3 T4 C H E M 
beautiful a polifh, that it has been recommended by 
Hellpt to make mirrors of telefcopes, not being iubjeitt 
to tarnifli-. When the zink is feparated from the gold 
by calcination, the oxyd which this metal affords is red- 
difli, and carries up a finall quantity of gold with it. 
Or the mixed metal may be put into nitric acid, which 
diffoi vcs the zink without affecting the gold. 
Gold has a ftronger affinity with mercury than with 
other metallic fubftances, and is capable cf decbmpofing 
their amalgams ; it unites with mercury in every propor¬ 
tion ; but La Grange recommends to form amalgam of 
one part of leaf-gold with feven parts and a half of 
mercury ; put thefe into a marble mortar, and triturate 
with a glafs peftle till the amalgam is properly formed. 
The gold may be feparated again from the mercury by 
heat, which occafions the mercury to volatilize. This 
amalgam is employed in water-gilding. 
Though gold is not capable of oxydationby the action 
of the fire of our furnaces with accefs of air, it never- 
thelefs becomes fo when heated together with mercury : 
if mercury, with one forty-eighth of its weight of gold, 
be heated in a flat-bottomed matrafs, whofe neck is 
drawn out into a capillary tube, the two metallic fub- 
ffances become oxydated at the lame time, and are.con¬ 
verted into a deep red powder. This compound oxyd, 
according to Baume, is obtained in much lefs time than 
that of mercury alone. We here fee a metal, which, 
though very difficult to oxydate alone, afllfts and facili¬ 
tates the.oxy'dation of another metallic matter, which is 
likewife very difficultly oxydated. 
Gold is ealily alloyed with tin and lead; thefe two me¬ 
tals deprive it of all its ductility, rendering it brittle and 
fragile. Gold unites with iron by fufion : equal parts 
of gold and iron or flee] form a grey mixture ; but three 
or four parts of iron to one of gold produce a metal al- 
rnoft as white as filver. This alloy with iron is very hard, 
and may be ufed to form cutting inftruraents, much Su¬ 
perior to thofe made with pure fieel. Lewis propofes 
gold as a very proper and firm folder for (mail pieces of 
fteel. Gold alfo.combines with copper, which gives it a 
red colour, and greater firmnefs, at the flame time that it 
renders it more fufibie : this alloy is mixed in different 
proportions for money, plate, and toys; it is called red 
alloy, and is ufed as a folder for gold. 
Gold and filver unite in all proportions. The metals, 
when mixed, feem to lofe very little of their ductility ; 
but they acquire firmnefs and elafticity. A twentieth 
part of filver renders gold confiderably paler ; but filver 
may be mixed with a fourth, or even a third, part of 
gold, without any apparent change of colour. This al¬ 
loy,. is not, however, made without a certain degree of 
difficulty, on account of the different ipecific gravities 
of thefe two metals, as Homberg obferves, who flaw them 
feparate during their fufion. The alloy of gold with fil¬ 
ver forms the green gold of the jewellers and gold-beaters. 
As gold is of the mofl extenfive ufe, and, by the con¬ 
vention of mankind, is become, together with filver, the 
price of all the other produdtions of nature and of art, it 
is of importance to afoertain the degree of purity of this 
precious metal,' in order to prevent the deceptions which 
covetoufnefs might produce, and to caufe the value of 
all the maffes or pieces of gold difperfed in commerce to 
be the fame, equal weights being fuppoled. Severe la ws, 
founded in juftice, have therefore been made, effablifhing 
the quantity of alloy neceffary to be ufed, in order to give 
the due degree of hardnefs and rigidity to gold intended 
to form utenfils in which thefe properties are neceffary. 
Ch&miftry affords methods of afcertaining the quantity 
of imperfect metals mixed with gold: the operation by 
which this knowledge is obtained, is called the ajfay of 
gold. Twenty-four grains of the gold intended to be af- 
fay-ed is cupelled with forty-eight grains of filver and 
four drachms of pure lead ; the latter, in its vitrification, 
carries along with it the bafer metals, fuch as copper, 
&c. and the gold remains combined with the filver after 
5 . 
S T R Y. 
the cupellation is finiflted. Thefe two metals are fepa-. 
rated by an operation called parting ; the parting of gold 
and filver confifts in the feparating of the two metals by 
a folvent, which afts on filver without affetling gold: 
aqua-fortis is commonly ufed.' Silver is added to the 
geld, becaufe experience has fhown that it is neceffary 
the gold fliouid be mixed with at leaft double its weight 
oi filver, in order that the nitric acid may perfectly dif- 
folve the latter metal. As three parts of filver are ulually 
added to one of gold, this procefs is called quartation, 
the gold being one-fourth of the weight of the alloy. 
This proportion, however, as Vauquelin remarks in his 
late accurate experiments, is proper only when the gold 
is very fine, as 997, 998, 01-999, parts fine in 1000; for, 
fhould it contain 200, 250, or 300, parts of copper, two 
parts of fine filver will fuflice •, but, as it is neceffary for 
the quantity of filver to diminifh in an inverfle ratio to 
the purity of the gold, the lead, on the contrary, mult 
increafe in a direft proportion. It is ea-fy to conceive, 
indeed, that, when the gold is fine, or almoft fine, the 
lead is more neceffary for favouring the fufion of the fil¬ 
ver and the gold, than for refining the gold ; but, when 
the gold contains much copper, the cafe is otherwife; 
if, for example, the gold is only 0-75 fine, then twenty- 
four times its weight of lead would be requifite for it» 
purification, and fio in proportion. The fame remarks; 
relative to the quantity to be allayed at a time, will hold 
good here, that were made on the cupelling of filver. 
A greater heat is neceffary for allaying gold than for fil¬ 
ver ; but there is no danger of its f'ubliming like that 
metal, nor are the precautions there directed neceffary here. 
When the aflay is complete, and the matter has cooled, 
the metallic button is to be hammered flat, and then 
roafted again, either by placing it on a piece of charcoal 
heated from underneath, or upon burning coals, or by 
putting it into the muffle of a cupelling furnace, taking- 
care it does not melt. Then it is to be paffed through 
an inflrument, to form it into a plate, not more than one 
fourth of .a line' thick ; this metallic plate is to be roaft- 
ed again, and then rolled up fpirally. The plating, and 
the re-roofing, are'two operations neceffary to the liiccefs 
of the procefs, and which require iorae precautions. 1. 
The plate muff be neither too thick nor too thin ; if too 
thin, it anight break during the agitation caufle'd by the 
boiling in aqua-fortis, which it is to undergo; if too thick, 
the aqua-fortis would not perhaps penetrate to its centre, 
fb as to feparate the very latft particles of filver. 2. The 
re-roafting of the plate, while it gives it more pliability, 
and promotes its folding up, without breaking or crack¬ 
ing, opens the pores of it, which the prefl'ure of thetnffru- 
rnent had elofe'd, and favours theadtion of the aqua-fortis. 
Now put the rolled plate into a pear-fhaped matrafs, 
that is, one whofe neck diminifhes from the belly to the 
top ; pour in pure aqua-fortis of 22 0 tiil the matrafs, 
which ufually holds about three ounces, be half or three 
parts full. Then place it on burning coals, covered with 
a flight layer of allies, left the veil'd fhould break by the 
fudden heat; from the inffant the liquor boils, till the 
end of the operation, fliouid be about fifteen or twenty 
minutes. This is called humid parting : at this time a red 
vapour is di(engaged, proceeding from the folution of 
the filver by the nitric acid: the rolled plate changes co¬ 
lour; it becomes brownifh, lofes its fblidity and confif- 
tence, as is eafy to conceive. ' When the aqua-fortis has 
thus boiled for twenty minutes upon the gold, the folu¬ 
tion is to be carefully decanted off, taking care the gold 
does not fall; then pour on the fame quantity as at firff 
of aqua-fortis at 32 0 , to feparate the laft portions of filver 
that might adhere to the gold. This fecond operation 
is called the repetition. Let it boil this time for about 
feven or eight minutes; decant this aqua-fortis like the 
firff, and then fill the matrafs with diftilied water, or very 
pure river-water. Invert a flmall crucible upon the aper¬ 
ture of the matrafs ; turn the matrafs very carefully bot¬ 
tom upwards ; the fpiral gold finks in the crucible through 
the 
