jecture, and all treatment blind and base empiricism. The French school of medicine is famous for the stress it lays on diagnosis; and students from other parts of Europe, and more especially from the United States, flock to Paris to acquire the elements and practice of this most essential branch of their profession.

DIAGORAS OF MELOS, surnamed the Atheist, a Greek philosopher, lived in the time of Socrates and Aristophanes, but neither the date of his birth nor that of his death is known. He must have removed from his native island to Athens before the performance of the "Clouds " of Aristophanes (424 B.C.), for he is alluded to in that piece as one well known to the Athenians. He was a disciple of Democritus of Abdera. He ridiculed the popular religion, and attacked especially the Eleusinian mysteries, on account of which he was accused of impiety (411 B. C.). Fearing the result of a trial, he made his escape from the city. He was condemned to death by the court, and a price set upon his head. Notwithstanding this, after living for a time at Pallene, he finally died at peace in Corinth. His works are all lost.

DIAL. Sun dials are among the most ancient of human inventions, and, although sometimes said to have been invented in Lacedæmon, were more probably derived by the Greeks from eastern nations. The dial of Ahaz, the king of Judah, is one of the earliest mentioned in the history of the East, and it is probable that the Jews learned the use of this invention from the Babylonians. According to Wilkinson, there are no indications in the sculptures to prove the epoch when the dial was first known in Egypt." The modern improvements in artificial modes of measuring time are so great, that sun dials are now more a matter of curiosity than of use. They may be divided into 2 essentially different kinds, one of which we may call geometrical, the other algebraical. In order to comprehend the first, we need only observe, that if a rod or gnomon be placed parallel to the axis of the earth, its shadow, conceived of as a sheet of darkness passing in a plane from the rod on the opposite side of the sun, would swing steadily and equally round the rod as a hinge, so long as the sun shone upon it. Upon whatever surface this shadow fell, whether horizontal, vertical, or inclined, its place could be used as a means of measuring time. And if upon this surface lines were drawn, marking the place of the shadow at definite hours of the day, the rod might be made as short as we pleased, reduced indeed to a single ball, held in the place where the extremity of the rod had been, and the shadow of this ball would mark the time upon the lines of the shadow of the rod. The other sort of dial, the algebraical, is more difficult to explain without the use of a diagram. It is drawn upon a piece of card, to which is attached a plumb line with a bead sliding upon it; the card being held in such a manner that the upper edge shall point at the sun, its plane being vertical, the bead marks the

hour upon the face of the card. This dial has the advantage of being portable. Dials of the first kind, of a rude nature, may be made portable by having the rod and the dial surface light enough to be balanced upon a compass needle. Beautifully engraved sun dials, for the regulating of clocks, are manufactured by the electrotype process in copper.-The term dial is also applied to any graduated surface, such as a clock or watch face, upon which time is marked out.

DIALLAGE (Gr. diadλayn, change, alteration), a mineral of the augite family, so named from its tendency to cleave in different directions. It is a variety of hornblende, in thin foliæ, of various shades of green, gray, brown, and bronze colors, and is found in serpentine and greenstone. Its specific gravity is 3.25. Diallage rock, also called euphotide, is a compound rock of diallage and feldspar.

DIAMAGNETISM. In the native magnet (an ore of iron) a peculiar force resides, which, if a mass of this body be suspended freely, turns or directs it into a line varying slightly from the course of a meridian on the earth's surface. The same end of the magnet being always directed toward the north, this has been termed its N. pole; the opposite, its S. pole. Certain bodies, especially iron, brought near to a magnet, have the magnetic condition induced in them, the extremity nearest either magnetic pole becoming a pole of the opposite name, that most remote a pole of the same name. This result is in accordance with the law that like poles repel, while unlike attract each other. A soft iron bar, around which the electrical current is made to circulate upon a coiled conductor, or helix, becomes magnetic for the time, but loses its magnetism when the current ceases. Small magnetizable particles, as iron filings, dusted upon a surface on which a magnet rests, or agitated near it, become arranged in lines which, between unlike poles that are presented to each other, run across in straight lines, while about these on either side they form curves, making larger and larger sweeps into space. The lines thus indicated have been named magnetic curves, or lines of force. Until recently, the number of magnetic bodies was supposed to be very small. Becquerel, in 1827, found that a needle of wood playing freely on a pivot took a direction across, not in, the magnetic curves; and in 1829 Le Bailli also observed that bismuth repelled the magnetic needle. But the significance of these facts was not understood until an accidental discovery of Faraday, in 1845, led that philosopher into a full investigation of the phenomenon. In the course of his experiments on magnetic rotary polarization, he observed that a bar of so-called "heavy glass," suspended between the poles of an electro-magnet, moved, as soon as by the passage of the electrical current magnetism was induced in the latter, into a position crossing the lines of force, or at right angles to the line joining the poles. Terming the position assumed by a soft iron bar which is lengthwise between the two poles,

or from one to the other, axial, Faraday gave to the new direction assumed by the glass the name of equatorial. The glass was not merely thus directed, it was repelled by either pole; and if, reduced to the form of a small mass or cube, it was thrown out of the line joining the poles to one side or the other, it moved into the position of weakest magnetic action. This new-found property of certain bodies Faraday termed diamagnetism; and in contrast with this, he denominated the familiar form of magnetic action paramagnetism. His experiments warrant the conclusion that, with a sufficiently powerful electro-magnet, all substances whatever can be shown to exhibit one or other of these properties. Liquids and solutions were examined by being suspended in glass vials, the known influence of the glass being allowed for. Among paramagnetic substances, by far the most powerful is iron, then nickel and cobalt, and, in a slight degree, manganese, palladium, crown glass, platinum, osmium, and some others. Vacuum serves as zero in the scale. Then, passing from the less to the more diamagnetic bodies, are found arsenic, ether, alcohol, gold, water, mercury, flint glass, tin, heavy glass, antimony, phosphorus, and, by far the most powerful, bismuth. Flames are diamagnetic, being so strongly repelled by the poles that they divide and pass up on either side, a descending current of air going down in the middle. Most organic substances are diamagnetic; wood, starch, sugar, leather, bread, and even animal tissues and blood, are instances. Oxygen, and perhaps nitrous gas, are the only gases which are known to be ordinarily magnetic; and when it is added that oxygen loses in a degree, though not wholly, its magnetic condition by increase of temperature, it will be seen that the properties of this constituent of our atmosphere probably have important bearings on the production of terrestrial magnetism. Green glass is magnetic in consequence of the iron it contains; and to render wood ordinarily magnetic, it is only necessary to cut a chip of it with a common knife. The magnetic condition of any compound body is found to be determined by what may be called the algebraic sum of the magnetic and diamagnetic powers of its constituents. Thus a compound or solution containing much iron will always be paramagnetic in greater or less degree; but if the iron be blended with comparatively large amounts of water and other diamagnetics, it may be brought to the neutral point, or the compound may be actually diamagnetic. Under all ordinary circumstances, the decidedly magnetic or diamagnetic bodies give to combinations their own character. Another important point is the influence of enveloping material. Certain substances that are repelled, and take the equatorial position in air, are attracted and set axially in water; and even a solution of iron, magnetic in air, if weaker than another solution in which it is immersed, will stand equatorially, and act as a diamagnetic. In terming a body magnetic

or diamagnetic, then, we mean that it is such with reference to the medium in which it is tried; and as this medium is commonly air, in which the magnetism of the oxygen dominates over the opposite property of the nitrogen, it is evident that some so-called diamagnetics are only relatively such. With reference to the theory, Faraday now considers that the diamagnet is not rendered polar, as is the magnet, but simply repelled. Prof. W. Thomson has supposed the diamagnet to be simply a body less magnetizable than air, but still polar. In this case it would move away for the more magnetic air, just as in gravitation smoke makes way and ascends above the more strongly gravitating cold air. Plücker, Tyndall, and others adhere to a modified form of Prof. Faraday's earlier view, namely, that the diamagnet is a body susceptible in greater or less degree of a double polarity opposite in character to the double polarity of the magnet; or, in the language of Ampère's theory, that as the currents induced in soft iron are parallel to the currents in the inducing magnet or battery wire, so, in bismuth and other diamagnetics, the currents are induced in contrary directions, so that these bodies become inverted magnets, and place themselves across the magnetic lines of force.

DIAMANTINA, formerly TEJUCO, a city of Brazil, and capital of the diamond district, situated in a valley of the province of Minas Geraes, at an elevation of 5,700 feet above the sea; lat. 18° 28' S., long. 43° 50′ W.; pop. about 6,000. It is built in the form of an amphitheatre, with wide, ill-paved streets, and handsome churches, one of which, belonging to negroes from the coast of Africa, contains an image of a black Virgin. Most of the houses are surrounded by pleasant gardens, and the environs of the city are adorned with orange and banana trees. The climate is mild. The inhabitants are employed chiefly in the gold or diamond trade.

DIAMETER, a straight line passing through the centre of a circle, terminated at each end by the circumference. Straight lines holding an analogous relation to curves, such as the conic sections, are also called diameters of those curves.

DIAMOND (from adamant, and this from Gr. a privative and daμaw, to subdue), so named on account of its extreme hardness and indestructibility, a gem distinguished above all other precious stones for its brilliant lustre and hardness. It is met with in solid pieces of small size in alluvial deposits which are worked for gold. In a few instances diamonds have been found attached to loose pieces of brown hematite, and one was discovered in a kind of conglomerate rock, composed of rounded silicious pebbles, quartz, and chalcedony, cemented together by ferruginous clay; but no positive knowledge is had of the particular rock in which they originated, more than that it is one of those belonging to the metamorphic group, which yield gold. In the districts where they occur, a peculiar variety of light yellowish and white quartz rock, of laminated structure, called itacolumite,

is very commonly met with in these rocks. It is remarkable for its flexibility, and the peculiar manner in which the long strips yield to a slight pressure without parting, as if broken in their interior. It is found in Brazil, and in Georgia and North Carolina, in the vicinity of the localities that furnished the few diamonds discovered in these states. In the Golconda district the diamond is found in a black carboniferous boggy earth, in which the natives seek for it by feeling with their feet. The belief is current with them that in this material it grows. In Brazil diamonds have been found massive, in the form of pebbles. Their color is black; specific gravity, 3.012 to 3.416; composition carbon, with sometimes 2 per cent. foreign matter. This quality is valued at 75 cents the carat of 4 grains nearly. The brilliancy and indestructibility of the diamond attracted attention to it at very early periods, and caused it to be highly esteemed as a gem. It was long known in Asia before it was discovered in any other quarter; and the greater part of the supplies have been from that part of the world. Indeed, it was not until the early part of the last century that diamonds were known to exist elsewhere. The mines of Brazil were then discovered, and from 1730 to 1814, according to Baron d'Eschwege, their production was at the rate of 36,000 carats per annum. After 1814 it fell off greatly; but from 1845 to 1858 there has been an enormous increase, the statistics of which have been already furnished in the article BRAZIL. In the gold region of Siberia a few have been obtained, and within the last 20 years a few also in that of North Carolina and Georgia. In Asia, the most noted localities were the island of Borneo, Bengal, and the famous mines of the kingdom of Golconda in Hindostan. The city of this name was the repository of the diamonds collected in the territory of the kings of Golconda. These mines, celebrated as having produced some of the most valued precious stones in the world, have for some time past been unproductive, and are not now worked. The diamond is pure crystallized carbon. Its hardness 10, the highest number of the scale; but the external coat is harder than the internal portion, and may be rated at 10.5 or 11. The following are ascertained specific gravities of different varieties: Brazilian, 3.444; Brazilian yellow, 3.519; oriental, 3.521; oriental green, 3.524; oriental blue, 3.525. The primitive form of the crystal, and that into which the numerous secondary forms may be converted by cleavage, is the regular octahedron, consisting of 2 four-sided pyramids joined at their bases. The faces of the crystals are often rounded off, so as to present a convex surface, and the edges are also often curved. The cleavage planes greatly facilitate the cutting of the diamond, and also present the most brilliant natural surfaces. Some diamonds found of a spherical figure are deficient in these planes, or they lie in a concentric arrangement which renders their cutting almost impracticable by any known process. The diamond is not acted upon by acids or alkalies, and when pro

tected from the action of the air may be heated to whiteness without injury. Exposed to the intense heat produced by a powerful Bunsen's battery, or by a condensed mixture of carbonic oxide and oxygen gas, it fuses, and is converted into a mass resembling coke, and its specific gravity is reduced in some cases to 2.678. "Heated in the open air, it burns at the temperature of 14° Wedgwood, or about that of melting silver, and is dissipated in the form of carbonic acid gas, thus proving its composition to be pure carbon. Its inflammability was suspected by Boetius de Boodt in 1607, and in 1673 Boyle discovered that it was dissipated in vapor at a high heat. Its combustibility was first proved by the Florentine philosophers in 1694, by subjecting the gem to the solar rays concentrated in the focus of the large parabolic reflector made for Cosmo de' Medici, when it burned with a blue lambent flame. The experiment has been several times repeated by Sir Humphry Davy with the same speculum, and by Lavoisier, Mr. Tennant, and others, by different processes. Sir George Mackenzie made use of the diamond for furnishing the carbon to convert iron into steel. The property of phosphorescence has been attributed to the diamond after it has been exposed to a heat approaching redness, or to the action of the solar rays, especially the blue rays; and it has been stated that when the phenomenon is produced by the latter method the effect continues some time after the stone is removed from the light. But this is not confirmed by late authorities. Experiments conducted through several months in 1858 at Messrs. Tiffany and company's, of New York, failed to develop any save negative evidence; and when they were renewed in January, 1859, for the purposes of this article, they were abruptly terminated through the carelessness of a workman, by the unfortunate destruction of a valuable gem obligingly lent by them. In no instance did any symptom of phosphorescence appear; but a remarkable increase in refraction was several times observed, and this appeared to be permanent. The diamond possesses single or double refraction according to its different crystalline forms; and it has an extraordinary power of refracting light, the index of refraction being 2.44, which led Sir Isaac Newton to suspect its inflammable composition. The dispersive quality of diamond is high; its index is equal to 0.0109. Its refraction index (exceeded only by that of chromate of lead) equals 2.439; of some brown stones it has been observed to be 2.470, 2.487, and 2.775.-Diamonds are found of various colors, as well as colorless and perfectly transparent. The latter are most esteemed, and are distinguished as diamonds of the first water from their semblance to a drop of clear spring water. When of a rose tint and of clear water, they are also highly valued. A yellow shade is objectionable, as is a cinnamon color, a stone having these rarely being clear and sound. Next to the rose, a green color is the least objectionable; many very fine diamonds have this

tint; and some are found of a bluish color, and some black. For the valuation of diamonds an arbitrary rule has been given, which is, however, little regarded in actual sales of the most costly of these gems. Purchasers for such being few, the only real rule adopted, as in the sale of many other commodities, is to demand the highest price there is the least probability that one may be induced to pay. The mere statement of the rule is sufficient to show its indefiniteness. It is to multiply the square of the weight in carats by a sum varying according to the state and quality of the stone. If clear and of good shape, this sum is £2; if perfect and well cut, £6 or £8 for the brilliant or rose, but a lower figure for the table. The rate is now $15 in place of the £2 above, and a specimen brilliant is worth $75. For diamonds of moderate size the rates vary as little as those of exchange between countries. They follow from the natural proportions in which diamonds are found. Diamonds weighing over 10 carats are generally esteemed at a higher proportional rate than the smaller sizes; yet the latter can commonly be sold at higher proportional rates, on account of the few purchasers for those of large size. In the great sale of jewels in London in 1837, on the distribution of the Deccan booty obtained by the army of the marquis of Hastings, the splendid Nassuck diamond, weigh ing 357 grains, and of the purest water, brought only £7,200. The present value of diamonds may be inferred from the price paid in Dec. 1858, for a stone weighing 61 carats, £33,000. A pair of drop-shaped diamonds for ear-rings were bought at the same sale for £15,000. A steady increase has taken place since the last century, and a much more rapid increase in the price of fine gems is to be expected, from the increased demand and diminished supply. It is an interesting fact that the finest gems of commerce are now in great part supplied by the old jewels of Portuguese, Spanish, French, and English families, the proportions from each nation in the order named; and that the best market for them is now the United States. The art of cutting and polishing probably originated in Asia, at a very early period, but was first introduced into Europe about the middle of the 15th century by Louis Berquen of Bruges, who accidentally discovered that by rubbing 2 diamonds together their surfaces might be abraded. The powder obtained in this way is used for polishing the stone. Diamond cutting was for a long time a monopoly in Holland, and the business is at the present day mostly confined to Amsterdam. The process, which consists of grinding down the surfaces as well as cutting, is slow and tedious, and being done entirely by hand, occupies for a single stone the continual labor of months. The Pitt diamond indeed required 2 years for the completion of the process. Two diamonds are employed, each cemented into the end of a stick or handle, a model in lead being taken of the one to be cut, by which the faces are determined. The stones are then rubbed together with a strong pressure, being

held over a metal box having a double bottom, the upper one perforated with small holes, through which the diamond dust falls. This is afterward carefully collected, mixed with vegetable oil, and used for polishing the gem upon a revolving cast-iron disk. When a large piece is to be removed from the stone, it is sometimes cut off by means of a steel wire covered with diamond powder, and sometimes by the use of a chisel and hammer, though in this way there is danger of destroying the stone. The workman should understand perfectly the position of the cleavage planes, as it is only upon them that pieces can be removed by the chisel. The forms usually adopted in cutting the diamond are the brilliant, the rose, and the table. The first shows the gem to the best advantage. It is composed of a principal face called the table, surrounded by a number of facets, which is all that is visible above the bezil when set. The stone in depth below the bezil should be equal to half its breadth. On the under side it terminates in a small table, which is connected with the upper surface by elongated facets. As the brilliant is the most economical of material, and shows the stone most advantageously, it is usually preferred to any other. The rose, which is very brilliant, is flat below and cut into facets entirely over the upper surface. The table is least beautiful, and is used mostly in India for thin stones with a large surface, which are ornamented by being cut into facets at the edges.-Among the most cel ebrated diamonds known, that obtained by Mr. Pitt, governor of Madras, is perhaps one of the finest and most perfect. It is known as the regent. Its weight before cutting was 410 carats, and by this process, which occupied 2 years, it was reduced to 136 carats, and was purchased by the regent duke of Orleans in 1743 for $675,000. Its present value is estimated at $1,000,000. It was placed by Napoleon in the hilt of the sword of state. A splendid diamond recently found in Brazil, and imported into France, is called the "Star of the South." It weighs in its rough state 254 carats. Its general form is a rhomboidal dodecahedron, and upon its faces are impressions which appear to have been made by other diamonds, so that the whole was probably a group of diamond crystals. The famous diamond in possession of the king of Portugal is also from Brazil. If genuine, of which there is some doubt, its value, according to the rule of computation, should be $28,000,000, weighing as it does in the rough 1,680 grains.-The famous Koh-i-noor or "Mountain of Light" is now in possession of the queen of England. This wonderful stone, interesting alike for its historical associations and for its intrinsic beauty, was, according to Indian tradition, obtained before the Christian era from one of the mines of Golconda. From the rajah of Oojein, who seems to have possessed it at the beginning of the Christian era, it passed to successive sovereigns of central India, and in the early part of the 14th century was added to the treasures of Delhi by the Patan monarch Aladdin. It remained in possession of the rul

ing families of the empire until the irruption of the Persian conqueror Nadir Shah, who saw it glittering in the turban of the vanquished Mohammed Shah, and proposing an exchange of head dress as a mark of friendship, bore it away with him, and gave it the name by which it is still known. After the assassination of Nadir it passed through the hands of Ahmed Shah of Cabool to Shah Soojah, who paid it as the price of his liberty to his conqueror Runjeet Singh, the "lion of the Punjaub," in 1813. On the annexation of the Punjaub to the East India company's territory in 1849, it was stipulated that the Koh-i-noor should be surrendered to the queen of England, to whom it was accordingly delivered by the company, July 3, 1850. At this period its weight was 186 carats. It was exhibited at the crystal palace in London in 1851, where it attracted universal attention; but when found to exhibit an inferior display of colors to its glass model, and that it was necessary to surround it with a number of vivid lights to develop its colored refractions, multitudes turned away disappointed from the worldrenowned "Mountain of Light." An examination was made of it by scientific gentlemen, with reference to the propriety of recutting the gem. After obtaining the opinions of skilful cutters at Amsterdam, it was decided that the attempt should be made, though some fears were entertained as to its success. Being the largest diamond cut in Europe for a long time, it was a work of no common interest. Orders were given that the proper machinery should be prepared, and a small steam engine was procured for the purpose. The duke of Wellington commenced the work. The stone being imbedded in lead, excepting only that portion which was first to be cut, he held it firmly against a rapidly revolving wheel, which by its friction removed the angle exposed, and thus the first facet of the fresh cutting was accomplished. The operation was continued and successfully completed by careful and experienced workmen. Now, the splendid Koh-i-noor, freed from all blemishes and defects, blazes brilliantly among the crown jewels of the sovereign of England. The diamond so long in possession of the sultan of Matan, of the island of Borneo, is remarkable for its size and purity. It weighs 367 carats, and should be worth at least $3,500,000. It is shaped like an egg with an indented hollow in the smaller end. It was discovered at Landak. The Orloff diamond purchased for the empress Catharine of Russia is about the size of a pigeon's egg, and weighs 195 carats. It is said to have formed the eye of a famous idol in a temple of Brahma at Pondicherry. A French deserter robbed the pagoda of this valuable stone. After passing through the hands of various purchasers, it came into the possession of a Greek merchant, who received for it from the empress $450,000, an annuity of $20,000, and a title of nobility. The Austrian diamond is of a beautiful lemon color, and cut in rose; its weight is 139 carats. Its

value is less than it would be but for its color and the form in which it is cut, ranking as worth $500,000 instead of $750,000. The most valuable diamond found in the United States was picked up by a workman at Manchester, on the banks of the James river, opposite Richmond, in 1856. The locality is in the tertiary formation, and the diamond originally belonged, no doubt, to the gold region up the river. It is of curvilinear octahedral form, specific gravity 3.503, and weighs 23.7 carats. It is lightly chatoyant, and would probably cut white; but an original flaw was increased by the rough treatment it received from those into whose hands it fell, so that its value was greatly deteriorated. It is now in possession of Prof. Dewey. Another valuable diamond, owned by Thomas G. Clemson, Esq., was found among the gold washings of North Carolina about the year 1842. This was of curvilinear form, and was set without cutting. Others of less importance have been found in Georgia.-The imitation of diamonds has been carried to an astonishing degree of perfection among the French. M. Bourguignon has been especially successful in this manufacture. The sand employed for the production of his splendid diamonds is procured from the forest of Fontainebleau, and forms a considerable article of trade. The setting of these mock stones is always of pure gold, and of the newest fashion, and the ornaments when completed rival in delicacy and lustre the purest diamonds which nature has produced, and only by the closest inspection can the difference be detected. The chief objection to them is their liability to become dull in time by deliquescence.The process of collecting diamonds is similar to that of collecting gold in the alluvial deposits. The coarse gravel and rolled pebbles derived from the primary and metamorphic rocks, form the lowest stratum among the sands and clays of the alluvium. This stratum, resting upon the surface of the rock, is the repository alike of gold and of diamonds. It is laid bare in the beds of the streams, when these cease to flow in the dry season, or are drawn off by sluices made for the purpose. From these beds, as well as from excavations in the bottom, the gravel is removed, to be washed when convenient. This in Brazil is usually in the rainy season, and the work is done in a long shed, through which a stream of water is conveyed, and admitted into boxes in which the gravel is washed. A negro works at each box, and inspectors are placed to watch the work, and to prevent the laborers from secreting the diamonds. It is the custom to liberate the negro who finds a diamond weighing 17 carats. Dr. Beke, in a paper read at a meeting of the British association, relates that a slave in Brazil seeking for diamonds in the bed of a river broke with his iron bar through a crust of silicious materials, cemented together by oxide of iron, in which he discovered a bed of diamonds, which were afterward sold for $1,500,000. This immense quantity, being carried to England, so