This historic book may have numerous typos and missing text
Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1813. Excerpt: ... CHAP. VI. TURQUOIS LAPIS-LAZULI CHRYSO PRASE SEMI-OPAL CARNELIAN SARDONYX AGATE--JASPER. TURQUOIS. THERE are two kinds of Turquois, one a variety of fossil ivory, the other, probably, a peculiar mineral substance. The former (called also Turquoise de nouvelle roche,) is of a sky blue colour, passing into greenish-blue and apple-green, often presenting black dendritical delineations, which greatly impair the beauty and value of the substance. The remains of animal organization are visible in its texture, which presents slender fibres of a lighter tinge than the rest of the mass, either parallel, or crossing each other, so as to form a kind of net-work, according as the section has been made parallel to the length.or thickness of the tusk. Its hardness is somewhat superior to that of glass; it effervesces slightly with acids, and its specific gravity varies from 3. to 3.5. It appears from analysis, to be bone or ivory, coloured by phosphate of iron. The other kind, called Turquoise de vieille roche, although nearly resembling the former in colour, may be readily distinguished by the following characters:--Its texture is perfectly even and uniform; its specific gravity varies from 2. to 2.5: it does not effervesce with acids. It is composed, according to Descotils, chiefly of alumine, coloured by phosphate of copper. In commerce, however, the two varieties of Turquois, are for the most part, confounded together, the distinction of oriental and occidental, in this, as in many other analogous cases, serving only to discriminate the finest from the inferior specimens, without any reference to the real or supposed place of their origin, or the difference in their chemical composition. The most valued kinds of Turquois, come from Persia, but the best specimens ra...