Other portions of the stone were found to be
comparatively soft, and became harder as the outside was cut away.
The intense hardness of the diamond can be illustrated by the
following experiment. On the flattened apex of a conical block
of steel place a diamond, and upon it bring down a second cone
of steel. On forcing together the two steel cones by hydraulic
pressure the stone is squeezed into the steel blocks without
injuring it in the slightest degree.
In an experiment I made at Kimberley the pressure gauge showed 60
atmospheres, and the piston being 3·2 inches diameter, the absolute
pressure was 3·16 tons, equivalent on a diamond of 12 square mm.
surface to 170 tons per square inch of diamond.
The use of diamond in glass-cutting I need not dwell on. So hard is
diamond in comparison to glass, that a suitable splinter of diamond
will plane curls off a glass plate as a carpenter’s tool will plane
shavings off a deal board. The illustration (Fig. 17) shows a few
diamond-cut glass shavings.
DENSITY OR SPECIFIC GRAVITY
The specific gravity of the diamond varies ordinarily from 3·514
to 3·518. For comparison, I give in tabular form the specific
gravities of the different varieties of carbon and of the minerals
found on the sorting tables:
SPECIFIC
GRAVITY.
Amorphous carbon 1·45-1·70
Hard gas coke 2·356
Hard graphite 2·5
Quartzite and granite 2·6
Beryl 2·7
Mica 2·8
Hornblende 3·0
Boart 3·47-3·49
Carbonado 3·50
Diamond 3·514-3·518
Garnet 3·7
Corundum 3·8
Zircon 4·4
Barytes 4·5
Chrome and titanic iron ore 4·7
Magnetite 5·0
There is a substance, the double nitrate of silver and thallium,
which, while solid at ordinary temperatures, liquefies at 75° C.
and then has a specific gravity of 4·5. Admixture with water lowers
the density to any desired point.
If a glass cell is taken containing this liquid diluted to a
density of about 3·6, and in it is thrown pieces of the above-named
minerals, all those whose density is lower than 3·6 will rise to
the surface, while the denser minerals will sink. If now a little
water is carefully added with constantly stirring until the density
of the liquid is reduced to that of the diamond, the heterogeneous
collection sorts itself into three parts. The graphite, quartz,
beryl, mica, and hornblende rise to the surface; the garnet,
corundum, zircons, etc., sink to the bottom, while the diamonds
float in the middle of the liquid. With a platinum landing-net I
can skim off the swimmers and put them into one dish; with the
same net I can fish out the diamonds and put them in a second
dish, while by raising a sieve at the bottom I can remove the heavy
minerals and put them into a third. The accurate separation of
diamonds from the heterogeneous mixture can be effected in less
time than is taken to describe the experiment.
The table shows that diamonds vary somewhat in density among
themselves, between narrow limits. Occasionally, however, diamonds
overpass these figures. Here is an illustration. In a test-tube of
the same dense liquid are three selected diamonds. One rises to the
top, another floats uncertain where to settle, rising and falling
as the temperature of the sorting liquid is raised or lowered,
whilst the third sinks to the bottom. Allowing the liquid to cool
a degree or two slightly increases the density and sends all three
to the surface.
PHOSPHORESCENCE OF DIAMOND
After exposure for some time to the sun many diamonds glow in
a dark room. Some diamonds are fluorescent, appearing milky in
sunlight. In a vacuum, exposed to a high-tension current of
electricity, diamonds phosphoresce of different colours, most South
African diamonds shining with a bluish light. Diamonds from other
localities emit bright blue, apricot, pale blue, red, yellowish
green, orange, and pale green light. The most phosphorescent
diamonds are those which are fluorescent in the sun. One beautiful
green diamond in the writer’s collection, when phosphorescing in a
good vacuum, gives almost as much light as a candle, and you can
easily read by its rays. But the time has hardly come when diamonds
can be used as domestic illuminants! The emitted light is pale
green, tending to white, and in its spectrum, when strong, can be
seen bright lines, one at about λ 5370 in the green, one at λ 5130
in the greenish blue, and one at λ 5030 in the blue. A beautiful
collection of diamond crystals belonging to Professor Maskelyne
phosphoresces with nearly all the colours of the rainbow, the
different faces glowing with different shades of colour. Diamonds
which phosphoresce red generally show the yellow sodium line on
a continuous spectrum. In one Brazilian diamond phosphorescing a
reddish-yellow colour I detected in its spectrum the citron line
characteristic of yttrium.
The rays which make the diamond phosphoresce are high in the
ultra-violet. To illustrate this phosphorescence under the
influence of the ultra-violet rays, arrange a powerful source
of these rays, and in front expose a design made up of certain
minerals, willemite, franklinite, calcite, etc.--phosphorescing
of different colours. Their brilliant glow ceases entirely when a
thin piece of glass is interposed between them and the ultra-violet
lamp.
I now draw attention to a strange property of the diamond, which
at first sight might seem to discount the great permanence and
unalterability of this stone. It has been ascertained that the
cause of phosphorescence is in some way connected with the
hammering of the electrons, violently driven from the negative
pole on to the surface of the body under examination, and so great
is the energy of the bombardment, that impinging on a piece of
platinum or even iridium, the metal will actually melt. When the
diamond is thus bombarded in a radiant matter tube the result is
startling. It not only phosphoresces, but becomes discoloured,
and in course of time becomes black on the surface. Some diamonds
blacken in the course of a few minutes, while others require an
hour or more to discolour. This blackening is only superficial,
and although no ordinary means of cleaning will remove the
discolouration, it goes at once when the stone is polished with
diamond powder. Ordinary oxidising reagents have little or no
effect in restoring the colour.
[Illustration: FIG. 17. DIAMOND-CUT GLASS AND SHAVINGS.]
[Illustration: FIG. 18. DIAMONDS IN RÖNTGEN RAYS.
A. BLACK DIAMOND IN GOLD FRAME.
B. PINK DELHI DIAMOND.
C. PASTE IMITATION OF B.
To face p. 98.]
The superficial dark coating on a diamond after exposure to
molecular bombardment I have proved to be graphite. M. Moissan has
shown that this graphite, on account of its great resistance to
oxidising reagents, cannot have been formed at a lower temperature
than 3600° C.
It is thus manifest that the bombarding electrons, striking the
diamond with enormous velocity, raise the superficial layer to the
temperature of the electric arc and turn it into graphite, whilst
the mass of diamond and its conductivity to heat are sufficient to
keep down the general temperature to such a point that the tube
appears scarcely more than warm to the touch.
A similar action occurs with silver, the superficial layers of
which can be raised to a red heat without the whole mass becoming
more than warm.
CONVERSION OF DIAMOND INTO GRAPHITE
Although we cannot convert graphite into diamond, we can change
the diamond into graphite. A clear crystal of diamond is placed
between two carbon poles, and the poles with intervening diamond
are brought together and an arc formed between. The temperature of
the diamond rapidly rises, and when it approaches 3600° C., the
vaporising point of carbon, it breaks down, swells, and changes
into black and valueless graphite.
TRIBO-LUMINESCENCE
A few minerals give out light when rubbed. In the year 1663 the
Hon. Robert Boyle read a paper before the Royal Society, in which
he described several experiments made with a diamond which markedly
showed tribo-luminescence. As specimens of tribo-luminescent bodies
I may instance sphalerite (sulphide of zinc), and an artificial
sphalerite, which is even more responsive to friction than the
native sulphide.[6]
Mrs. Kunz, wife of the well-known New York mineralogist, possesses,
perhaps, the most remarkable of all phosphorescing diamonds. This
prodigy diamond will phosphoresce in the dark for some minutes
after being exposed to a small pocket electric light, and if rubbed
on a piece of cloth a long streak of phosphorescence appears.
ABSORPTION SPECTRUM OF DIAMOND
On passing a ray of light through a diamond and examining it in a
spectroscope, Walter has found in all colourless brilliants of over
1 carat in weight an absorption band at wave-length 4155 (violet).
He ascribes this band to an impurity and suggests it may possibly
be due to samarium. Three other fainter lines were detected in the
ultra-violet by means of photography.
REFRACTIVITY
But it is not the hardness of the diamond so much as its optical
qualities that make it so highly prized. It is one of the most
refracting substances in nature, and it also has the highest
reflecting properties. In the cutting of diamonds advantage is
taken of these qualities. When cut as a brilliant the facets on
the lower side are inclined so that light falls on them at an
angle of 24° 13´, at which angle all the incident light is totally
reflected. A well-cut brilliant should appear opaque by transmitted
light except at a small spot in the middle where the table and
culet are opposite. All the light falling on the front of the
stone is reflected from the facets, and the light passing into the
diamond is reflected from the interior surfaces and refracted
into colours when it passes out into the air, giving rise to the
lightnings, the effulgence, and coruscations for which the diamond
is supreme above all other gems.
The following table gives the refractive indices of diamonds and
other bodies:
REFRACTIVE INDICES FOR THE D LINE
Chromate of lead 2·50-2·97
Diamond 2·47-2·75
Phosphorus 2·22
Sulphur 2·12
Ruby 1·78
Thallium glass 1·75
Iceland spar 1·65
Topaz 1·61
Beryl 1·60
Emerald 1·59
Flint glass 1·58
Quartz 1·55
Canada balsam 1·53
Crown glass 1·53
Fluor-spar 1·44
Ice 1·31
In vain I have searched for a liquid of the same refraction as
diamond. Such a liquid would be invaluable to the merchant, as
on immersing a stone the clear body would absolutely disappear,
leaving in all their ugliness the flaws and black specks so
frequently seen even in the best stones.
THE DIAMOND AND POLARISED LIGHT
Having no double refraction, the diamond should not act on
polarised light. But as is well known, if a transparent body which
does not so act is submitted to strain of an irregular character
it becomes doubly refracting, and in the polariscope reveals the
existence of the strain by brilliant colours arranged in a more or
less defined pattern, according to the state of tension in which
the crystal exists. I have examined many hundred diamond crystals
under polarised light, and with few exceptions the colours show how
great is the strain to which some of them are exposed. On rotating
the polariser, the black cross most frequently seen revolves round
a particular point in the inside of the crystal; on examining
this point with a high power we sometimes see a slight flaw, more
rarely a minute cavity. The cavity is filled with gas at enormous
pressure, and the strain is set up in the stone by the effort of
the gas to escape. I have already said that the great Cullinan
diamond by this means revealed a state of considerable internal
stress and strain.
So great is this strain of internal tension that it is not uncommon
for a diamond to explode soon after it reaches the surface, and
some have been known to burst in the pockets of the miners or when
held in the warm hand. Large crystals are more liable to burst than
smaller pieces. Valuable stones have been destroyed in this way,
and it is whispered that cunning dealers are not averse to allowing
responsible clients to handle or carry in their warm pockets large
crystals fresh from the mine. By way of safeguard against explosion
some dealers imbed large diamonds in raw potato to ensure safe
transit to England.
The anomalous action which many diamonds exert on polarised light
is not such as can be induced by heat, but it can easily be
conferred on diamonds by pressure, showing that the strain has not
been produced by sudden cooling, but by sudden lowering of pressure.
The illustration of this peculiarity is not only difficult, but
sometimes exceedingly costly--difficult because it is necessary to
arrange for projecting on the screen the image of a diamond crystal
between the jaws of a hydraulic press, the illuminating light
having to pass through delicate optical polarising apparatus--and
costly because only perfectly clear crystals can be used, and
crystals of this character sometimes fly to pieces as the pressure
rises. At first no colour is seen on the screen, the crystal not
being birefringent. A movement of the handle of the press, however,
gives the crystal a pinch, instantly responded to by the colours on
the screen, showing the production of double refraction. Another
movement of the handle brightens the colours, and a third may
strain the crystal beyond its power of resistance, when the crystal
flies to pieces.
THE DIAMOND AND RÖNTGEN RAYS
The diamond is remarkable in another respect. It is extremely
transparent to the Röntgen rays, whereas highly refracting glass,
used in imitation diamonds, is almost perfectly opaque to the rays.
I exposed for a few seconds over a photographic plate to the X-rays
the large Delhi diamond of a rose-pink colour weighing 31½ carats,
a black diamond weighing 23 carats, and a glass imitation of the
pink diamond (Fig. 18). On development the impression where the
diamond obscured the rays was found to be strong, showing that
most rays passed through, while the glass was practically opaque.
By this means imitation diamonds can readily be distinguished from
true gems.
ACTION OF RADIUM ON DIAMOND
The β-rays from radium having like properties to the stream of
negative electrons in a radiant matter tube, it was of interest to
ascertain if they would exert a like difference on diamond. The
diamond glows under the influence of the β-radiations, and crushed
diamond cemented to a piece of card or metal makes an excellent
screen in a spinthariscope--almost as good as zinc sulphide. Some
colourless crystals of diamond were imbedded in radium bromide and
kept undisturbed for more than twelve months. At the end of that
time they were examined. The radium had caused them to assume a
bluish-green colour, and their value as “fancy stones” had been
increased.
This colour is persistent and penetrates below the surface. It
is unaffected by long-continued heating in strong nitric acid and
potassium chlorate, and is not discharged by heating to redness.
To find out if this prolonged contact with radium had communicated
to the diamond any radio-active properties, six diamonds were put
on a photographic plate and kept in the dark for a few hours. All
showed radio-activity by darkening the sensitive plate, some being
more-active than others. Like the green tint, the radio-activity
persists after drastic treatment. To me this proves that
radio-activity does not merely consist in the adhesion of electrons
or emanations given off by radium to the surface of an adjacent
body, but the property is one involving layers below the surface,
and like the alteration of tint, is probably closely connected with
the intense molecular excitement the stone had experienced during
its twelve months’ burial in radium bromide.
A diamond that had been coloured by radium, and had acquired
strong radio-active properties, was slowly heated to dull redness
in a dark room. Just before visibility a faint phosphorescence
spread over the stone. On cooling and examining the diamond it was
found that neither the colour nor the radio-activity had suffered
appreciably.
BOILING- AND MELTING-POINT OF CARBON
On the average the critical point of a substance is 1·5 times its
absolute boiling-point. Therefore the critical point of carbon
should be about 5800° Ab. But the absolute critical temperature
divided by the critical pressure is for all the elements so far
examined never less than 2·5; this being about the value Sir James
Dewar finds for hydrogen. So that, accepting this, we get the
maximum critical pressure as follows, viz. 2320 atmospheres:
(5800° Ab.)/CrP = 2.5, or CrP = (5800 Ab.)/2.5,
or 2320 atmospheres.
Carbon and arsenic are the only two elements that have a
melting-point above the boiling-point; and among compounds carbonic
acid and fluoride of silicium are the only other bodies with
similar properties. Now the melting-point of arsenic is about
1·2 times its absolute boiling-point. With carbonic acid and
fluoride of silicium the melting-points are about 1·1 times their
boiling-points. Applying these ratios to carbon, we find that its
melting-point would be about 4400°.
Therefore, assuming the following data:
Boiling-point 3870° Ab.
Melting-point 4400°
Critical temperature 5800°
Critical pressure 2320 Ats.
the Rankine or Van der Waals formula, calculated from the
boiling-point and critical data, would be as follows:
log. P = 10·11 - 39120/T,
and this gives for a temperature of 4400° Ab. a pressure of 16·6
Ats. as the melting-point pressure. The results of the formula are
given in the form of a table:
Temperature Pressure
Ab. Ats.
3870° 1·00 Boiling-point.
4000° 2·14
4200° 6·25
4400° 16·6 Melting-point.
4600° 40·4
4800° 91·2
5000° 193
5200° 386
5400° 735
5600° 1330
5800° 2320 Critical point (15 tons per square inch).
[Illustration: FIG. 19. CURVE OF VAPOUR PRESSURE OF CARBON]
If, then, we may reason from these rough estimates, above a
temperature of 5800° Ab. no amount of pressure will cause carbon
vapour to assume liquid form, whilst at 4400° Ab. a pressure
of above 17 atmospheres would suffice to liquefy some of it.
Between these extremes the curve of vapour pressure is assumed to
be logarithmic, as represented in the accompanying diagram. The
constant 39120 which occurs in the logarithmic formula enables
us to calculate the latent heat of evaporation. If we assume the
vapour density to be normal, or the molecule in vapour as C_{2},
then the heat of volatilisation of 12 grms. of carbon would
be 90,000 calories; or, if the vapour is a condensed molecule
like C_{6}, then the 12 grms. would need 30,000 calories. In the
latter case the evaporation of 1 grm. of carbon would require
2500 calories, whereas a substance like zinc needs only about 400
calories.