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DIAMOND
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DIAMOND , a See also:mineral universally recognized as See also:chief among See also:precious stones; it is the hardest, the most imperishable, and also the most brilliant of minerals.l These qualities alone have made it supreme as a See also:jewel since See also:early times, and yet the real brilliancy of the See also: But one or other of the remaining varieties mentioned by Pliny (the Macedonian, the Arabian, the See also:Cyprian, &c.) may be the true diamond, which was in great See also:request for the See also:tool of the See also:gem-engraver. Later See also:Roman authors mentioned various See also:rivers in India as yielding the Adamas among their sands. The name Adamas became corrupted into the forms See also:adamant, diamaunt, diamant, diamond; but the same word, owing to a See also:medieval misinterpretation which derived it from adamare (compare the See also: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 See also:power of averting See also:insanity, and of rendering See also:poison harmless; and in the See also:middle I Diamonds are invariably weighed in carats and in i1, of a See also:carat. One (See also:English) carat =3.17 grains= .2o54 See also:gram. One See also:ounce =1511 carats. (See CARAT.) ages it was known as the " pietra della reconciliazione," as the peacemaker between See also:husband and wife. Scientific Characters.—The See also: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 See also:attachment to any See also:matrix; the stones were, until the See also:discovery of the See also:South See also:African mines, almost entirely derived from sands or gravels, but owing to the hardness of the mineral it is rarely, if ever, See also:water-worn, and the crystals are often very perfect. The crystals belong to the cubic See also:system, generally assuming the See also:form of the See also:octahedron (fig. I), but they may, in accordance with the principles of See also:crystallography, also occur in other forms symmetric-ally derived from the octahedron,—for example, the See also:cube, the I2-faced figure known as the rhombic See also: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 See also:Brazil, but is very rare among the S. African stones; and the dodecahedron is perhaps more See also:common in Brazil than elsewhere. '..'""" "' =.""" There is often a furrow See also:running along the edges of the octahedron, or across the edges of the cube, and this indicates that the apparently See also:simple crystal may really consist of eight individuals meet- See also:ing at the centre; or, what comes to the same thing, of two individuals inter-penetrating and projecting through each other. If this be so the form of the diamond is really the See also:tetrahedron (and the various figures derived symmetrically from it) and not the octadehron. Fig. 5 shows how the octahedron with furrowed edge may be constructed from two interpenetrating tetrahedra (shown in dotted lines). If the grooves be See also: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 See also:regular interpenetrations are known FIG. 6. in crystallography as " twins." There are also twins of diamond in which two octahedra (fig. 6) are See also:united by contact along a See also:surface parallel to an octahedron See also:face without interpenetration. On account of their resemblance to the twins of the mineral See also:spinel (which crystallizes in octahedra) these are known as " spinel twins." They are generaIly flattened along the See also:plane of See also: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 See also:alum, and have probably been produced, like them, by the action of some solvent. Similar, but somewhat different markings are produced by the See also:combustion of diamond in See also:oxygen, unaccompanied by any rounding of the edges.
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 See also:appearance of drops of See also:gum. Absolutely colourless stones are not so common as cloudy and faintly coloured specimens; the usual tints are See also:grey, See also: Diamond may break with a conchoidal fracture, but the crystals always cleave readily along planes parallel to the octahedron faces: of this See also: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 See also:tantalum, or an alloy of tantalum now competes with it) and is chosen as to in the mineralogist's See also:scale of hardness; but the difference in hardness between diamond (10) and corundum (9) is really greater than that between corundum (9) and See also:talc (1); there is a difference in the hardness of the different faces; the See also:Borneo stones are also said to be harder than those of See also: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 7500, and diminishes very rapidly at See also:low temperatures; the maximum See also:density is attained about -42° C. The very high refractive power (See also:index = 2.417 for See also:sodium See also:light) gives the stone its extraordinary brilliancy; for light incident within a diamond at a greater See also:angle than 241° is reflected back into the stone instead of passing through it; the corresponding angle for See also:glass is 402°. The very high See also:dispersion (index for red light = 2.402, for blue light = 2.460) gives it the wonderful" See also:fire " or display of spectral See also: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 See also:Rontgen rays, whereas See also:paste imitations are opaque. It is a See also:good conductor of heat, and therefore feels colder to the See also:touch than glass and See also:imitation stones. The diamond has also a somewhat greasy feel. The specific heat increases rapidly with rising temperature up to 6o° C., and then more slowly. Crystals belonging to the cubic system should not be birefringent unless strained; diamond often displays See also:double See also:refraction particularly in the neighbourhood of inclusions, both liquid and solid; this is probably due to See also:strain, and the spontaneous See also:explosion of diamonds has often been observed. Diamond differs from graphite in being a See also:bad conductor of electricity: it becomes positively electrified by See also:friction. The See also:electrical resistance is about that of ordinary glass, and is diminished by one-See also:half during exposure by Rontgen rays; the See also:dielectric See also:constant (16) is greater than that which should correspond to the specific gravity. The See also:phosphorescence produced by friction has been known since the See also:time of See also:Robert See also:Boyle (1663); the diamond becomes luminous in a dark See also: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 See also: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 See also:Isaac See also:Newton on account of its high refractive power; it was first established experimentally by the Florentine Academicians in 1694. In oxygen or See also:air diamond See also:burns at about 85o°, and only continues to do so if maintained at a high temperature; but in the See also: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 ofdiamond were made by See also:Smithson See also:Tennant (1797) and Sir See also:Humphry See also:Davy (1816), with the See also:object of proving that it is pure See also:carbon; they showed that burnt in oxygen it yields exactly the same amount of carbon dioxide as that produced by burning the same See also:weight of carbon. Still more convincing experiments were made by A. See also:Krause in 1890. Similarly Guyton de Morveau showed that, like charcoal, diamond converts soft See also:iron into See also:steel. Diamond is insoluble in See also:acid and alkalis, but is oxidised on See also:heating with See also:potassium bichromate and sulphuric acid. See also: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 "). See also:Carbonado or " black diamond," found in See also:Bahia (also recently in See also: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 See also: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 See also: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 See also:wheel for faceting diamonds was introduced by L. von Berquen of See also:Bruges in 1476. Diamonds are now employed not only for faceting precious stones, but also for cutting and drilling glass, See also:porcelain, &c,; for See also:fine engraving such as scales; in See also:dentistry for drilling; as a turning tool for electric-light carbons, hard See also:rubber, &c.; and occasionally for See also:finishing accurate turning See also:work such as the See also:axle of a transit See also:instrument. For these tools the stone is actually shaped to the best form: it is now electroplated before being set in its See also:metal See also:mount in See also:order to secure a See also:firm fastening. It is also used for See also:bearings in watches and electric meters. The best glaziers' diamonds are chosen from crystals such that a natural curved edge can be used. For See also: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 See also: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 £10 to £14 a carat. It has been found that the " carbons " in drills can safely be subjected to a pressure of over 6o kilograms per square millimetre, and a See also:speed of 25 metres per second. A See also:recent application of the diamond is for See also:wire See also:drawing; a hole tapering towards the centre is drilled through a diamond, and the metal is See also:drawn through this. No other tool is so endurable, or gives such See also:uniform thickness of wire. See also:Distribution and See also:Mining.—The most important localities for diamonds have been: (1) India, where they were mined from the earliest times till the See also:close of the 19th See also:century; (2) South See also:America, where they have been mined since the middle of the 18th century; and (3) South See also:Africa, to which almost the whole of the diamond-mining See also:industry has been transferred since 1870. India.—The diamond is here found in See also:ancient sandstones and conglomerates, and in the See also: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 See also:quartzite, See also:jasper, See also:sandstone, See also:slate, &c. The mines fall into five See also:groups situated on the eastern See also:side of the See also:Deccan See also:plateau about the following places (beginning from the south), the first three being in See also:Madras. (1) Chennur near See also:Cuddapah on the river See also:Pennar. (2) See also:Kurnool near Baneganapalle between the rivers Pennar and See also:Kistna. (3) Kollar near See also:Bezwada on the river Kistna. (4) See also:Sambalpur on the river See also:Mahanadi in the Central Provinces. (5) See also:Panna near See also:Allahabad, in See also:Bundelkhand. The mining has always been carried on by natives of low See also:caste, and by See also:primitive methods which do not differ much from those described by the French See also:merchant See also:Jean See also:Baptiste See also: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 See also: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 See also:hand. The buying and selling was at that period conducted by See also:young See also:children. In more See also:modern times there has been the same excavation of shallow pits, and sluicing, sifting and sorting, by hand labour, the only machinery used being See also:chain pumps made of earthen See also: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 See also: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 See also:group, where Tavernier found 60,000 workers in 1645 (?), the mines having, according to native accounts, been discovered about too years previously. See also:Golconda was the fortress and the See also: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 moo carats were being raised annually in the neighbourhood of See also:Hyderabad. The Sambalpur group appear to have been the most ancient mines of all, but they were not worked later than 185o. 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 678 carats was found at Wajra Karur in the Chennur group in 1881, and one of 2101 carats at See also:Hira Khund in 1809. Other Indian localities besides those mentioned above are See also:Simla, in the N.W. Provinces, where a few stones have been found, and a See also:district on the Gouel and the Sunk rivers in See also:Bengal, which V. See also:Ball has identified with the Soumelpour mentioned by Tavernier. The mines of Golconda and Kurnool were described as early as 1677 in the twelfth See also:volume of the Philosophical Transactions of the Royal Society. At the See also:present time very few Indian diamonds find their way out of the See also:country, and, so far as the See also:world's See also:supply is concerned, Indian mining of diamonds may be considered See also:extinct. The first See also: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 See also:Diamantina) in Minas Geraes, and the mining became important about 1740. The chief districts in Minas Geraes are (I) 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 See also:head See also:waters of the Rio de See also:San Francisco and its tributaries; (3) Diamantina, on and about the See also:watershed separating the Rio de San Francisco from the Rio Jequitinhonha; and (4) Grao See also: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 See also:itacolumite, and in parts a See also:conglomerate; it is interbedded with See also:clay-slate, See also:mica-schist, See also:hornblende-schist and See also:haematite-schist, and intersected by See also:veins of See also:quartz. This See also:series is overlain unconformably by a younger quartzite of similar See also:character, and itself rests upon the crystalline See also: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 See also:oxide of See also:titanium (See also:rutile, See also:anatase and See also:brookite), oxides and hydrates of iron (See also:magnetite, See also:ilmenite, haematite, See also:limonite), oxide of See also:tin, iron See also:pyrites, See also:tourmaline, See also:garnet, xenotime, See also:monazite, kyanite, See also:diaspore, See also:sphene, See also:topaz, and several See also:phosphates, and also See 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 See also: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 super-See also:vision of overseers; the cascalho is dug out in the dry See also: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 See also:rain water. It is difficult to obtain an estimate of the actual See also:production of the Minas Geraes mines, for no See also: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 See also:week in sluices of running water, where it was turned over with the See also:hoe, and finally washed in wooden basins and picked over by hand; sometimes also the diamantiferous material is scooped out of the See also:bed of the shallow rivers by See also: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 See also:port of Canavieras in the S.E. corner of Bahia. The enormous development of the South Africanmines,which supplied in 1906, about 9o%of the world's produce, has thrown into the shade the Brazilian production; but the Bulletin for Feb. 1909 of the See also:International See also:Bureau of See also: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. See also:Atherstone, who identified as diamond a pebble obtained from a See also:child in a See also:farm on the See also:banks of the See also:Orange river and brought by a trader to Grahamstown; it was bought for £500 and displayed in the See also:Paris See also:Exhibition of that year. In 1869 a stone weighing 831 carats was found near the Orange river; this was See also:purchased by the See also:earl of See also:Dudley for £25,000 and became famous as the " See also:Star of South Africa." A See also:rush of prospectors at once took See also:place to the banks of the Orange and See also:Vaal rivers, and resulted in considerable discoveries, so that in 1870 there was a mining See also: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' See also:committee limited the See also:size of claims to 3o ft. square, with See also:free See also:access to the river See also:bank; the gravel and See also: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 See also: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 See also:Kimberley mine in the neighbourhood of the latter. At each of these spots the diamantiferous See also: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 See also: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 See also:diameter; that at Kimberley occupying to acres, that at Dutoitspan 23 acres. There were soon 50,000 workers on this See also: 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 See also:cradle and rocking-trough were replaced by puddling troughs stirred by a revolving See also:comb worked by horse power; reservoirs were constructed for the scanty water-supply, bucket See also: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 floc, to more than £4000 in value, began to be consolidated, and a Mining See also:Board was introduced. In a very few years, however, the open pit mining was rendered impossible by the mud rushes, by the falls of the masses of barren rock known as " See also:reef," which were left See also:standing in the mine, and by landslips from the sides, so that in 1883, when the pit had reached a depth of about 400 ft., mining in the Kimberley crater had become almost impossible. By 1889, in the whole group of mines, Kimberley, Dutoitspan, De Beers and Bultfontein, open pit working was practically abandoned. Meanwhile mining below the bottom of the pits by means of shafts and underground tunnels had been commenced; but the full development of modern methods See also:dates from the year 1889 when See also:Cecil See also:Rhodes and See also:Alfred See also:Beit, who had already secured See also:control of the De Beers mine, acquired also the control of the Kimberley mine, and shortly afterwards consolidated the entire group in the hands of the De Beers See also:Company. (See KIMBERLEY.) The See also:scene of native mining was now transferred from the open pit to underground tunnels; the vast network of wire ropes (Plate II. fig. 12) with their ascending and descending buckets disappeared, and with it the See also:cosmopolitan See also:crowd of busy miners working like ants at the bottom of the pit. In place of all this, the visitor to Kimberley encounters at the edge of the town only a huge crater, silent and apparently deserted, with no visible sign of the great mining operations which are conducted nearly half a mile below the surface. The aspect of the Kimberley pit in 1906 is shown in fig. 13 of Plate II., which may be compared with the See also:section of fig. 8. In fig. 13, Plate II., the sequence of the See also:basalt, shale and melaphyre is clearly visible on the sides of the pit; and fig. 8 shows how the crater or" See also:pipe " of blue ground has penetrated these rocks and also the underlying quartzite. The workings at De Beers had extended into the still more deeply seated See also:granite in 1906. Figure 9, elate I., shows the See also:top of the De Beers' crater with basalt over-lying the shale. Figure 8 also explains the modern system of mining introduced by See also:Gardner See also:Williams. A vertical See also:shaft is sunk in the vicinityof the mine, and from this See also:horizontal tunnels are driven into the pipe at different levels separated by intervals of 4oft. Through the blue ground itself on each level a series of parallel tunnels about 120 ft. apart are driven to the opposite side of the pipe, and at right angles to these, and 36 ft. apart, another series of tunnels. When the tunnels reach the side of the mine they are opened upwards and sideways so as to form a large chamber, and the overlying mass of blue ground and debris is allowed to See also:settle down and fill up the See also:gallery. On each level this process is carried somewhat farther back than on the level below (fig. 8) ; material is thus continually withdrawn from one side of the mine and extracted by means of the rock shaft on the opposite side, while the superincumbent debris is continually sinking, and is allowed to fall deeper on the side farthest from the shaft as the blue ground is withdrawn from beneath it. In 1905 the See also:main shaft had been sunk to a depth of 2600 ft. at the Kimberley mine. For the extraction and treatment of the blue ground the De Beers Company in its great winding and washing plant em-ploys labour-saving machinery on a gigantic scale. The ground is transferred in trucks to the shaft where it is automatically tipped into skips holding 96 cubic ft. (six See also:truck loads) ; these are rapidly hoisted to the surface, where their contents are automatically dumped into side-tipping trucks, and these in turn are drawn away in a continual procession by an endless wire rope along the tram lines leading to the vast " distributing floors." These are open tracts upon which the blue ground is spread out and left exposed to See also:sun and rain until it crumbles and disintegrates, the process being hastened by harrowing with steam ploughs; this may require a period of three or six months, or even a year. The stock of blue ground on the floors at one time in 1905 was nearly 4,500,000 loads. The disintegrated ground is then brought back in the trucks and fed through perforated cylinders into the washing pans; the hard blue which has resisted disintegration on the floors, and the lumps which are too big to pass the cylindrical See also:sieves, are crushed before going to the pans. These are shallow cylindrical troughs containing muddy water in which the diamonds and other heavy minerals (concentrates) are swept to the rim by revolving toothed arms, while the lighter stuff escapes near the centre of the See also:pan. The concentrates are then passed over sloping tables (pulsator) and shaken to and fro under a stream of water which effects a second concentration of the heaviest material. Until recently the final separation of the diamond from the concentrates was made by hand picking, but even this has now been replaced by machinery, owing to the remarkable discovery that a greased surface will hold a diamond while allowing the other heavy minerals to pass over it. The concentrates are washed down a sloping table of corrugated iron which is smeared with grease, and it is found that practically all the diamonds adhere to the table, and the other minerals are washed away. At the large and important Premier mine in the See also:Transvaal the Elmore process, used in See also:British See also:Columbia and in See also:Wales for the separation of metallic ores, has been also introduced. In the Elmore process oil is employed td See also:float off the materials which adhere to it. while the other materials remain in the water, the oil being separated from the water by centrifugal action. Additional information and CommentsThere are no comments yet for this article.
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