GEL

GEL

( 473 )

tion of true gelatin in jjj times its weight ofwater, or equally dilute with the prefent folu-tion, would have given an immediate andcopious precipitate with tan, a chara£terfufficiently difcriminative. On the other handgelatin will not be precipitated by acetitedlitharge, whereas mucus is abundantly affedtedby this teft. Thefe differences, therefore,together with the abfence of the gelatinizingproperty, will, for the prefent, entitle us toconfider mucus as a diftindt animal principle.

GELBERDE. See Yellow Earth.

GEMS. Gems or precious ftones form a clafsof earthy minerals very diftinguifhable in com-merce by their fuperior price, but of which it isnot eafy to give a fcientific definition. Thatfurnifhed by Boetius de Boot 1 is, perhaps, uponthe whole, the beft which is to be found.“ Lapis ” fays he, “ alius a Natura magnus,“ alius parvus producitur; parvus alius rarusf< inventu, alius frequens. Rarus alius durus,“ alius mollis. Durus alius pulcher afpcdtuqueu gratus, alius turpis. Pulcher Gemma nomen“ meretur. Erit itaque gemma lapis parvus,u rarus, durus, et pulcher, a Natura procreatus.”Nature forms fame fanes large and others fmall;of the fmall , fame are of rare occurrence , others arecommon ; of the rare, fame are hard , others arefoft ; of the hard , fame are beautiful and pleafant tothe ftghty others are mean. The beautiful merit thename of Gems . The word gem thereforeftgnifes anaturalfone of fmallfze, rare , hard, and beautiful.

The ancients feem to have made a differencebetween precious ftones and gems, comprehend-ing under the former term thofe hard andbeautiful, but for the molt part uncryftallizedftones, fuch as agate, carnelian, onyx, chalce-dony, &c. upon which they were accuftomedto engrave, reftridting the word gem to thofeftones which, on account of their minutenefs,extreme hardnefs, and brilliant luftre, wereworn in rings and other ornaments, either inthe ftate in which nature prefents them, or, atmoft, after being cut and polifhed. Someftones, however, fuch as the topaz and ame-thyft, being commoner and in larger pieces thanmoft of the other gems, held a kind of middlerank between thefe and the precious ftones,being fometimes employed as materials toengrave upon, and fometimes ufed plain fornecklaces, rings, & c .

Bergman in his excellent effay on the earthof gems has fhown, contrary to the generalopinion of his day, that the hardnefs and crys-talline texture of the gems is not owing to

their confifting almoft entirely of vitrifiableearth or filex ; fince many of the gems, efpeciallythofe called oriental, appear from analyfis tocontain a much larger quantity of alumine thanof filex. On the other hand, however, thisable chemifl himfelf is deceived when he laboursto thew that all the moft perfect gems, includ-ing the diamond, are indebted for their hard-nefs, exceeding fpecific gravity and luftre tothe prefence of alumine in their compofition tit being now fully afcertained that the diamondwhen pure is wholly combuftible, and thereforecontains no earth whatever.

The particular gems are defcribed at large intheir proper places; we lhal 1 therefore onlygive a lift of them here, arranging them in fourclaffes, according to the relative eftimation inwhich when perfect they are held by jewellers ;premifing, however, that this order is fubjectto fome variations from the cafual plenty orfcarcity of any particular kind, and from thecaprice of fafliion.

The diamond and the oriental ruby conftitutethe firft clafs. Of thefe, a very fmall diamondis more valuable than a ruby of equal weight;but rubies of ten carats or more being muchrarer than diamonds of equal weight bear a.fomewhat higher price in the market. Thefecond clafs of gems contains the emerald, ftar-ftone, oriental girafol, fapphire, fpinel and balaisrubies, oriental topaz, oriental ametliyft, andnoble opal. In the third rank are found thejargoon, cat’s-eye, oriental chryfolite, hyacinth,and peridot. Thofe in the fourth and loweftclafs are often engraved upon, and form themoft valuable feal-ftones; they are the beryllor aqua-marine, Brazilian topaz, Saxon topaz,Sirian garnet, Bohemian garnet, and Europeanamethyft.

GEMS ARTIFICIAL. See Glass (coloured.)

GILDING , art of. I..'Art de Dorer, Fr.

Die Kuttf zu Vergolden , Germ.

The art of gilding or of laying a thin fuper-ficial coating of gold on wood, metal and otherfubftances has been long pradtifed, and highlyefteemed both for its utility and the fplendideffect which it produces. Gold, from the ex-treme beauty of its colour, and from the lengthof time during which it may be expofed to theaction of the air without tarnifhing, is perhapsthe moft valuable of all fubftances for the pur-pofe of decoration ; but on account of its dear-nefs and weight it cart very feldom be employedin fubftance, and its ornamental ufe would belimited indeed if it was not at the fame time

«De Gemnus, p. rj.

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VOL. I.