576
GILDING
Diamonds, sapphires, emeralds, rubies, topazes, hyacinths, and chrysoberyls, are reck'oned the most valuable gems.
Cyrstalline quartz, pellucid, opalescent, or of various hues, amethyst, lapis lazuli,malachite, jasper, agate, &c., are.ranked in the much more numerous and inferior clasof ornamental stones. These distinctions are not founded upon any strict philosophic®principle, but are regulated by a conventional agreement, not very well defined; fi> r Iis impossible to subject these creatures of fashion and taste to the rigid subdivisions oscience. We have only to consider the value currently attached to them, and take car^not to confound two stones of the same color, but which may be very differently prizeby the virtuoso. ,
Since it usually happens that the true gems are in a cut and polished state, or even sin gold or silver, we are thereby unable to apply to them the criteria of mineralogical anchemical science. The cutting of the stone has removed or masked its crystalline cha^acter, and circumstances rarely permit the phenomena of double or single refractionbe observed; while the test by the blowpipe is inadmissible. Hence the only scientiresources that remain are the trial by electricity, which is often inconclusive; the degtof hardness, a criterion requiring great experience in the person who employs it; an , ’lastly, the proof by specific gravity, unquestionably one of the surest means of distinguiing the really fine gems from ornamental stones of similar color. This proof can be \plied only to a stone that is not set; but the richer gems are usually dismounted, ^offered for sale. ,
This character of specific gravity may be applied by any person of common 1 g,, egence,. with the aid of a small hydrostatic balance. If, for example, a stone of acrimson-red color be offered for sale, as an oriental ruby; the purchaser must ascer jp 0if it be not a Siberian tourmaline, or ruby spinel. Supposing its weight in air to begrains, if he finds it reduced to 69 grains, when weighed in water, he concludes 6 > a ]
bulk is equal to that of 31 grains of water, which is its loss of weight. No'V, 3 .sapphire which weighs 100 grains in air, would have weighed 76-6 in water; 3 r'jflOruby of 100 grains would have weighed 72'2 in water, and a Siberian tourmalin ® 0 neS .grains would have weighed only 69 grains in water. The quality of the stone > n 1, e jtion is, therefore, determined beyond all dispute, and the purchaser may be thus P rotfrom fraud. e of
The sard of the English jewellers ( Sardoine , French ) is a stone of the n at j n t oagate, having an orange color more or less deep, and. passing by insensible shadeyellow, reddish, and brown; whence it has been agreed to unite under this denomiall the agates whose color verges upon brown. It should be remarked, however, •*>sard presents, in its interior and in the middle of its ground, concentric zones, ornebulosities, which are not to be seen in the red cornelian, properly so ca ^ e “'„ ra v.e<I>ancients certainly knew our sard, since they have left us a great many of them en j? reI) clbbut they seem to have associated under the title sarda both the sardoine of the ^ r .and our cornelians and calcedonies. Pliny says that the sarda came from the neI ’ ;hehood of a city of that name in Lydia, and from the environs of Babylon. ' p ° va le ofengraved sards which exist in the collection of antiques in the Bibliotheque Boy ^ 0ll eParis, there is an Apollo remarkable for its fine color and great size. When g^s,forms a part of the agate-onyx, it is called sardonyx. For further details upoand the art of cutting and engraving them, See Lapidary. descr'K
GEOGNOSY, means a knowledge of the structure of the earth; Geology, oY ; nC e oftion of the same. The discussion of this subject does not come within the Pthis Dictionary.
GERMINATION (Eng. and Fr.; Das Keimrn, Germ.) is the firstseed after it is sown, or when, after steeping, it is spread upon the malt floor.
GIG MACHINES are rotatory drums, mounted with thistles or wire teeth *°cloth. See Woollen Manufacture. f s
GILDING (Dortwe, Fr.; Vergoldung , Germ.) is the art of coating s “^, a . ^onj £*1.thin film of gold. For a full discussion of this subject, See Gold. Mr. LI ^ ea ps oItoy maker, obtained a patent in June, 1836, for gilding copper, brass, & c *> .j jye, s ?7potash or soda combined with carbonic acid, and with a solution of gold. Lj 1 foil 0 '** 0 ?he, 5 oz. troy of fine gold in 52 oz. avoirdupois of nitro-muriatic acid of t 1 p]oH atlproportions: viz. 21 oz. of pure nitric acid, of spec. grav. 1*45, 17 oz. of P #
acid, of spec. grav. 1*15; with 14 oz. of distilled water. , j, ea ted J ?he
■ The gold being put into the mixture of acids and water, they are to e con tia ue .glass or other convenient vessel till the gold is dissolved; and it is usual geS to r1 ®,^
application of heat after this is effected, until a reddish or yellowish vapor ' gei>er a >■
The clear liquid is to be carefully poured off from any sediment - n ailoy^appears and results from a small portion of silver, which is generally f° u ^yare is Pgold. The clear liquid is to be placed in a suitable vessel of stone, P^^ e nll nds of ^ lCferred. Add to the solution of gold 4 gallons of distilled water, and 20 P
of*
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