Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/814

 G O L

G O L

"Was laid on a clean glafs, and examined by the micro- fcope, the falts foon coagulated, but appeared very irregular in their manner of concretion. They fometimes affumed a regular hexangular form, but more frequently they were rendered irregular, by the application of other fubftances, in form of fmaller cryftals upon them. One thing in this is very remarkable, that is, the cryftals formed at different times feem very differently impregnated with the metal, thofe which concrete firft being ufually large, and perfectly clear and tranfparent, and thofc which afterwards form themfelves up- on thefe, and alter and injure their figures, being fmaller and feemingly much more impregnated with the metal, as being all of a fine gold yellow. Sometimes alfo it happens, that the fmall falts fated with the particles of the gold fhoot firft, and when the other clear and tranfparent cryftals of fait form themfelves afterwards, it not unfrequently happens that they form about the others, and enclofe them within their own bodies. Thus it is not uncommon to fee one of thefe large and tranfparent cryftals with a fmall gold coloured cryftal immerfed in its center.

This is a very elegant and entertaining fight by the micro- fcope, but it is only to be obferved in dry feafons, for in moift weather the cryftals either do not form themfelves at all, or elfe it is done but very confufedly, and they melt and diffolve away on the accefs of frefh damp air. If a fmall piece of copper be put into this folution of goldln aqua regia, the liquor immediately operates upon it, and not only diftblves the copper, but precipitates the gold it had before held in a ftate of folution upon the furface of the copper, in the place of what copper it had taken away. The fame thing happens when a piece of copper is put into a fo- lution of filver in aqua fortis, but in this laft cafe the effect is more beautiful ; for the filver particles precipitated on the copper arrange themfelves in the form of trees and fhrubs, like the arbor Dianse; [feeDi ANiE Arbor} but in the cafe be- fore us, the gold is only precipitated in form of irregular fi- laments, or fmall granules, and feems rather to imitate that procefs obferved inthe famous water of theZiment, or Cement Springs in Hungary ; where the water being highly impreg- nated with a folution of blue vitriol, containing a very large portion of copper, if a piece of iron be put into it, imme- diately it works upon the iron, and diffolving it, precipitates copper in fmall irregular granules in its place : So that in the opinion of the vulgar, the copper is made of the iron, but in reality the iron is all diffolved, and the copper left in a folid form by the menftruum which diffolved it j as in this experiment, the copper put into the folution of gold is dif- folved, and taken up into the liquor, and the gold which was before diffolved is precipitated in its place. Philof. Tranf. N u 286. p. 1438.

If the afliftance of art be called in, and the folution of gold be made according to the rules of chemiftry in a fand heat, and the menftruum fo fated with it, that it can diffolve no more, then it readily moots into cryftals, on being expofed to dry in drops before the microfcope on plates of glafs ; and the cryftals which it forms are in this cafe all of one kind, all fated with gold, of a regularly angular figure, and their angles very fharp.

Thefe cryftals appear only like fo many pieces of pure gold, of a fine luftre and brightnefs, and feem maffes of native gold of this form juft taken from fome mine. In order to procure thefe gold cryftals as regularly formed as can be, the fmall drop, laid on the plate of glafs, mult be poured off again at one fide, and by this means the liquor being fpread extremely thin, the cryftals will be formed extremely regular ; but then they will be fo fmall, that they are only vifible through very powerful glaffes; when thus managed, however, they fhew their true form, which is regularly hexangular, but that Hot in form of a column, as in the hexangular cryftals ; but in a fhort and thick mafs, refembling a die, only more long ftiaped, not fo exactly fquare. Notwithftanding that thefe fmall cryftals are yellow, they are as tranfparent as cry- ftal, and the liquor remaining about them is alfo yellow and tranfparent. Some of thefe cryftals, though of the fame figure with the others, appear not of the fame even and plain rftructure, but are plainly feen to be formed of a great num- ber of rings, or concentric circles, extended round each other. Copper being put into this folution, fo highly fated with gold, is not preyed upon fo faft as in a weaker folution ; fome few air bubbles, however, are driven to the furface, but without any great effect j but if the folution be diluted with a little water, the acid then preys with violence upon a piece of copper put into it. A part of the copper is foon dif- folved, and the liquor becomes green, and a fmall portion of the gold, out of the folution, is precipitated on the copper, in the place of that part of it which was diffolved. In this cafe alfo the gold, in fome meafure, emulates the appearance of the filver in its concretion ; for though it does not form regular trees and branches, it forms long and flender filaments, and thefe do not look of a pure fine yellow, or like pure gold, but feem dufky, and to have fome copper among them. If a fmall parcel of this folution .be evaporated on a glafs plate over a gentle heat, the nature of the metal, and its ef- Ijs&s on vitrified bodies, are more eafily difcovergd than onel

would expect in fuch fmall quantities. Where the Hquoe has lain the thinneft in this cafe, the glafs is tinged red by the entrance of the particles of gold into its furface, which they do eafily; becaufe in thofe places where the liquor lay thin, they formed themfelves into extremely minute concre- tions, according to the foregoing obfervatiohs ; and it is well known, that red is the colour which gold imparts inthe earth to cryftals and other ftones, and in the experiments of the chemifts, to glafs of any kind. In fuch places as the liquor has lain thicker in, and has been confequently evaporated in larger quantity, and from a larger portion of the gold, the glafs is not tinged red, but there is feen a remainder of the ib- lution fticking to it : This however is not formed into cry- ftals, but is in this cafe pure gold, left from the evaporation in its proper metalline form. This however was in fuch minute concretions, that where they lay fingle, or feparate, they were not vifible, according to Mr. Lewenhoek's ac- count, to the naked eye, and lie computes that many thou- fand millions of them were not equal in fize to a fingle grain of fand : In moft places they were not however thus fepa- rate, but were formed into oblong and flender concretions, which were fometimes fimple, and fometimes ramofe, and the microfcope fhews thefe concretions very elegantly, fome in form of fimple threads, fome of branches of trees or plants. Philof. Tranf. N" 286. p. 1441. The denfity of gold is no way better feen than in this ex- periment ; for though it may eafily he conceived, that fuch particles of thefe as were but of a thouland millionth part, or much lefs than that, of the bignefs of a grain of fand, muff, needs be of an almoft inconceivable thinnefs, yet not the fmalleft of thefe would tranfmit any light through it, but all were as perfectly opake as a piece of flieet lead. A fmall drop of a folution of gold in aqua regia being laid over a ftrong fire, approaching it by degrees, till being expofed to its utmoft fury, both the gold, and the glafs on which it lies, begin to melt; in this cafe the gold is found concreted in like manner into filaments compofed of globules, or of particles nearly round ; and if it be taken from the fire juft at that time, when the glafs only begins to run, it affords to the microfcope a very diftinet way of obferving the ra- mifications of the gold and filver ores in their firft ftate, and fhews how they are formed, ibid.

Mr. Homberg found by experiment, that gold properly ma- naged in the focus of the great burning glafs, after the eva- poration of fome of its conftituent parts, was vitrifiable, like other metals. That part of this metal which flies off in fume, he fays, is the mercury which entered its compofiti- on, and what then remains, which is an earth penetrated by, fulphurs, runs into glafs, as all other fubftances of a like kind muft with a like fire. As the fubftances expofed to the focus of the burning glafs are laid upon a piece of char- coal, and that charcoal is always in part reduced to afhes about the body in trial, and thefe afhes often fly over and all about it; it had been objected to Mr. Homberg, by fome who faw his experiments, that it was thefe afhes alone which vitrified on the gold, and not any part of the gold itfelf 3 but Mr. Homberg very judicioufly anfwers, that if this were the cafe, thefe afhes would alfo be vitrified upon filver in the like trials and drcumftances, and give an appearance of the filver's vitrifying ; but often repeated experiments have proved, that filver expofed to the focus of the burning glafs, gives not the leaft fign of vitrification in any circumftance, unlefs it has been refined with antimony ; that is to fay, unlefs there has been given to it by art more fulphurs than it is naturally poffened of; but in that cafe, thefe unnatural abundant fulphurs will always vitrify a part of its earth. Mem. Acad. Par. 1707.

Mimic Gold. See Aurum fopbifliaim.

Gold, in medicine, is now of little other ufe, than as a co- vering for pills. Some mechanical phyficians have had a notion, that if the particles of gold could be rendered fine enough, fo as to circulate with the blood, that it would prove more effectual in the cure of difeafes than mercury. But this is mere hypothefis. The aarum fuhninam has been ufed, and other* preparations of gold are to be met- with in authors ; but they feem to meet with little credit among modern practitioners.

Gold colour. The manner of giving this colour to glafs is this. Take cryftal fritt two parts, and rochetta fritt one part, both made with tarfo, not fand ; mix thefe well, and to every hundred weight of this add of tartar in lumps, but well powdered afterwards, and of manganefe prepared, each one pound, mix thefe well together and put them into the fritt ; when this is done put all into the furnace, and let it ftand there four days in a common fire not too violent, and at the end of this time work it. It will be thus very beauti- ful, but if all cryftal fritt be ufed, it will be yet much finer. It may be made deeper or paler, by adding to, or diminifh- ing the quantity of the ingredients, in proportion to that -of the fritt. Neri's Art of Glafs, p. 89.

Gold coloured glaxivg. See Glazing.

Gold on china-ware. The gold on this ware is never laid en

alone, but managed in the following manner : they grind it

in water to a fine powder, and leave it to dry in the lliade,

4. They