3io CHE M I 
weight, it is called filver of eleven pennyweights fine'; if 
it has loft only a twenty-fourth, it is called filverof eleven 
pennyweights twelve grains fine, and io forth. The 
cuppel, after this operation, is found to be much heavier, 
and contains the oxyd of vitreous lead, and that of the 
other metals, which were united with the filver, and have 
been feparated by the lead. As the lead itfelf almoft 
always contains a fmall quantity of filver, it is ne- 
ceflary firft to cuppel it by itfelf, in order to determine 
the quantity of the filver it contains, and a deduction 
rauft be made from the button of fine filver obtained of 
the fmall portion knowm to be contained in the lead made 
ufe of, which is called the w itnefs. Cuppellation is at¬ 
tended with a phenomenon, by which the artift \s adver- 
tifed of the Hate of the procefs as it goes forward. In 
proportion as the filver becomes pure by the vitrification 
and feparation of the lead, it appears much more bril¬ 
liant than the portion wdiich is not yet fine; the brilliant 
part increafes by degrees, and wdien all the furface of 
the metal become pure and luminous, the inftant in 
which it pafies to this ftate exhibits a flafh or fulguration, 
w hich denotes that the operation is finifhed. Cupelled 
filver is very pure with relpedt to the bafer metals it may 
have contained, but it may contain gold ; and, as it al¬ 
ways contains a certain quantity, another operation muft 
be made to leparate thefe two perfeft metals. As gold is 
much lefs changeable than filver by moft folvents, the 
filver is diflolved by the addition of the nitric or muri¬ 
atic acids, or by lulphur; and the gold, on which thefe 
folvents have little or no adlion remains pure. This me¬ 
thod of feparating filver from gold is called parting . 
The large works where filver is extradfed from its ores 
and purified,' are fimilar to thole we have defcribed for 
the affay of the ores of this metal. There are in general 
three methods of treating filver in the large way ; the 
firft confifts in triturating virgin filver wdth mercury ; 
this amalgam is wafhed to leparate all the earth ; it is 
then palled through the pores of bags of leather, and dif- 
tilled in iron retopts ; after which the filver is melted and 
call into ingots. This procefs cannot be ufed w r ith filver 
ores that contain fulphur: thefe are roafted and mixed 
with lead, to refine the lilver by cuppellation. Rich lil- 
ver ores are treated in this manner, but the poorer ores 
are melted without previous roafting, with a fmall quan¬ 
tity of pyrites. This fufion, which is called the crude 
fujion , affords a mat of copper in combination with filver, 
which is treated with lead in the way of eliquation ; the 
latter, which carries down the filver during the fufion, is 
afterwards fcorified on the cupel, and the filver remains 
pure. Cuppellation in the large way differs from that 
which is made in the fmall way, in this circumftance, 
that in the firft, the fcorified lead is driven off by the 
adtion of a bellows, whereas, in tlie latter, the oxyd of 
vitreous lead is abiorbed by the cuppel. 
The filver obtained by the procefies here defcribed is, 
in general, much lefs liable to alteration than all the me¬ 
tals hitherto defcribed. The contaft of light does not 
at all change this metal, however long it be expofed to 
it; heat melts it, caufes it to boil, and to become vola¬ 
tilized, but without alteration. It does not melt in lefs 
than a white heat, but is more fufible than copper. When 
it has been held in fufion for a certain time, it boils and 
emits vapours, which confift of filver volatilized. This 
fadt is proved by theexiftence of the metal in the funnels 
of chimneys, under which large quantities are continu¬ 
ally melted. It is likewife confirmed by the capital ex¬ 
periment of the Academicians of Paris, who expofed 
very pure filver to the focus of Trudaine's lens. Thefe 
philolophers obferved, that the melted metal emitted a 
thick fume, which completely filvered a piece of gold 
held over it. 
The following additional remarks on the fubjedt of 
cuppellation, are extradfed from the new work of Vau- 
quelin. With regard to the proportion of lead to be 
ufed, if the lilver contains a twentieth part, or 0‘05, of 
S T R Y. 
copper, then four times and a half as much lead as of 
filver will he required; hut, if it contains o’ao, at leaft 
eleven times as much will be neceftary.' If- the filver k 
fo alloyed with copper as to require fifteen or fixteen parts 
of lead to feparate it, not more than eight grains can be 
allayed at a time, unlefs you have a cuppel twice or thrice 
as large as for filver containing but one twentieth of cop¬ 
per; for, the cuppels can hardly abforb more than their own 
weight of the oxyd of lead; then the remainder would 
lie at the furface of the water, which would be inconve¬ 
nient. It is proper to have the heat greateft at the begin¬ 
ning of the operation ; but a very great heat is improper 
towards the end, as part of the filver might be volatiliz¬ 
ed, and the metallic button become too hard, which 
are two great inconveniences, where perfedt exadtnefs is 
required as to the finenefs of the lump ; therefore, when 
the operation is about two-thirds over, draw the crucible 
towards the front of the furnace, that it may have pre- 
cifely the quantity of heat neceftary to bring on the ful¬ 
guration or corrufcation fpoken of before: for at the 
moment the laft portions of lead evaporate, the fur¬ 
face is covered with ftreaks exhibiting all the colours of 
the rainbow. It will be known that the affay has been 
-well made, when the remaining button is round, of a 
bright white colour, cryftallized above and below, and 
laftly, if it is eafily looiened from the cuppel when cold. 
It is very difficult, however, without much pradlice, to 
adjuft a proper degree of heat to lilver of different fine¬ 
nefs; but, in general, filver much alloyed requires a 
ftronger beat, ef'pecially towards the beginning, than 
fine filver; and that fine filver will require but one part 
and half of lead, and lefs fire, particularly towards the 
conclufion of the operation. 
Mr. Keir has defcribed, in the Philofophical Tranfac- 
tions for 1790, a method of feparating thefe metals from 
each other, which appears to be particularly ufeful in 
the arts. It confifts in putting the mixture of filver and 
copper, or copper plated with filver into an earthen glaz¬ 
ed pan, and pouring on them a mixture of fulphuric 
acid andnitrat of potafli, in the proportion of from eight 
to ten parts of the acid to one part of the nitrat. The 
mixture is to be ftirred fo that the furface of the pieces 
of metal may be frequently expofed to frefh portions of 
the liquor. The folution is to be affifted by a gentle beat 
of from 100 to 200 of Fahrenheit’s fcale. When the li¬ 
quor is nearly faturated, the filver is to be precipitated 
from it by the addition of muriat of foda. A muriat of 
filver is formed, eafily reducible in a crucible by melting 
it with a lufficient quantity of potafli; and laftly, by re¬ 
fining the melted filver, if neceftary, with a little nitrat 
of potafli thrown on it. In this manner the filver will be 
got fufficiently pure, while the copper remains unchanged. 
Silver is diflolved by the fulphuric acid when very 
concentrated .or boiling, and the metal is greatly divided. 
Much fulphureous acid gas is difengaged during this 
folution ; the filver is Converted into a white matter, on 
which fulphuric acid muft be poured, in order to hold it 
in folution : very fmall needles of fulphat of filver are 
obtained by evaporating this liquor; Fourcroy obtained 
this fait in plates, formed by the union of thefe needles 
length-wife. This fait melts in the fire, and is decom- 
poled : the acid is driven off by heat, then the oxygen, 
and the filver is left in its metallic ftate. It is decom- 
pofable by alkalis, iron, copper, zink, mercury, &c. All 
the precipitates obtained by alkalis, are reducible with¬ 
out addition, and become converted into fine filver, in 
clofed veffels. The hydro-fulphures precipitate the filver 
from its fulphuric folution, of a fine black' colour; the 
muriatic acid, and all the muriats, decompofe this fait. 
Nitric acid diffolves filver; but the proportion is not 
eafy to be known, as it depends on the degree of con¬ 
centration of the acid : fometimes it requires one part 
and a half, at other times two parts, to one of filver. For 
a very clear and pure nitric folution of filver, it will be 
neceftary to ufe the cuppelled filver, without which the 
nitric 
