1902 Encyclopedia > Diamond > Diamonds: Optical Properties, Chemical Character and Composition. Action of Heat on Diamonds.

(Part 3)

Optical Properties of Diamonds. Chemical Character and Composition of Diamonds. Action of Heat on Diamonds.

The optical properties of the diamond are also very remarkable. The purest stones, or those of the first 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 or 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 luster 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 angels above 25° being totally reflected. Like all crystals of the same system it possesses only simple refraction, but Dr (Sir David) Brewster 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, Boetius de Boot conjectured that the diamond was an inflammable 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 confimed in 1694 and 1695 by experiments 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 combustibility of the diamond when freely expose in a strong heat has been often repeated, and is 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 corresponded 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 experiments have been made. Before the blowpipe it can 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 temperature at which pig-iron melts, or to the strongest heat produced in the porcelain kiln, undergo no change; but 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. 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.

The faces also during burning became marked with peculiar triangular hollows, with their sides parallel to the edges of the octahedron. Seen in a strong light they appear as faces of an icositetrahedron, whilst other regular triangular impressions on the faces of natural crystals of diamond are produced by faces of the dodecahedron. (Rose, "Ueber das Verhalten des Diamants," &c., in Berlin Monatsberichte, June 1872).

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