Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/813

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is large, and the jaws have each two rows of teeth. The belly fins join together into one, and make a fort of funnel fhaped hollow, by which the fifh is fuppofed to apply itfelf, and flick faft to the rocks, whence it has its name of rock fifh. It is very foft and tlippery, yet is covered with fcales, but t!iey ire very (lender and foft. It is caught about the Englifh and Italian fhores, and lives in fhallow water. ■ It is conftantly in the markets of Rome, Uc. IVillughby, Hift. Pifc. p. 206.

GOHDAKADURA, in botany, a name given by fome au- thors to the tree which produces the mix vomica. Hirm. Muf. Zeyl. p. 41.

GOHKATHU, in botany, a name given by fome authors to the tree which produces the gamboge, or gittta gamba of the fhops. Dale, Pharm. p. 327.

GOING, in the manege, called in French alkure, is the pace or gate of a horfe.

GOLD" (Cyel.) — This metal is found in many places mixed in fmall grains among the fand of rivers. The way of fepa- rating it is this : They have a long trough made with a declivity, and lined at the bottom with flannel ; the fand is received into this, and while it is ftirred about in the water by the hand, the fand is wafhed off, and the fmall particles or gold left in the pores, and among the woolly matter of the flannel, and is afterwards wafhed out from thence. This is the method where the fand is fine, but when it is mixed with large gravel, they fift it through wire fieves, whofe mefhes are of one determinate fize ; by which means all the gold is fure to go through with the fand, and the gravel or larger ftones arc left behind, and thrown away. The fand thus feparated is treated as before, and there is no danger of any gold being thrown away with the gravel, be- caufe it is extremely rare to meet with any one grain of gold in this ftate fo large as a barley corn ; and therefore as the mefhes of their fieves are kept to more than that fize, there is no fear of lofing any of the metal. Sbaiv's Lectures,

P- 2 53-

Mr. Boyle mentions an artift in Holland, who by digefting go'ldwhh aqua fort'ts, had feparated the tincture, or yellow fulphur from it, and made it volatile; the remaining body growiiig white; and that with this, golden tincture he had turned lilver into very perfect gold, with confiderable profit. Mr. Boyle » feems to credit this account ; and fince that Mon- fieur Homberg b declares, he has actually converted filver into go/ibyheat. — ['WorksAbr. Vol. 1. 'p. 166. b Mem. de FAcad. des Sciences 1709.]

Monfieur Homberg is of opinion, that there is in filver fome parts, which, though not yet gold, eafily become fo by means cf fire ; and he thinks that in gold naturally pale, there is a whitifh matter, which fire brings to the true colour of gold. He obferves, that this pale gold does not afford filver, and therefore thinks there is a mean metal between gold and filver. See Mem. de l'Acad. Loo cit.

Mr. Boyle mentions an anti-elixir he tried, that is, one that would dehafe gold, and which changed its malleabili- ty, colour, homogeneity, and fpecific gravity, fo that tho' the powder did not weigh the thoufandth part of the gold, yet the fpecific gravity of the metal to which it was debafed was to that of water, but as 15* to 1. inftead of 19 to 1. See his Works Abr. Vol. t. p. 78.

The folution of gold in vegetable acids has not yet fucceeded according to Mr. Marggraf, but he thinks it poffible, as well as that of filver and mercury. See the articles Silver and Mercury.

It is the received opinion, that whatever has been done to this metal, the common way of purifying it by the copcl reftores it cafily to itfelf again ; but this is not always fo certain, or fo eafy, as is generally imagined. Mr. Homberg put into the copel an ounce of gold; which he had uled in feveral chemical experiments, in order to rcftore it to its original ftate ; hut after four times copelling it, he found that whatever quantity of lead he ufed, ftill the metal, tho' of the right and beautiful colour, was brittle, and not at all in the ductile ftate of pure gold. Finding therefore that lead would not anfwer his purpofe, he incorporated the gold with four times its quantity of filver, and having performed the operation of the depart in the common manner, he fufed it with borax ; but after all this the gold was as brittle as before, though ftill of the fame beautiful colour. He then melted it again without any addition, but ftill it remained in the fame brittle ftate. His next trial was by antimony, not doubting but that the particles, whatever they were, that had flood the operation of the depart, and of lead, would be abforbed by this powerful femi-meral, and the gold left in its natural malleable ftate : He melted it twice to this purpofe with eight ounces of antimony ; but after feparating the an- timony, and then melting the gold feveral times with fait petre, and many times without any mixture, it was found • of the moft perfect and beautiful colour imaginable, but ftill brittle, and not in the leaft malleable. Surprifed to fee all the common methods fail; he melted it again with fix ounces of antimony, and afterwards melted this rcgulus with three times its own weight of lead, and fet the whole in a copcl in a proper degree of fire, to evaporate both tile anti- Suppt. Vol. I.

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mony and the lead. But when the fire was out, he was iurprifed to find the gold covered with a brown'ifh fubftance; refembhng a mufhroom, which fell to powder on being touched by the fingers. The gold itfelf was become of a grey colour, and was full of wrinkles on the upper furface, from whence this fungous matter had been thrown out ; but on the under furface, where it ftuck to the copel, it was ftill of a fine and beautiful gold colour. He then melted feveral times the gold and the fungous fubftaiice together, and every time there appeared the fame fort of fungous cruft at the top. At laft he gathered this fungous fubftance carefully off from the gold, and then melting the metal by itfelf, there was no more any fungous matter thrown out upon it, but only a thin layer of a powder of the fame nature and colour with the former. Three times it was melted, and at every time threw off a quantity of this powder ; and after this being fufed with borax, it at length became pcrfefl malleable gold.

He then fufed together the fungous matter, and all the quantities of powder which he had faved, there appeared a new fungus on the matter, and that for feveral times melt- ing ; but at laft this difappeared, and there was found a fmall lump of pure gold in the copel. It is not eafy to guefs at the caufe of this obftinate brittlenefs of the gold, becaufe it had paffed feveral experiments, and been mixed with fe- veral falts and feveral metals, particularly at laft with iron and with emery. The emery feemed moft to be fufpefled as the caufe, but this brittlenefs cannot be given to gold by emery alone, and muft have been owing to the mixed ef- fect: of fome of the falts and emery together, as by means of the falts, the particles of emery are thrown farther, and more intimately mixed With thofe of the gold than they could otherwife be. We have been told of the degradatiori of gold. This gold would have been declared degraded, and robbed of one of its greateft qualities, its malleability, had it fallen into any hands but thofe of fo great a chemift ; and it is much to be doubted, whether Mr. Hombeig would not have been able to reduce the degraded gold of any other chemift, and reftore it to its natural ftate and purity, though the common methods of operating on it fhould all have fail- ed, as they did in this cafe. Mem. Acad. Par. 1693. The method of rendering gold perfectly pure from filver, by means of aqua regia, is as follows. The mixture of gold and filver muft firft be copelled with lead in the ufual way

which feparates from it all other metals. Out of the

remaining rows fire,

regulus hammer thin plates, and when the metal gi rigid under the hammer, make it red hot in a gentle ... by which means its duaility will be rcftored. When th. plates are fufEciently thin, make them red hot for the laft time, and then cut them into fmall pieces with a pair of fheers. Put thefe pieces into a cucurbite of clear glafs fet in a warm place, and pour on them a fufficient quantity of pure and ftrong aqua regia, flopping the orifice of the cucur- bite with a cornet of paper, to keep out duft. When the diffolution is finifhed the filver will remain at the bottom, in form of a white powder. Pour off the clear fo- lution, and put on the remaining calx fome phlegm of the fpirit of fait ; make the whole boil a good while, to incor- porate all the remaining folution of gold; and pour it off clear to the former folution : This done, infpiffate the whole over a very gentle fire till quite dry ; put what remains into a crucible; cover it with powder of borax, melted beforehand with a little nitre, cover the crucible clofe with a tile, and when the f'ufion is perfect, pour it into an ingot. Cramer's Art of Allaying, p. 260.

The purifying of gold by cementation is to be performed in this manner. Chufe fome tiles or bricks not vitrified by an execflive fire, nor too much tempered with fand; the oldeft alfo are the beft; clean them well from lime and all other filthinefs, pound them in an iron mortar, arid then fift them through a coarfe fieve ; take four parts of this powder, and one part of colcothar not wafhed, and the fame quantity of common fait ; mix thefe thoroughly together, grind them in a mortar; and wet them with a little water or urine, fo that they may cohere when preffed between the hands. Then take a clean earthen veffel of a proper fize (it muft be ouite found) fufriciently thick, and not glazed ; ftrcw the bottom of this with the moiftened powder, or cement, and diftributc it evenly all over with a finger, and prefs in down very gently, fo that the thicknefs of the cement may be half an inch ail about; put upon this the geld in final! pieces, and in thin plates, made perfectly clean by heating them red hot in the fire ; cover the furface of the cement with thefe pieces, then lay fuch another bed of cement over thefe, and over that another of gold, and fo on alternately, till the veflel is full within the breadth of a finger; let this laft fpace be filled with cement ; lute well a tile upon the veffel, and keep it moderately red hot in a furnace for fixteen or twenty hours then open the pots and wafh the purified gold. Cramer's Art of Allaying, p. 271.

A folution of gold in aqua regia affords a very curious object for the microfcope. Mr. Lcwenhoek took sreat .pains to obferve the configurations it would put on in its concretions : He found that in warm weather, if a drop of the folution

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