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called Adamas Gemma by the Latins, Demant parts transparent, and in others charged with foulness,
water; or by rubbing it with a piece of glass. By
. In confirmation of this, weight *.
n° 386. he mentions a large diamond worth upwards of 5000l. viii. art. 9. we may read a relation of experiments Sterling, which burst into nine pieces while polishing made on precious stones, by order of the grand duke on the wheel at Venice.
of Tuscany, with a burving lens, the diameter of which The finest diamonds are those of a complexion like was two thirds of a Florentine ell, near the focus of that of a drop of pure water. It is likewise a valuable pro- which was placed another smaller lens. By these experty if they are of a regular form and truly made; as periments we find, that diamonds were more altered by also that they be free from stains, spots, specks, flaws, solar heat than most of the other precious stones, aland cross veins. If diamonds are tinctured yellow, though not the leait appearance of a commencing fublue, green, or red, in a high degree, they are next fion was observable. A diamond weighing 30 grains, in efteem; but if they are tin&tured with these colours thus expofed during 30 seconds, loit its colour, luftre, only in a low degree, the value of them is greatly di- and transparency, and became of an opaque white. In minished. There are also diamonds of other com- five minutes, bubbles appeared on its furface; soon afplexions ; such as brown, and those of a dark hue: terwards it burst into pieces, which were dissipated ; the first resembling the browneft sugar-candy, and the and the small fragment which remained was capable of latter dusky iron. In the Philosophical Commerce of being crushed into fine powder by the presiure of the Arts, Dr Lewis tells us of a black diamond that he blade of a knife. Neither the addition of glafi, flints, himself had seen. At a distance, it looked uniformly fulphur, metals, or falt of tartar, prevented this diffiVol. VI. Part I.