G O L

G O L

C 532 )

filings, and heated, although each particle isperfeftly fluid, it is extremely difficult to makethem unite into one mafs without having re-courfe to fome faline flux, and none is foefficacious for this purpofe as borax, whichaccordingly is employed by the goldfmiths onall occafions, even when apparently it is not atall wanted. If cooled very flowly it ffiews aftrong tendency to cryftallization in the appear-ance of brilliant facets on its furface like thepetals of a flower, and by pouring off the fluidpart when it is half congealed, it may beobtained in fhort tetrahedral prifms. No vapourarifes from gold when in fufion, and its remark-able fixity is proved by Kunckel who kept aquantity for almoft thirty weeks in the hightemperature of a glafs-houfe furnace withoutthe lofs of a fingle grain, or any other alteration.When a fmall portion however is expofed in thefocus of a very powerful lens, or fubjefted tothe aftion of a ftream of oxygen gas, it is vola-tilized unaltered and will gild a plate of filverheld over it.

The poffibility of altering gold by expofureto heat and air has long been a fubjeft ofcontroverfy, nor do modern chemifts feem byany means agreed on this point: there appearshowever no good reafon to doubt that gold maybe thus oxydated by particular management,though many of the facts ufually brought inproof of it, are without queftion erroneous.Homberg affirms that a fmall quantity of goldplaced upon charcoal and fubjefted to the actionof a very powerful lens, began to fmoak, and wasin part vitrified. Of the faft there is no reafon todoubt, but the vitrefied portion in all probabilityconfifted of the allies of the charcoal, colouredby fome of the alloy of the gold, as indeed wasthe opinion of fome of the affiftants at this veryexperiment. We have already cited the tefti-monv of Kunckel to fhow the unalterability ofgold, by the continued aft ion of a high tem-perature, but the fame cherr.ift affures us, thatthis metal when prepared in a particular way,and expofed in fine powder to the aftion of areverberatory furnace not fufficiently hot tomelt it, is at length converted into an irreduci-ble calx. It appears likely, that the preparationhere alluded to, was alloying the metal, proba-bly with tin: for in this fiate it is pulverizeable,and by a treatment fimilar to the one herementioned, is entirely convertible into a fpungyoxyd of a purple colour, and difficult reduftion.This effeft of the tin does not appear to beowing to any chemical aftion on the gold, but

merely to its feparating the particles of thismetal from each other, in which ftate it isreducible into a purple oxyd without muchdifficulty: for “ if gold leaf be divided by grind-“ ing with an admixture of earthy powders, as“ calcined hartfhorn and chalk, or with faline“ ones of the more fixed and lefs fufible kind,“ as vitriolated tartar, and expofed for fixteen<e or twenty hours to a moderate heat, fcarce“ fufficient to keep the veflel red hot: the gold“ wholly lofes its metallic brightnefs,and changes“ its yellow colour to a red or purple. ” kFrom this mixture the fait may be diffolved outby water, or the chalk by a weak acid, and thepure oxyd of gold remains behind. Gold mayin like manner be converted into a purple oxydby a ftrong difcharge of eleftricity, as is mani-feft from the common experiment of palling aIhock through gold-leaf laid between two platesof glafs, and thus tinging the glafs purple; andftill more decidedly from an experiment ofVan Marum, in which a gold wire was ignitedby eleftricity, and volatilized with a greenflame; the vapour being condenfed, proved tobe the purple oxyd of gold. The relative pro-portions of the two ingredients in this oxydhave not been afcertained. When expofed to afull red heat it is decompofed, the oxygenflying off, and the gold in its metallic ftateremaining behind.

If gold is precipitated by a fixed alkali fromits folution in nitro-muriatic acid, a yellow•oxyd falls down, which is fuppofed by Dr.Thomfon to differ from the purple oxyd incontaining a larger proportion of oxygen: noexperiments have however as yet been inftitu-ted to afcertain this point. The yellow oxydis reducible per fe with the fame eafe as theother.

Gold in its metallic ftate appears to be aftedon only by the following among the clafs ofacids, viz. the nitric, nitro-muriatic, and oxy-muriatic acids.

Pure colourlefs nitric acid is incapable of dif-folving reguline gold alfo pure: but if the acidis highly charged with red nitrous vapour, andin this ftate is digefted on gold-leaf, in a retortor other clofe veflel, the metal is readily takenup. This faft was firft difcovered by Dr.Brandt in the following manner. Wiffiing tofeparate the filver from an alloy compofed of 16parts of filver and 3 of gold, he placed themixture in an alembic with an attached receiver,and boiled on it fucceffive portions of nitricacid, increafing the ftrength of it as he procecd-

k Lewis, p. 74.