Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/789

 GEM

GEM

t!c notch, and is bent upward toward the other line, I together with which it forms a fort of irregular fenii- circle. Both thefe mufcles have alfo a fmall infertion in the infide of the ifchium, where being united together, by a particular membrane, one of them joins the upper fide, and the other the lower fide of the obturator internus a little after it has paffed over the notch. They enclofe it as in a bag, and continue to be fixed to it by fleflry fibres all the way to its extremity. The fuperior mufcle terminates wholly with the tendon of the obturator internm, but the inferior being broader, is inferted likewife by flefhy fibres, in the orbi- cular ligament, and under the tendon of the fame ob- turator. WinJIoxtfs Anat. p. 210. GEMS (Cycl.) — Mr. Boyle fpeaks of tinctures from gems, and of his having diffolved them; and feems to think it not im- probable, that they may have medicinal ufes exhibited in fub- ftance. Boyle's Works, Abr. Vol. I. p. 62. Gems of the antients. The gems, on which the antients en- graved, and which are commonly called intaglios, are ufu- ally of a long oval figure inclining to a point at each end, convex as well upon the engraved furface as on the others, and having a ridge running from end to end on the under- fide, which is thereby as it were divided into two faces ; both which are alfo, though not fo difti nelly, parted from the upper face by another ridge running quite round the oval. The ftone moft frequently engraved on, is what the jewellers call the beryl, a ftone very different from that called the beryl by the antients, which was the gem we call the aquamarine.

This beryl of the jewellers is only a finer kind of cornelian, and is of three kinds ; the red, the yellow, and the white. The firft is much the moft valuable of thefe, and is of the ■ greateft luftre ; that which is moft frequent next this beryl, is the prafius, or as fome call it, the plafma de fmaraldo. This our jewellers call the preme or prime emerald. It is of a fomewhat dufky green, and is fometimes tolerably clear, but moft ufually it is full of black and white fpecks, and rather opake.

The moft frequent next to this is the jacynth or hyacinth. This alfo is not the gem known by the antients under this name, that having been a fort of amethyft, but the ftone we now call the hyacinth, which is of a tawny red, or a fort of fire colour, and is very bright and tranfparent. The chryfolite is fometimes but rarely found engraved on. It is of a greenifti colour, and is tranfparent but not lively. Sometimes there are alfo met with the oriental peb- ble cryftal, which is much more tranfparent and lively than the common cryftal, and indeed but little inferior in beauty to the white fapphire: the garnet which is of a deep red, and not fo lively as the hyacinth ; and finally the amethyft which is of a deep purple, and is very lively and tranfpa- rent. Thefe feem to have been all the ftones ufed in the earlier times of the Roman empire. There are indeed fome- times found fome other fpecies, but thefe are rarely of any time but the latter ages of the empire, when the art it'felf was greatly upon the decline. The beforementioned ftones are faid to have been brought to Rome from the borders of the Nile or of the Ganges. The onyx agate, onyx fardonyx, and alabafter of two colours were alfo engraved on by the Romans in a different manner. Thefe, as alfo certain fhells of different coats, were frequently engraved in relief, and thefe fort of engravings are commonly called cameos; they alfo fometimes engrafted a head or fome other figure in re- lief of gold upon a bloodftone. Philof. Tranf. N°. 483. Bcfide thefe there are alio fome antiques, moftly cornelians, which are covered with a ftratum of white. This ftratum has by fome been looked on as natural ; but it was really only a fort of coat or enamel that was laid on : this was ufed only in the times of the lower empire. The antients engraved many of their ftones juft as they were found with their native polifh, and in their native fhape ; but the beauty of the feveral fpecies of the onyx could only be fhewn by a proper cutting. The antique gems of Greek work are of all others the moft efteemed, and next to them the Roman in the time of the higher empire. Philof. Tranf. N 9 483. Imitation of antique Gems. There has been at different times a method pra&ifed by particular perfons of taking the im- preffions and figures of antique gems, with their engravings in glafs of the colour of the original gem. This has always been efteemed a very valuable method, and graatly preferable to the more ordinary ones of doing it on fealing wax or brim- ftone ; but to the misfortune of the world, this art being a fecret only in the hands of fome particular perfons, who got their bread by it, died with them, and every new artift was obliged to new invent the method, till at length Mr. Hom- berg having found it in great perfection, gave the whole procefs to the world to be no more forgotten or loft, and fince that time it has been very commonly pra&ifed in France, and fometimes in other places.

Mr. Homberg was favoured in his attempts with all the en- graved gems of the king's cabinet, and took fuch elegant impreffions, and made fuch exact refemblances of the origi- nals, and that in glades fo artfully tinged to the colour of the gems themfelves, that the niceft judges were deceived in Suppl. Vol. I.

them, and often took them for true antique ftone!!. Thefe coun- terfeit gems will ferve as well as the original ones to make more copies from afterwards ; fo that there is no end of the num- bers that may be made from one ; and there is this farther ad- vantage, that the copy may be eaftiy made perfect, though the original fhould not be fo, but fhould have fuftained fome damage from a blow orotherwife. Mem. Acad. Par. 17 12. The great care in the operation, is to take the impreffion of the gem in a very fine earth, and to prefs down upon this a piece of! a proper glafs foftened or half melted at the fire, fo that the figures of the impreffion made in the earth, may be nicely and perfectly expreffed upon the glafs. In general, the whole procefs much refembles that of the com- mon founders. But when it is brought to the trial, there are found a number of difficulties which were not to be forefeen, and which would not at all affect the common works of the founder. For his purpofe every earth will ferve, that is fine enough to receive the impreffions, and tough enough not to crack in the drying ; thefe all ferve for their ufe, becaufe the metals which they caft are of a nature incapable of mixing with earth, or receiving it into them, even if both are melted together, fo that the metal always eafily and perfectly fepa- rates itfelf from the mould ; but it is very difficult in thefe cafts of glafs. They are compofed of a matter which differs in nothing from that of the mould, but that it has been run into this form by the force of fire, and the other has not yet been fo run j but is ©n any occafion ready to be fo run, and will mix itfelf infeparably with the glafs in a large fire; and coufequently if there be not great eare ufed, as well in the choice of the glafs, as in the manner of uimg it, when the whole is fmifhed, there will be found great difficulty in the feparating the glafs from the mould, and often this can- not be done without wholly destroying the impreffion. All earths run more or lefs eafily in the fire, as they are more or lefs mixed with faline particles in their natural formation. As all falts make earths run into glafs, and as it is neceffary to ufe an earth on this occafion for the making a mould, it being alfo neceffary to the perfection of the experiment, that this earth fhould not melt or run ; it is our bufinefs to fearch out for this purpofe fome earth, which naturally contains very little fait. It might be fuppofed, that earths artificially diverted of their falts by means of fire or water, or by what- ever other procefs, might ferve this purpofe ; but experiment proves this not to be the cafe. Thefe earths, though divefted of their falts, yet retain thofe cellules where their faline par- ticles were once lodged, and are therefore ready to receive them on occafion again ; and glafs being only a compofition of earth, and falts run together as foon as it becomes hot, its faline parts are lefs firmly united than before to its earth, and readily run into the cellules of a foreign body, which are open to receive them ; and thence when fuch earths are em- ployed as mould, the glafs and the moulds are always found fo blended together that the whole is fpoiled. Whereas on the other hand, thofe earths which naturally contain little or no fait, have no pores, or cellules ready to receive any, and never blend themfelves with the glafs, unlefs the whole is kept too long over a great fire, in which cafe the falts of the glafs will get loofe and ferve as a flux to the reft, and fo deftroy the whole. Of all the fpecies of earth which Mr. Homberg examined on this occafion, none proved fo much divefted of falts, or fo fit for the purpofe as the common tripela, or tripoly, ufed to polifh glafs and ftones. Of this earth there are two common kinds, the one reddifh, and compofed of feveral flakes or ftrata ; the other yellowifh, and of a fimple Structure. Thefe are both to be had in the fhops. The latter kind is from the Levant, and the former is found in England, France, and many other places. This tri- pela muft be chofen foft and fmooth to the touch, and not mixed with fandy or other extraneous matter. The yel- low kind is the beft of the two, and is commonly called Venetian Tripoli. This receives the impreffions very beau- tifully, and never mixes with the glafs in the operation, which the red kind fometimes does. Mr. Homberg ufually employed both kinds at once in the following manner. Firft powder a quantity of the red tripela in an iron mortar, and fitting it through a fine fieve, fet it by for ufe j then fcrape with aknifea quantity of the yellow tripela into a fort of powder, and afterwards rub it till very fine in a glafs mortar with a glafs peftle. The finer this powder is, the finer will be the impreffion, and the more accurately perfect the caft. The artificer might naturally fuppofe that the beft method to ob- tain a perfedly fine powder of this earth, would be by wafh- ing it in water ; but he muft be cautioned againft this. There is naturally in this yellow tripoly a fort of unauofity, which when it is formed into a mould keeps its granules to- gether, and gives the whole a uniform glofly furface : now the wafhincr the powder takes away this unauofity, and though it render it much finer, it makes it leave a granu- lated furface, not this fmooth one in the mould, and this muft render the furface of the caft lefs fmooth. Mem. Acad.

Par. ibid. , ,■■■,-!■

When the two tripelas are thus feparately powdered, the red kind muft be mixed with fo much water, as will bring it to the confidence of a pafte, fo that it may be moulded like 12 E a lump