Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/181

163 DIAMOND JG3 ing it, that the diamond slowly regained the first place among gems. Even in the 1 6th century (1550), Benvenuto Cellini (Trattato dell orificerio, cap. i.) assigns it only the third rank in value, estimating a perfect ruby of one carat weight as worth 800 scudi d oro (each equal to about 4s.), a similar emerald at 400, an equal diamond at 100, and a sapphire at 10 scudi. In the same century the use of the diamond for cutting glass and engraving gems seems also to have become known. ineralo- The diamond always occurs in crystals of the tesseral ysical octahedron, or double four-sided pyramid (fig. 1), the u-acters. ^^i^ dodecahedron with twelve faces (fig. 2), and others with twenty-four (fig. 3), and forty-eight faces FIG. 2. Rhombic Dodecahedron FIG. 3. Triakisoctahedron. FIG. 4. Hexakisoctahedron. (fig. 4). The first form is most common in stones from India, the second in those from Brazil. Cubes also occur, but are rare, whilst the icositetrahedron has not been observed. Hitherto the diamond has been described as hemihedric, but Sadebeck from his own and G. Rose s researches shows it to be holohedric (in the Berlin Monatsberichte, Oct. 1876). The faces are often curved, strongly striated, or marked by stair-like inequalities, hiding the true form. Many of the crystals also are round almost like spheres (fig. 5), or the smaller ones like grains of sand. This does not arise, however, from attrition during transport by water, but is the original shape of the stones. Macles. or twin-crystals, specially of two octahedrons, are common, and the striae due to this structure appear even on the polished facets. The diamond has ?% ^ a perfect cleavage parallel to the faces of the octahedron, and breaks readily both in this and other directions. Con trary to the old and still common opinion, it is rather brittle, and is easily injured by a slight blow or fall. Its hardness 10 in the mineralogical scale far surpasses that of all other known stones, and was used even by the ancients to discriminate it from other gems. In specific gravity, 3 52 (or 3 -515 to 3 525), it is considerably higher than rock crystal, but nearly the same as the topaz, which may thus be mistaken for it. According to Fizeau, it has its greatest density at - 42 3 C., and below this begins to expand, a property seen in very few other solid bodies. Its expansion by heat is very small, the volume from the freezing to the boiling point of water only rising from 1 to 1 -00000354. By friction it becomes positive electric. The so-called compact diamond or carbonado of the stone polishers, found as round grains or masses of one or two pounds weight in the washings near Babia, of a brownish black colour and sp. g. = 3 012 to 3 416, is porous diamond mixed with a small amount of other matter. The optical properties of the diamond are also very Optical remarkable. The purest stones, or those of the first proper! water are highly transparent and colourless. But more generally it is less transparent, and shows various tints, specially white, grey, or brown ; more rarely blue, red, yellow, green ; and very seldom black. Such stones, when the colours are pure, are often highly valued. It is also distinguished by its brilliant adamantine lustre. Newton, two centuries ago, remarked its high refractive power, and from this conjectured that it was a substance of a peculiar nature. The index of refraction is 2 4135 for the red rays, 2-4195 for the yellow, and 2 -4278 for the green. This high refractive power, and the strong reflection at both surfaces, render it seldom completely transparent, but give it the high lustre for which it is valued as an ornament. They also produce the numerous internal reflections seen in the interior of cut stones, all the rays of light falling on the posterior surfaces at angles above 25 being totally reflected. Like all crystals of the same system it possesses only simple refraction, but Dr (Sir David) Brews ter found that many showed traces of double refraction by their action on polarized light. This he ascribed to a peculiar tension produced in the interior of the stone during its formation, and a somewhat similar explanation is still adopted. In a history of gems published early in the 17th century, Chemi Boetius de Boot conjectured that the diamond was an in- charac flammable body. Robert Boyle, who in 1664 described its property of shining in the dark, or phosphorescing after being exposed to the light of the sun, a few years later observed that a part of it was dissipated in acrid vapours when subjected to a high temperature. This combustibility of the diamond was confirmed in 1694 and 1695 by ex periments with a powerful burning glass or lens made in the presence of Cosmo III., grand duke of Tuscany, by the Florentine Academicians. The experiment of the com bustibility of the diamond when freely exposed in a strong heat has been often repeated, and its true character was proved by Lavoisier, who determined that the product was carbonic acid gas. Sir George Mackenzie converted iron into steel by powdered diamonds ; whilst Mr Smithson Tennant showed that the carbonic acid produced cor responded to the oxygen consumed. No doubt, therefore, now remains that the diamond is only pure carbon in the crystallized condition, and like it insoluble in acids. In regard to the action of heat on the diamond, various Actioi experiments have been made. Before the blowpipe it is heat - infusible, and closely packed in powdered charcoal it can resist a very high temperature. But when oxygen is present it burns slowly at a temperature usually given at about that of melting silver. Gustaf Rose lately found that when air is excluded diamonds exposed to a tempera ture at which pig-iron melts, or to the strongest heat pro duced in the porcelain kiln, undergo no change ; b;it at a higher temperature, like that at which bar-iron melts, they begin, whilst retaining their form, to be converted into graphite. He further observed that when diamonds and graphite were exposed together in the same muffle, foliated graphite was far more difficult to burn than the diamond, but compact graphite was consumed more readily. In the current of air the diamond gradually became smaller and smaller, but retaining its brilliancy till it finally vanished.
 * al and O r cubical system. Its most frequent forms are the