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t “ ] 23. Some ftones of this clafs are of a regular angular form, fine tranfparency, brightnefs of colours, and fuperior hardnefs, and are then called gems. The diamonds, which, on ac count of their bright radiant fplendor and immenfe hardnefs, (that will yield to no file, tool, or fubftan.ce whatfoever, except dia- mond-powder), are the firft: in this genus of ftones, will, however, in a long-continued violent fire, firft grow opaque, then lamel- lated, and laftly fo volatile as entirely to fly off*. The ruby is next in rank in point of hardnefs, but is unalterable in regard to lize, colour, and texture. The reft are all fofter-, fome will even be attacked by a file (viz. the emerald, chryfolite, amethyft, hyacinth, berill, and garnet), and fome loie colour and texture in a violent fire. 24. Quartz, one of the flinty ftones, is, according to the experiments mentioned No. 21, a com pound of the acid of fiuor and fleams of water meeting together, This circumftance makes it highly probable, that diamonds are likewife formed by the moft fubtle fteam of water and of acid of fluor, becaufe they are fo volatile in a violent long-continued fire. 25. Stones of a lamellated, glofly texture, whofe lamellae are commonly pliable, which, accord ing to experiments IV.—VIII. are not afled upon by acids or fire, except that its lamellae Jofe their flexibility, and Ihiver into thinner * The experiments of the late emperor Francis are mentioned in Hcnckel'i Oeuvres, Paris, 1760, 410. vol. IL p. 413. note; and thofe of Mr. Darcet, on Diamonds, are found in his Me- ptoire fur un feu 'violent tsf continue pendant un long terns; and in his Mepioire fecond. leaves,