OR, A DICTIONARY O F ARTS, SCIENCES, AND MISCELLANEOUS LITERATURE; BY WHICн THE DIFFERENT SCIENCES AND ARTS TREATISES OR COMPREHENDING SYSTEMS, The HISTORY, THEORY, and PRACTICE, of each, AND FULL EXPLANATIONS GIVEN OF THE VARIOUS DETACHED PARTS OF KNOWLEDGE, WHETHER RELATING TO NATURAL and ARTIFICIAL Objects, or to Matters ECCLESIASTICAL, Including ELUCIDATIONS of the most important Topics relative to RELIGION, MORALS, TOGETHER WITH A DESCRIPTION of all the Countries, Cities, principal Mountains, Seas, Rivers, &c. A General HISTORY, Ancient and Modern, of the different Empires, Kingdoms, and States; AND An Account of the LIVES of the most Eminent Perfons in every Nation, from the earlieft ages down to the prefent times. Compiled from the writings of the beft Authors, in feveral languages; the most approved Dictionaries, as well of general fcience as of its parti THE THIRD EDITION, IN EIGHTEEN VOLUMES, GREATLY IMPROVED. ILLUSTRATED WITH FIVE HUNDRED AND FORTY-TWO COPPERPLATES. INDOCTI DISCANT, ET AMENT MEMINISSE PERIT 1. EDINBURGH, PRINTED FOR A. BELL AND C. MACFARQUHAR MDCCXCVII, ENCYCLOPEDIA BRITANNICA. Diamond. DIA Though the diamond is commonly clear and pellu- The fineft diamonds are thofe of a complexion like that of a drop of pure water. It is likewife a valuable property if they are of a regular form and truly made; as alfo that they be free from ftains, fpots, fpecks, flaws, and crofs veins. If diamonds are tinctured yellow, blue, green, or red, in a high degree, they are next in efteem; but if they are tinctured with thefe colours only in a low degree, the value of them is greatly diminished. There are alfo diamonds of other complexions; fuch as brown, and thofe of a dark hue: the first refembling the browneft fugar-candy, and the latter dufky iron. In the Philofophical Commerce of Arts, Dr Lewis tells us of a black diamond that he himfelf had feen. At a diftance, it looked uniformly VOL. VI. Part I. DIA black; but on closer examination appeared in fome Diamond. Thefe gems are lamellated, confifting of very thin The names of oriental and occidental, given by jewellers to this and all other precious ftones, have a different meaning from the obvious fenfe; the finest and hardeft being always called oriental, whether they be produced in the eaft or not. Thofe called occidental are of inferior value; but according to Mr Jefferies, who has written a treatife on the subject, the diamonds of Brafil equal the finest oriental ones. The art of cutting these gems was invented in 1476 by Louis de Berquen a native of Bruges in the Auftrian Netherlands. This ftone becomes luminous in the dark, by expofure during a certain time to the rays of the fun; by heating it in a crucible; by plunging it in boiling water; or by rubbing it with a piece of glass. By friction it acquires an electrical property, by which it attracts the fubftance ufed for foils called black maflic, and other light matters. The author of the Chemical Dictionary fays, that diamonds are refractory in the fire, and even apyrous. Nevertheless, experiments have been made, which prove that diamonds are capable of being diffipated, not only by the collected heat of the fun, but also by the heat of a furnace. Mr Boyle fays, that he perceived certain acrid and penetrating exhalations from diamonds expofed to fire. A diamond by expofure to a concave speculum, the diameter of which was 40 inches, was reduced to an eighth part of its weight*. In the Giornale de Letterati d'Italia, tom. viii. art. 9. we may read a relation of experiments made on precious ftones, by order of the grand duke of Tuscany, with a burning lens, the diameter of which was two thirds of a Florentine ell, near the focus of which was placed another fmaller lens. By these experiments we find, that diamonds were more altered by folar heat than most of the other precious ftones, although not the least appearance of a commencing fufion was obfervable. A diamond weighing 30 grains, thus expofed during 30 feconds, loft its colour, luftre, and tranfparency, and became of an opaque white. In five minutes, bubbles appeared on its furface; foon afterwards it burft into pieces, which were diffipated; and the fmall fragment which remained was capable of being crushed into fine powder by the preffure of the blade of a knife. Neither the addition of glafs, flints, fulphur, metals, or falt of tartar, prevented this diffi A pation Plilof. Tranfac n° 386. |