DIA 



DIA 



all those on the other side. This is New- 

 ton's sense of a diameter. 



Hut, according to some, a diameter is 

 that line, whether right or curved, which 

 bisects all the parallels drawn from one 

 point to another of a curve. So that in 

 this way every curve will have a diame- 

 ter; and hence the curves of the second 

 order have, all of them, either a right- 

 lined diameter, or else the curves of 

 some one of the conic sections for diame- 

 ters. And many geometrical curves of 

 the higher orders may also have, for dia- 

 meters, curves of more inferior orders. 



l)i VMETER of a upltere, is the diameter 

 of the semicircle by whose rotation the 

 sphere is generated ; in which sense it is 

 the same with axis. 



DIAMKTKR of gravity, in any surface 

 or solid, is that line in which the centre 

 of gravity is placed. See CEXTKH. 



DiAMKTKR, in astronomy. The dia- 

 meters of the planets are either apparent 

 or real : the apparent diameters are such 

 as they appear to the eye ; and being 

 measured by a micrometer, are found 

 different in different circumstances and 

 parts of their orbit. See ASTIMM>M \ . 



DIAMOND. The diamond has always 

 been regarded a^ the most \aluable of 

 the gems, and, consequently, as the most 

 valuable production of the mineral world, 

 :i superiority which it derives from its 

 very high lustre, it transparency, and 

 hardness. The first quality arises from 

 its greater refractive power, which is such 

 as to cause all the light to be reflected 

 which falls on it at an angle of incidence 

 greater than 24 J degrees ; and it is capa- 

 ble of being rendered still more brilliant 

 by its surface being cut into facets, which 

 multiply the reflections of light. From 

 .ts hardness, too, its lustre remains unin- 

 jured: this hardness is such, that it can 

 be cut, or rather worn down, only by 

 rubbing one diamond against another, 

 and is polished only by the finer diamond 

 powder. 



This substance is found in India, in the 

 districts of Visapore and Clolconda, and 

 likewise in Bengal, and in lira/il in South 

 America. It is not found in its original 

 situation, but in the beds of streams, or 

 in a loose ferruginous sand beneath the 

 soil. The Brazilian diamonds arc infe- 

 rior in transparency and purity to the 

 Oriental. 



The diamond is found crystalli/ed, 

 being either in perfect crystals, or in 

 fragments often encrusted with a hard 

 coating. The usual form is an octahe- 

 dron, composed of two four- sided pyra- 



mids joined by the base, the faces being 1 

 somewhat convex. Of this form there 

 ap- some modifications : the angles being 

 replaced by triangular tares, so as to give 

 rise to a dodecahedron of twenty-four 

 faces, likewise a little convex. These are 

 the crystallizations of the Oriental dia- 

 mond. The Brazilian is generally a do- 

 decahedron, with rhomboidal faces. These 

 crystalline forms are often imperfect, pro- 

 bably from the attrition which they have 

 suffered, and frequently the fragments 

 are altogether indistinct 



The diamond is colourless, or tinged 

 of various shades of white or grey, and 

 sometimes also, though more rarely, of 

 brown, green, yellow, blue, and red, fre- 

 quently with darker coloured spots. It is 

 generally transparent, though not perfect- 

 ly so, and has the property of single re- 

 fraction ; its fracture is lamellated, and it 

 can be split by striking it in the direction 

 of the plates. Its specific gravity is from 

 3500 to 3600. 



The diamond is phosphorescent, or, 

 when it has been exposed to the light, is 

 luminous in the dark. It is rendered 

 electrical by rubbing, the electricity be- 

 ing positive. 



From the qualities of the diamond it 

 was long ranked with the other gems, 

 and considered as analogous to them in 

 its chemical construction. Newton, by a 

 happy application of a physical princi- v 

 pie,, conjectured that it was an inflam- 

 mable substance. Transparent bodies, 

 which are uninflammable, refract light 

 nearly in the ratio of their densi: 

 while those which arc inflammable have 

 refractive powers which are greater than 

 their densities ; and the diamond having 

 this great refractive power led Newton 

 to conclude, that it "probably is an unc- 

 tuous substance Coagulated. (Optics, 

 Monk II. Prop. 10.) In 1(>9J experiments 

 had been made at Florence, which prov- 

 ed the diamond to be dissipated by the 

 intense heat in the focus of the power- 

 ful burning lens of Tschirnausen. After- 

 ward, in experiments made. 1 at Vienna, it 

 was found, that in the heat of a furn . 

 diamonds lost weight, and, if 

 for a sufficient length of time, entirely 

 disappeared, while the rubv and other 

 gems exposed to the same heat remain- 

 ed unaltered. At a later period Darcet 

 exposed diamonds to heat, enclosed in 

 balls of porcelain clay in various \\ .. 

 and always found that they wer 

 sipated by exposure to a strong h 

 These facts at the same time appeared in 

 contrad'cliou to the common pnr'J'-e ot 



