DIALLAGE.

tain varieties, especially those possessing a characteristic lustre, have been cut and polished.

The name diallage is also applied to the mineral enstatite and its ferriferous variety, bronzite.

DIALOGUE (Lat. dialogus, Gk. diáλoyos, dialogos, speech, from diá, dia, through + 2óyos, logos, speech, from eyew, legein, to speak). A conversation between two or more persons, implying, however, greater unity of subject and formality than an ordinary conversation. The Socratic dialogue is a conversation in the form of question and answer, so contrived that the person questioned is led himself to originate those ideas that the questioner wishes to bring before him. The dialogues of Plato are, as it were, philosophical dramas, in which the Socratic method of investigation is brought to bear upon speculative subjects. And in general among the ancients the dialogue was a favorite form for didactic literature. Aristotle employed it, but in less dramatic form than Plato; and Cicero chose the Aristotelian dialogue for his philosophical works. Lucian employs it for purposes approaching those of the drama. Of the more eminent modern writers in this form, we may mention Erasmus in Latin; Hutten, Lessing, Herder, Wieland, and Schelling among the Germans; Petrarch and Macchiavelli in Italy; Fénelon, Fontanelle, and Voltaire in France; and Berkeley, Hurd, and Harris in England. Landor's Imaginary Conversations also are a happy effort of this kind. When dialogue is combined with action, we have the drama. On the ancient dialogue, consult Hirzel, Der Dialog (Leipzig, 1895).

DIALOGUE OF DEATH. A religious work by William Bullein, written and published during the London Plague (1664-65) and professing to be "a goodly regimente against the fever Pestilence."

DIAMAGNETISM (from Gk. diá, dia, through + máyvns, magnes, magnet, from Máy vs, Magnes, Magnesian, from Maymola, Magnésia, a city of Thessaly, in Greece, where magnetic stones abounded). A name given certain magnetic phenomena observed in the case of bismuth and some other substances. If a small piece of bismuth is brought near one pole of a strong magnet, it will be seen to be repelled. This is owing to two things: (1) the magnetic field at the end of the magnet is not uniform, being strong near the magnet and feeble at a distance; (2) the surrounding medium is air. If the field of magnetic force were uniform, there would be neither repulsion nor attraction, the piece of bismuth would simply turn and take up a definite position. Further, Faraday has shown that the question of attraction or repulsion depends not on the body itself, but on its magnetic properties with reference to those of the surrounding mediums. (See MAGNETISM.) Faraday showed that the following substances and a few others were diamagnetic when placed in air: bismuth, antimony, tin, zine, cadmium, mercury, silver, copper, gold, lead, sulphur, sugar, wood, water, alcohol.

DIAMANTE, dê'å-män'tå, JUAN BAUTISTA (c.1626-c.1690). A Spanish dramatist. He was born in Madrid, where about one-half of the fifty plays written by him and still extant were published in book-form (2 vols., 1670 and 1679).

DIAMOND.

His best work is the tragedy entitled El honrador de su padre. Among his other pieces La judía de Toledo has probably enjoyed the greatest popularity. He was a knight of Saint John of Jerusalem and probably, like Calderon, Lope de Vega, and several other celebrated poets, spent the closing years of his life in the midst of his brotherhood.

DIAMANTINA, dē'à-man-tē nå. An episcopal city in the State of Minas Geraes, Brazil, situated 146 miles northeast of Ouro Preto, and 4000 feet above sea-level (Map: Brazil, J 7). It is the centre of a diamond-producing district, and has diamond-cutting and goldsmiths' establishments, as well as cotton and leather manufactures, and a trade in diamonds amounting almost to $1,000,000 annually. Diamantina was founded in 1730. It formerly bore the name Tejuco. Population, in 1890, 14,000.

DI'AME'NIA. A genus of elapine venomous snakes of Australia. See GRAY-SNAKE.

DIAMETER (Lat. diametros, from Gk, diáμέτρος, diameter, from dia, dia, through + uέrpov, metron, measure). A diameter of a conic section (q.v.) is any chord which connects the points of contact of parallel tangents. In the circle, ellipse, and hyperbola, all diameters pass through the centre (see CURVES) and are bisected by it. Only the circle has all diameters equal, and each bisects the chords perpendicular the minor and major axis bisect the chords perto it. In the ellipse only the diameters called pendicular to them. In the parabola all diameters are parallel to the axis. (For conjugate diameters, see CONJUGATE.) Certain solid bodies also have diameters. In the sphere and the ellipsoid, the lines passing through the centre are called diameters and are bisected by the centre. See AXIS.

DIAMOND (OF., Fr. diamant, It., Sp. diamante, from Lat. adamas, adamant, diamond, from Gk. ȧdápas, adamas. adamant, from a, a priv. + dauav, daman, Skt. dam, to subdue). A precious stone composed of pure carbon, remarkable for hardness, lustre, and flash of color. It crystallizes in the isometric system, usually in octahedrons, or in combinations of several forms, including the octahedron, cube, rhombic dodecahedron, trisoctahedron, and hexoctahedron; the faces and edges of the crystals frequently are curved. It has a very perfect cleavage, by which the primary form of the octahedron may be derived. The diamond is not acted upon by acids or alkalies, but if heated to a high temperature in the presence of oxygen it burns to carbon dioxide. Its brilliant lustre and display of prismatic colors are due to the property of refracting and dispersing the light-rays, a property possessed in the highest degree by colorless stones. The specific gravity of the diamond is 3.52; its hardness, which is assumed to be 10 on the mineral scale, exceeds that of any other known substance. By friction it becomes phosphorescent and positively electric. The diamond has a great variety of natural colorings; white, yellow, and brown specimens are the most common, but pure white and fine shades of brown and yellow are quite rare and command high prices. Red in rich, deep tints is perhaps the most prized of all colors, while blue and green are also highly val ued owing to their beauty and rarity. The amorphous steel-gray to black diamond, known

as 'bort' or 'carbonado,' which is unsuitable for gems, is extensively used on the cutting edge of drills for tunneling and prospecting.

OCCURRENCE. Previous to the discovery of the Brazilian mines in 1727, diamonds were found chiefly in India and Borneo. In India they have been obtained near Kadapa, Karnul, and Bellari in Madras Presidency; near Nagpur, in the Central Provinces; and at Panna, in Bundelkhand. The famous Golconda was a market in Madras Presidency where the diamonds were collected from the mines. Formerly the diamondmining industry gave employment to a large number of people, but, owing to the gradual exhaustion of the deposits and the crude methods of working, it has steadily declined. The diamondmines of Borneo, which supply about 3000 carats annually, are located in the western part of the island near Pontianak. The Brazilian diamonds come from a small district within the States of Bahia, Minas Geraes, Goyaz, and Matto Grosso, being found for the most part in placers. Diamantina, Bagagem, and Abaete in Minas Geraes are the principal localities for gem-stones, and Lençoes, Sincorá, and Santo Ignacio for carbonados. The alluvial deposits consist of waterworn quartz pebbles, and ferruginous clay, carrying rutile, hematite, ilmenite, and other minerals besides the diamonds. In one locality (Bagagem) the stones are found in a weathered, decomposed phyllite; they are also known to occur elsewhere in conglomerate. The total output of the Brazilian mines up to the year 1899 has been estimated by one authority at 13,105,000 carats. The most valuable specimen from this source is the "Southern Star," found in 1854, which weighed 254 carats in the rough, and 124 carats after cutting. The South African diamond-fields yield about 98 per cent. of the total product. Their discovery in 1867 came from the purchase of a stone that had been picked up by the children of a Boer farmer, and which was subsequently sold in Paris for $2500. The diamonds are found in both river diggings and dry diggings. River diggings occur along the Vaal River from Potchefstroom down to the confluence with the Orange, and up the latter river as far as Hopetown. Mining is conducted in a similar manner to gold-washing, the operations usually being on a limited scale. The dry dig gings are in Griqualand West, on the borders of the Orange River Colony, about 640 miles northeast of Cape Town. They comprise a number of small areas of circular or oval form, ranging in diameter up to one-fourth of a mile. They are worked by a single company, the De Beers Consolidated Mines, Limited. The mines are opened on a large scale, and yield enormous profits. It is stated that the output from 1867 to 1897 was over 33,000,000 carats, or about 7% tons, valued after cutting at $450,000,000. Quite recently another diamond field has been opened in Transvaal, near Pretoria. The largest mine is the Premier which has been proved to be very rich. The South African diamonds in the average do not equal the Brazilian for purity, although some of the most valuable stones, remarkable for quality as well as for size, have been found in these mines. Other countries where diamonds are known to occur are the United States, British Guiana, Russia, China, Sumatra, and Australia. In the United States there are no deposits, so far as discovered, of sufficient

value to warrant mining operations, the stones being found only occasionally in alluvial materials and drift. More than fifty specimens have been identified, of which the largest, weighing nearly 24 carats, was found at Manchester, Va., in 1855. Their occurrence is generally confined to the regions of the southern Appalachians, the Great Lakes, and to the Coast and Sierra Nevada ranges of California and Oregon. The diamond-fields of British Guiana are about 250 miles up the Mazaruni River, and, while new, they give some promise of future importance. New South Wales produces about 25,000 carats annually from alluvial washings near Mudgee, and on the Macquarie River. In Russia, diamonds occur on the western slopes of the Urals, and are obtained in the washing of sands and gravels for gold.

ORIGIN OF DIAMONDS. The problem as to the origin of the diamond, once a favorite subject of speculation, has been studied recently by scientists with some success. In this connection, the South African fields are most significant. As previously stated, the deposits are circular or oval in form, and it has been found that they are inclosed by a wall of carbonaceous shale. For a considerable distance below the surface, the deposits consist of a yellowish, friable material called by the miners yellow ground,' but when, with depth, the limit of atmospheric weathering has been reached, they pass into a partially serpentinized olivine-pyroxene rock of igneous character, known as blue ground.' These facts indicate that the deposits occupy the vents or pipes of ancient volcanoes, and it is therefore probable that the presence of the diamond is connected with volcanic activity, the stones either having been brought up from the interior of the earth, or having been formed in place under the influence of the molten rock in contact with_the_carbonaceous shale. The French geologist De Launay has argued that the diamonds have come up from below. This view seems to be supported by the fact that their occurrence is apparently independent of the inclosing rock, whether this be the carbonaceous shale found at the surface or the underlying diabase and quartzite. The conditions necessary for the crystallization of carbon in the form of diamond would seem to be intense heat and great pressure, such as exist during the formation of igneous rocks. Successful attempts to reproduce these conditions artificially have been carried out by Professor Moissan of Paris, and also by the English chemist Professor Crookes. The method employed consists in heating pure prepared carbon and iron in an electric furnace. By sudden cooling of the molten iron, the surface contracts and exerts a powerful pressure upon the interior mass. When cold, the iron is dissolved in acid, and small black particles remain which exhibit the properties of genuine diamonds.

VALUE. The value of gem diamonds cannot be determined by absolute standards. Weight, cut, brilliancy, color, and perfection of the stones are factors that must be considered in estimating the value, which is, moreover, subject to fluctuation. Colorless stones bring higher prices than off-colored stones, but if decided tints of red, blue, or green are present they may increase the value abnormally. The value ordinarily increases in a ratio increasing with the weight,

REMARKABLE DIAMONDS. The most valuable gems, are generally owned by royal families or are treasured by governments as State jewels. One of the largest is the Orloff, owned by the Czar of Russia, which weighs 195 carats, and is cut in rose form. Its origin is doubtful; according to one account, it was stolen from a Brahman temple in India, while another indicates that it was owned by Nadir Shah of Persia, and came after his death into the hands of an American merchant. In 1772 it was purchased by the Empress Catherine II., at a price said to have been $450,000, besides an annuity of $4,000 and a Russian title. The Koh-i-nûr, weighing 102 carats, and owned by the royal family of England, was found, according to legend, in India long before the Christian Era. After having been handed down through a long line of Indian princes, it was seized during the British invasion of the Punjab, and carried to England. The Regent or Pitt diamond in the Louvre, in Paris, weighs 136 carats, and is valued at $2,500,000. It is perhaps the finest of large brilliants. The Hope diamond, weighing 444 carats, is a beautiful blue stone, valued at $125,000. Among the most valuable stones found in recent times are the Victoria, weighing 180 carats, which was purchased by the Nizam of Hyderabad; the De Beers, weighing 225 carats; and the Tiffany, weighing 125 carats.

The largest and in many respects the most remarkable diamond of which there is any record was found on January 26, 1905, in the Premier mine near Pretoria, Transvaal. The stone called the Cullinan diamond, from the name of its finder-weighed 303.2 carats in the rough and measured 4 inches by 2.5 inches by 1.25 inches. It shows five cleavage planes, indicating that it is only a portion of a large crystal. The color is reputed to be the purest of any of the large stones. Its value has been estimated at from $2,500,000 to $5,000,000.

DIAMOND-CUTTING. The art of diamond-cutting was not developed until the middle of the fifteenth century, although crude methods of polishing were in use long before that time. About 1470 a guild of lapidaries was established at Bruges, under the direction of Louis de Berquem, who is thought to have been the first to devise a regular plan for the arrangement of facets, so as to increase the color effects in gems. Other guilds were soon founded in Antwerp and Amsterdam, and these cities rapidly gained preeminence in the industry, a position they still

maintain.

In diamond-cutting, the operations are performed with the single object of producing the most valuable gem from a crude stone. The apparatus used is simple, but its manipulation requires both skill and experience. In the first place, the stone must be examined carefully to determine its shape, color, and the possible presence of flaws, as these features will govern the operation of cutting. If the shape is such that it cannot be reduced readily to one of the usual forms, the stone may be divided into two or more parts by taking advantage of the natural cleavage. The diamond is cemented to a wooden holder and a steel blade is applied in a direction

DIAMOND.

parallel to an octahedral face. A sharp blow upon the back of the blade is sufficient to accomplish the cleaving. The next process (bruting) brings out the facets. Two rough stones, fixed at the end of holders, are held in either hand of

Figs. 1-5-the Brilliant: 1 and 2, fundamental form; 3, 4, 5, side, top, and back views of the cutting. Figs. 6 and 7, top and side views of the Rose cutting. Fig. 8, view of the Briolette cutting.

the operator, who presses them against each other, at the same time giving them a rubbing motion. The friction wears away the surfaces gradually until they coincide in position to two of the facets. The diamonds are then cemented in new positions, and the operation is continued. The waste particles of dust are collected and used in the final process of polishing. For this the apparatus consists of a small disk of spongy cast-iron, turning on a vertical spindle at a speed of 2000 to 3000 revolutions per minute. The stone, imbedded in the apex of a metallic (lead-tin) cone is held by a clamp against the surface of the disk, which is smeared from time to time with a mixture of olive oil and diamonddust. This operation requires constant attention, as the stone may be permanently damaged by over-heating.

Of the several forms in which diamonds are cut, the brilliant is the favorite. It is derived from the fundamental octahedron and requires, when perfect, 58 facets. In the middle is an octagonal facet called the 'table,' which is surrounded by 32 smaller facets, reaching to the line of union (girdle) between the upper and lower portions. On the lower portion there are 24 facets symmetrically arranged around the 'culet.' The rose form is used for thin stones. It consists of a number of triangular facets, the apices of which meet in the crown, and whose bases are supported by a second row of facets extending to the girdle. The lower part is perfectly flat. In the briolette the stone is cut into triangular facets, but the form is pear-shaped and there is no girdle.

BIBLIOGRAPHY. Kunz, Gems and Precious Stones (New York, 1900): Streeter, Precious Stones and Gems (London, 1894); Streeter, The Great Diamonds of the World (London, 1882); Church, Precious Stones (London, 1883); De