D I A

D I A

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fomewhat inferior in this refpect to rock cryftal.The frafiture of diamond is ftrait foliated, itslamellas being parallel to the fides of a r*gu-lar odtohedron. Its hardnefs is fuperior to thatof all other bodies natural or artificial, it yieldshowever to corundum by Jong continued friction.It is brittle, and not difficultly frangible. Sp.

gr- 3-518—3.550.

Diamond, even when rough, acquires, byfriction, the vitreous or pofitive electricity. Itbecomes phofphorefcent when expofed either tothe entire rays of the fun, or to the blue onesalone when feparated by the prifm, and con-centrated on the diamond by means of a lens:a fimilar effedft is alfo produced by fixing it tothe end of a charged conductor, and taking afew electric fparks from it. Many diamondshowever are incapable of becoming phofphoref-cent, altliough agreeing in colour, tranfparency,&c. with thofe which readily become luminous.Thofe which are fmall acquire this property bya much fliorter expofure to the light than thelarger ones: fometimes a diamond that is notphofphorefcent by the fimple action of the folarlight, may be made fo by previoufly immerfingit for fome time in melted borax. b

Diamond when expofed to a current of air,and heated to the temperature of melting cop-per, is gradually but completely combuftible,and is furrounded, during the procefs, by aluminous areola. It is wholly converted intocarbonic acid, and therefore confifts of purecarbon, as we liave already mentioned underthat article.

The art of cutting and polifhing diamondswas.probably known to the inhabitants of Hin-doftan and China at a very early period; theonly material however that is ufed in the Eaftfor this purpofe, being corundum, and the appa-ratus being of extreme fimplicity, the jewellersof thefe countries are incapable of bringing outthe peculiar beauty of the diamond in a degreeat all comparable to what is readily effefted byEuropean artifts. The extreme hardnefs of thisgem had baffled the attempts of the jewellers ofEurope till the year 1456, when a young mannamed Louis de Berquin, a native of Bruges ,endeavoured to polilh two diamonds by rubbingthem againft each other ; he found that by thismeans a facet was produced on the furface ofthe diamonds, and in confequence of this hint,conftru&ed a poliffiing wheel, on which, bymeans of diamond powder, he was enabled tocut and polilh this fubftance in the fame manneras other gems are wrought by emery. Previous

v GroiTcr in Journ, ik Phyf. xx. p. 37

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to this difcovery, diamonds were fet in jewelleryprecifely in the Hate in which they arrived fromIndia , and hence the regular odtohedrons weremuch more efteemed than the reft, both onaccount of the regularity of the figure, and thefuperiority of its natural polilh.

Diamonds are cut and manufactured byjewellers 0 into brilliants and rofe diamonds; theformer being for the molt part made out of theodlohedral cryftals, and the latter from thefpheroidal varieties. To fafhion a rough dia-mond into a brilliant the firft ftep is to modifythe faces of the original octohedron, fo thatthe plane formed by the junction of the twopyramids lhall be an exact fquare, and theaxis of the cryftal, or a line connecting theapices of the pyramids, and perpendicular tothis common bafe, lhall be precifely twice thelength of one of the fides of the fquare. Theoctohedron being thus rectified, a feet ion is tobe made parallel to the common bafe or girdle,fo as to cut off X V of the whole heighth fromthe upper pyramid and T ’ T from the lower one :the fuperior and larger plane thus produced iscalled the Table, and the inferior and fmallerone is named the Collet in this ftate it is calleda complete fquare table diamond. To convert itinto a brilliant, two triangular facets are placedon each fide of the table, thus converting itfrom a fquare to an octagon, a lozenge-lhapedfacet is alfo placed at each of the four cornersof the table, and another lozenge extendinglengthways along the whole of each fide of theoriginal fquare of the table, which, with twotriangular facets fet on the bafe of each lozenge,completes the whole number of facets on thetable fide of the diamond, viz. eight lozengesand twenty-four triangles: on the collet fideare formed four irregular pentagons alternatingwith as many irregular lozenges, radiating from'the collet as a centre, and bordered by fixteentriangular facets adjoining the girdle. Thebrilliant being tlius completed is fet with thetable fide upwards, and the collet fide implantedin the cavity made to receive the diamond. Theregular rofe diamond is formed by inferibing aregular o£tagon in the centre of the table fideof the ftone, and bordering it by eight rightangled triangles, the bafes of which correfpondwith the fides of the odtagon; beyond thefe isa chain of eight trapeziums, and another of fix-teen triangles. The collet fide confifts alfo ofa minute central octagon, from every angle ofwhich proceeds a ray to the edge of the girdle,forming the whole furface into eight trapeziums,-1. c Jeffrie* on Diamond), p. 9,