precautions are to be taken against them. For instance, if the ballon
should break while the Phosphorus is distilling, and any of it should
fall on combustible bodies, it would set them on fire, and probably
burn the laboratory, because it is not to be extinguished without the
greatest difficulty. The furnace must therefore be erected under some
vault, or upon a bed of brick-work raised under some chimney that draws
well: nor must any furniture or utensil of wood be left near it. If a
little flaming Phosphorus should fall on a man's legs or hands, in less
than three minutes it would burn its way to the very bone. In such a
case nothing but urine will stop its progress.
"If the retort crack while the Phosphorus is distilling, there is an
unsuccessful end of your operation. It is easy to perceive this by the
stink of garlic which you will smell about the furnace; and moreover,
the flame that issues through the apertures of the reverberatory will
be of a beautiful violet colour. The Acid of Sea-salt always gives this
colour to the flame of such matters as are burnt along with it. But
if the retort break before the Phosphorus hath made its appearance,
its contents may be saved by throwing a number of cold bricks into the
fire-place, and upon them a little water to quench the fire at once."
All these useful observations we owe also to M. Hellot.
The Phosphorus here described was first discovered by a citizen
of Hamburgh, named Brandt, who worked upon urine in search of the
Philosopher's stone. Afterwards two other skilful Chymists, who knew
nothing more of the process, than that Phosphorus was obtained from
urine, or, in general, from the human body, likewise endeavoured
to discover it; and each of them separately did actually make the
discovery. These two Chymists were Kunckel and Boyle.
The former perfected the discovery, and found out a method of making it
in considerable quantities at a time; which occasioned it to be called
_Kunckel's Phosphorus_. The other, who was an English gentleman, had
not time to bring his discovery to perfection, and contented himself
with lodging a voucher of his having discovered it in the hands of the
Secretary of the Royal Society of London, who gave him a certificate
thereof.
"Though Brandt," says M. Hellot, "who had before this sold his secret
to a Chymist named Krafft, sold it afterwards to several other persons,
and even at a very low rate; and though Mr. Boyle published the process
for making it; yet it is extremely probable that both of them kept in
their own hands the master-key; I mean, _the particular management
necessary to make the operation succeed_: for, till Kunckel found it
out, no other Chymist ever made any considerable quantity thereof,
except Mr. Godfrey Hankwitz, an English Chymist, to whom Mr. Boyle
revealed the whole mystery.
"Nevertheless," continues he, "we are very far from alledging that
all those who have described this operation meaned to impose upon the
world: but we conceive that most of them having observed luminous
vapours in the ballon, and some sparks about the juncture of the
vessels, were contented with those appearances. And thus it came to
pass, that, after Kunckel and Boyle died, Mr. Godfrey Hankwitz was the
only Chymist that could supply Europe therewith; on which account it is
likewise very well known by the name of _English Phosphorus_."
Almost all the Chymists consider Phosphorus as a substance consisting
of the Acid of Sea-salt combined with the Phlogiston, in the same
manner as Sulphur consists of the Vitriolic Acid combined with the
Phlogiston. This opinion is founded on the following principles.
First, Urine abounds with Sea-salt, and contains also a great deal of
Phlogiston; now these are the ingredients of which they conjecture
Phosphorus to be composed.
Secondly, Phosphorus has many of the properties of Sulphur; such
as being soluble in oils; melting with a gentle heat; being very
combustible; burning without any soot; giving a vivid and blueish
flame; and lastly, leaving an acid liquor when burnt: sensible proofs
that it differs from Sulphur in nothing but the nature of its Acid.
Thirdly, this Acid of Phosphorus, being mixed with a solution of silver
in Spirit of Nitre, precipitates the silver, and this precipitate is
a true _Luna cornea_, which appears to be more volatile even than the
common sort; as M. Hellot tells us, who made the experiment. This
fact proves incontestably that the Acid of Phosphorus is of the same
nature with that of Sea-salt: for all Chymists know that the property
of precipitating silver in a _Luna cornea_ belongs to the Marine Acid
only.
Fourthly, M. Stahl observes, that, if Sea-salt be cast on live coals,
they instantly burn with great activity; then they emit a very
vivid flame, and are much sooner consumed than if none of this salt
had touched them; that Sea-salt in substance, which will bear the
violence of fire a considerable time when fused in a crucible, without
sustaining any sensible diminution, yet evaporates very quickly, and is
reduced to white flowers, by the immediate contact of burning coals;
and, lastly, that the flame which rises on this occasion is of a blue
colour inclining to violet, especially if it be not thrown directly
on the coals themselves, but kept in fusion amidst burning coals, in
a crucible so placed that the vapour of the Salt may join with the
enflamed Phlogiston as it rises from the coals.
These experiments of M. Stahl's prove, that the Phlogiston acts upon
the acid of Sea-salt, even while it is combined with its alkaline
basis. The flame that appears on this occasion may be considered as an
imperfect Phosphorus: and indeed its colour is exactly like that of
Phosphorus.
All the facts above related evince, that the Acid of Phosphorus is akin
to that of Sea-salt; or rather that it is the very same. But there are
other facts which prove that this Acid undergoes some change at least,
some peculiar preparation, before it enters into the composition of a
true Phosphorus, and that, when extricated therefrom by burning, it is
not a pure Acid of Sea-salt, but is still adulterated with a mixture of
some other substance, which makes it considerably different from that
Acid. For these observations we are obliged to M. Marggraff, of the
Academy of Sciences at Berlin, a celebrated Chymist. I shall presently
give an account of his principal experiments as succinctly as possible.
M. Marggraff hath also published a process for making Phosphorus, and
assures us, that by means thereof we may obtain in less time, with less
heat, less trouble, and less expence, a greater quantity of Phosphorus
than by any other method. His operation is this:
He takes two pounds of Sal Ammoniac in powder, which he mixes
accurately with four pounds of Minium. This mixture he puts into
a glass retort, and with a graduated fire draws off a very sharp,
volatile, urinous spirit.
We observed in our theoretical Elements, that some metallic substances
have the property of decomposing Sal Ammoniac, and separating its
volatile Alkali; concerning which phenomenon we there gave our opinion.
Minium, which is a calx of lead, is one of those metallic substances.
In this experiment it decomposes the Sal Ammoniac, and separates its
volatile Alkali; what remains in the retort is a combination of the
Minium with the Acid of the Sal Ammoniac, which is well known to be
the same with the Marine Acid; and consequently the residue of this
operation is a sort of _Plumbum corneum_.
The quantity thereof is four pounds eight ounces. Of this he mixes
three pounds with nine or ten pounds of urine, that has stood
putrefying for two months, evaporated to the consistence of honey.
These he mixes by little and little in an iron pan over the fire,
stirring the mixture from time to time. Then he adds half a pound of
charcoal-dust, and evaporates the matter, kept continually stirring,
till the whole be brought to a black powder. He next distils the
mixture in a glass retort with degrees of fire, which he raises towards
the end so as to make the retort red-hot, in order to expel all the
urinous spirit, superfluous oil, and ammoniacal salt. The distillation
being finished, there remains nothing in the retort but a very friable
_caput mortuum_.
This remainder he pulverises again, and throws a pinch thereof on
live coals, thereby to discover whether or no the matter be rightly
prepared, and in order for yielding Phosphorus. If it be so, it
presently emits an arsenical odour, and a blue undulating flame, which
passes over the surface of the coals like a wave.
Being thus assured of the success of his operation, he puts one half
of his matter in three equal parts, into three small earthen German
retorts, capable of holding about eighteen ounces of water a-piece.
These three retorts, none of which is above three quarters full, he
places together in one reverberatory furnace, built much like those we
have described, except that it is so constructed as to hold the three
retorts disposed in one line. To each retort he lutes a recipient
something more than half full of water, ordering the whole in such a
manner, that the noses of his retorts almost touch the surface of the
water.
He begins the distillation with warming the retorts slowly, for about
an hour, by a gentle heat. When that time is elapsed he raises the fire
gradually, so that in half an hour more the coals begin to touch the
bottoms of the retorts. He continues throwing coals into the furnace by
little and little, till they rise half way the heighth of the retorts;
and in this he employs another half hour. Lastly, in the next half hour
he raises the coals above the bowls of the retorts.
Then the Phosphorus begins to ascend in clouds: on this he instantly
increases the heat of the fire as much as possible, filling the furnace
quite up with coals, and making the retorts very red. This degree of
fire causes the Phosphorus to distil in drops, which fall to the bottom
of the water. He keeps up this intense heat for an hour and half, at
the end of which the operation is finished; so that it lasts but four
hours and an half in all: nay, he further assures us, that an artist
well versed in managing the fire, may perform it in four hours only. In
the same manner he distils the second moiety of his mixture in three
other such retorts.
The advantage he finds in making use of several small retorts, instead
of a single large one, is, that the heat penetrates them with more
ease, and the operation is performed with less fire, and in less time.
He purifies and moulds his Phosphorus much in the same manner as M.
Hellot does. From the quantity of ingredients above-mentioned, he
obtains two ounces and a half of fine crystalline moulded Phosphorus.
M. Marggraff considering, as a consequence of the experiments above
related, that a highly concentrated Acid of Sea-salt contributes
greatly towards the formation of Phosphorus, proceeded to try several
other experiments, in which he employed that Acid in a state of
combination with other bases. He mixed, for instance, an ounce of _Luna
cornea_ with an ounce and half of putrefied and inspissated urine, and
from the mixture obtained a very beautiful Phosphorus.
In short, the several experiments mentioned having thoroughly persuaded
him that the Acid of Sea-salt, provided it were highly concentrated,
would combine with the Phlogiston as readily as the Vitriolic Acid
does, he resolved to try whether he could not make Phosphorus with
matters containing that Acid and the Phlogiston, without making use of
any urine.
With this view he made a great number of different trials, wherein he
employed Sea-salt in substance, Sal Ammoniac, Plumbum corneum, Luna
cornea, fixed Sal Ammoniac, otherwise called _Oil of Lime_. These
several substances, all of which contain the Acid of Sea-salt, he mixed
with sundry matters abounding in Phlogiston, different vegetable coals,
and even animal matters, such as the oil of hartshorn, human blood,
_&c._ varying the proportions of these substances many different ways,
without ever being able to produce a single atom of Phosphorus: which
gave this able Chymist just cause to suspect, that the Marine Acid,
while pure and crude, is not capable of combining with the Phlogiston
in the manner requisite to form a Phosphorus; that for this purpose it
is necessary the Acid would have contracted a previous union with some
other matter; and that the Acid found in urine hath probably undergone
the necessary change. M. Marggraff is of opinion that the matter, which
by its union renders the Marine Acid capable of entering into the
composition of Phosphorus, is a sort of exceedingly subtle vitrifiable
earth. The experiments he made upon the Acid of Phosphorus, will shew
that his notion is not altogether groundless. M. Marggraff having let
some urine, evaporated to the consistence of honey, stand quiet in a
cool place, obtained from it, by crystallization, a Salt of a singular
nature. By distilling this urine afterwards, he satisfied himself that
it yielded him much less Phosphorus than urine from which no Salt had
been extracted; and as it cannot be entirely deprived of this Salt, he
thinks that the small quantity of Phosphorus, which this urine yielded
him, came from the Salt that was still left in it.
Further, he distilled this salt separately with lamp-black, and
obtained from it a considerable quantity of very fine Phosphorus. He
even mixed _Luna cornea_ with this Salt, in order to see whether it
would not increase the quantity of his Phosphorus; but without success:
whence he concluded, that in this Saline matter resides the true Acid
that is fit to enter into the composition of Phosphorus. This opinion
is confirmed by several experiments on the Acid of Phosphorus, which he
found to have some properties resembling those of this Salt.
The Acid of Phosphorus seems to be more fixed than any other: and
therefore if you would separate it, by burning, from the Phlogiston
with which it is united, there is no occasion for such an apparatus
of vessels as is employed for obtaining the Spirit of Sulphur. For
this Acid will remain at the bottom of the vessel in which you burn
your Phosphorus: indeed, if it be urged by the force of fire, its most
subtile part evaporates, and the remainder appears in the form of a
vitrified matter.
This Acid effervesces with fixed and volatile Alkalis, and therewith
forms Neutral Salts; but very different from Sea-salt, and from Sal
Ammoniac. That which has a fixed Alkali for its basis does not crackle
when thrown on burning coals; but swells and vitrifies like Borax. That
which has a volatile Alkali for its basis shoots into long pointed
crystals; and, being urged by fire in a retort, lets go its volatile
alkali, a vitrified matter remaining behind. This Salt is like that
above-mentioned, as obtained from urine and yielding Phosphorus.
It appears from the experiments adduced, that the Acid of Phosphorus
tends always to vitrification; which proves that it is not pure, and
gave M. Marggraff cause to think that it is altered by the admixture of
a very subtile vitrifiable earth.
M. Marggraff also obtained Phosphorus from several vegetable substances
which we use every day for food. This gives him occasion to conjecture,
that the Salt requisite to the formation of Phosphorus exists in
vegetables, and passes from thence into the animals that feed upon them.
Lastly, he concludes his dissertation by informing us of a very
important truth, _viz._ That the Acid obtained from Phosphorus, by
burning it, will serve to form Phosphorus anew; for which purpose it
need only be combined with some charred coal, such as lamp-black, and
distilled.
From what hath been said on this subject it is plain, that the Chymists
have a great many curious and interesting inquiries to make concerning
Phosphorus, and particularly concerning its Acid.
I shall conclude this article with an account of certain properties of
Phosphorus which I have not yet mentioned.
Phosphorus dissolves by lying exposed to the air. What water cannot
effect, says M. Hellot, or at least requires eight or ten years to
bring about, the moisture of the air accomplishes in ten or twelve
days; whether it be that the Phosphorus takes fire in the air, and
the inflammable part evaporating, almost entirely, leaves the Acid of
the Phosphorus naked, which, like all other Acids, when exceedingly
concentrated, is very greedy of moisture; or else that the moisture
of the air, being water divided into infinitely fine particles, is so
subtile as to find its way through the pores of the Phosphorus, into
which the grosser particles of common water can by no means insinuate
themselves.
Phosphorus heated by the vicinity of fire, or by being any way rubbed,
soon takes fire and burns fiercely. It is soluble in all Oils and in
Ether, giving to those liquors the property of appearing luminous when
the bottle containing the solution is opened. Being boiled in water,
it likewise communicates thereto this luminous quality. M. Morin,
Professor at Chartres, is the author of this observation.
The late Mr. Grosse, a celebrated Chymist of the Academy of Sciences,
observed, that Phosphorus being dissolved in essential oils
crystallizes therein. These crystals take fire in the air, either when
thrown into a dry vessel, or wrapt up in a piece of paper. If they
be dipped in Spirit of Wine, and taken out immediately, they do not
afterwards take fire in the air: they smoke a little, and for a very
short time, but hardly waste at all. Though some of them were left in
a spoon for a fortnight, they did not seem to have lost any thing of
their bulk: but when the spoon was warmed a little they took fire, just
like common Phosphorus that had never been dissolved and crystallized
in an essential Oil.
M. Marggraff, having put a dram of Phosphorus, with an ounce of highly
concentrated Spirit of Nitre, into a glass retort, observed, that,
without the help of fire, the Acid dissolved the Phosphorus; that part
of the Acid came over into the recipient which was luted to the retort;
that, at the same time, the Phosphorus took fire, burnt furiously, and
burst the vessels with explosion. Nothing of this kind happens when any
of the other Acids, though concentrated, are applied to Phosphorus.
PROCESS III.
_To decompose Sea-salt by means of the Vitriolic Acid. Glauber's Salt.
The Purification and Concentration of Spirit of Salt._
Put the Sea-salt from which you mean to extract the Acid into an
unglazed earthen pipkin, and set it amidst live coals. The Salt will
decrepitate, grow dry, and fall into a powder. Put this decrepitated
Salt into a tubulated glass retort, leaving two thirds thereof empty.
Set the retort in a reverberating furnace; apply a receiver like that
used in distilling the Smoking Spirit of Nitre, and lute it on in the
same manner, or rather more exactly if possible. Then through the
hole, in the upper convexity of the retort, pour a quantity of highly
concentrated Oil of Vitriol, equal in weight to about a third part
of your Salt, and immediately shut the hole very close with a glass
stopple, first ground therein with emery so as to fit it exactly.
As soon as the Oil of Vitriol touches the Salt, the retort and receiver
will be filled with abundance of white vapours; and soon after,
without lighting any fire in the furnace, drops of a yellow liquor
will distil from the nose of the retort. Let the distillation proceed
in this manner without fire, as long as you perceive any drops come:
afterwards kindle a very small fire under the retort, and continue
distilling and raising the fire by very slow degrees, and with great
caution, to the end of the distillation; which will be finished before
you have occasion to make the retort red-hot. Unlute the vessels, and
without delay pour the liquor, which is a very smoking Spirit of Salt,
out of the receiver into a crystal bottle, like that directed for the
smoking Spirit of Nitre.
_OBSERVATIONS._
Sea-Salt, as hath been already said, is a Neutral Salt composed of an
Acid, which differs from those of Vitriol and Nitre, combined with a
fixed Alkali that has some peculiar properties; but does not vary from
the others in its affinities. This Salt therefore, as well as Nitre,
must be decomposed by the Vitriolic Acid; which accordingly is the
case in the process here described. The Vitriolic Acid unites with the
Alkaline basis of the Sea-salt, and separates its Acid; and that with
much greater facility than it expels the Nitrous Acid from its Alkaline
basis, because the Acid of Sea-salt has not so great an affinity as the
Nitrous Acid with Fixed Alkalis.
As a highly concentrated Oil of Vitriol is used on this occasion, and
as the Sea-salt is previously dried and decrepitated, the Acid obtained
from it by distillation is very free from phlegm, and always smokes,
even more violently than the strongest Acid of Nitre. The vapours of
this Acid are also much more elastic and more penetrating than those
of the Nitrous Acid: on which account this distillation of the smoking
Spirit of Salt is one of the most difficult, most laborious, and most
dangerous operations in Chymistry.
This process requires a tubulated retort, that the Oil of Vitriol
may be mixed with the Sea-salt after the receiver is well luted to
the retort, and not before: for, as soon as these two matters come
together, the Spirit of Salt rushes out with so much impetuosity, that,
if the vessels were not luted at the time, the copious vapours that
would issue through the neck of the ballon would so moisten it, as well
as the neck of the retort, that it would be impracticable to apply the
lute and secure the joint as the operation requires. Moreover, the
operator would be exposed to those dangerous fumes, which, on this
occasion, rush out, and enter the lungs, with such incredible activity
as to threaten instant suffocation.
Having said so much of the elasticity and activity of the fumes of
Spirit of Salt, it is needless to insist upon the necessity of giving
vent to the vessels from time to time, by opening the little hole of
the ballon: indeed the best way to prevent the loss of a great many
vapours, on this occasion, is to employ adopters, and cover them with
wet canvas, which will cool and condense the vapours they contain.
When the operation is finished, we find a white, saline mass at the
bottom of the retort as in a mould. If this mass be dissolved in water,
and the solution crystallized, it yields a considerable quantity of
Sea-salt that hath not been decomposed, and a Neutral Salt consisting
of the Vitriolic Acid united with the Alkaline basis of that part
which hath been decomposed. This Neutral Salt, which bears the name of
_Glauber_ its inventor, differs from Vitriolated Tartar, or the _Sal de
duobus_, which remains after distilling the Nitrous Acid, especially in
that it is more fusible, more soluble in water, and hath its crystals
differently figured. But as in these two Salts the Acid is the same,
the differences that appear between them must be attributed to the
peculiar nature of the basis of Sea-salt.
Spirit of Salt drawn by the process above described is tainted with a
small mixture of the Vitriolic Acid, carried up by the force of fire
before it had time to combine with the Alkali of the Sea-salt; which
happens likewise to the Nitrous Acid procured in the same manner.
If you desire to have it pure, and absolutely free from the Acid of
Vitriol, it must be distilled a second time from Sea-salt, as the Acid
of Nitre was before directed to be distilled again from fresh Nitre, in
order to purify it from any Vitriolic taint.
Sea-salt, as well as Nitre, may be decomposed by any combination of
the Vitriolic Acid with a metallic or earthy substance: but it is
proper to observe, that if you distil Spirit of Salt by means of Green
Vitriol, the operation will not succeed so well as when Spirit of Nitre
is distilled in the same manner: less Spirit is obtained, and a much
fiercer fire is required.
The cause of this lies in the property which the Acid of Sea-salt
possesses of dissolving Iron, even when deprived of a part of its
Phlogiston by having contracted an union with another Acid; so that it
is no sooner dislodged from its own basis by the Vitriolic Acid, than
it unites with the ferruginous basis of the Vitriol, from which it
cannot be separated but by a most violent fire. This is the consequence
more especially when calcined Vitriol is made use of: for moisture,
as we shall presently see, greatly facilitates the separation of the
Marine Acid from those substances with which it is united.
When you do not desire a highly dephlegmated and smoking Spirit of
Salt, you may distil with the additament of any earth containing the
Vitriolic Acid; as Clay, for instance, or Bole. To this end one part
of Sea-salt, slightly dried and reduced to a fine powder, must be
accurately mingled with two parts of the earth you intend to employ
likewise pulverized; of this mixture make a stiff paste with a proper
quantity of rain water, and having formed little balls thereof about
the size of a hazel nut, let them dry in the sun; when dry, put them
into a stone or coated glass retort, leaving a third part thereof
empty; set this vessel in a reverberating furnace, covered with its
dome; apply a receiver, which need not be luted on for some time; and
heat the vessels very slowly. At first an insipid water will rise,
which must be thrown away: afterwards the Spirit of Salt will appear
in white clouds. Now lute your vessels, and raise the fire by degrees;
which, towards the end must be pushed to the utmost extremity. The
operation is known to be finished when no drops fall from the nose of
the retort, the receiver cools, and the white vapours that filled it
are seen no more.
The Spirit of Salt obtained by the process here delivered does not
smoke, and contains much more phlegm than that which is distilled by
means of the concentrated Oil of Vitriol; because the earth, though
dried in the sun, still retains a great deal of moisture, which
commixes with the Acid of the Sea-salt. Consequently it is much easier
to collect its vapours; so that this operation is attended with much
less trouble than the other. Nevertheless it is adviseable to proceed
gently; to apply but little heat at first, and to unstop every now and
then the small hole of the receiver: for a quantity of the vapours of
Spirit of Salt, even when weakened by the admixture of water, is very
apt to burst the vessels.
A much greater degree of fire is necessary to raise the Spirit of Salt
by this latter process, than by that in which the pure Vitriolic Acid
is employed: for, as fast as the Spirit of Salt is dislodged from its
own basis, by the Vitriolic Acid contained in the earth made use of,
part of it joins that earth, and cannot be separated from it without
the most violent heat.
A Spirit of Salt that shall not smoke may also be obtained by means of
the pure Vitriolic Acid. Spirit of Vitriol, or Oil of Vitriol, lowered
with a good deal of water, will do the business.
Some Chymists direct a little water to be placed in the receiver, when
Spirit of Salt is to be distilled by the intermedium of concentrated
Oil of Vitriol, in order to make the acid vapours condense more
readily. By this means indeed some of the inconveniencies attending
the distillation of smoking Spirit of Salt may be avoided: but, on the
other hand, the acid vapours being absolutely suffocated by the water
as fast as they come over, the Spirit of Salt obtained by this method
will be no less aqueous than that procured by the interposition of
earths: so that here is an expence to no manner of purpose. Therefore,
when a Spirit of Salt is desired that shall not smoke, it is best
to employ an additament of earth; and that so much the rather as
the Marine Acid obtained by this means is purer and freer from any
Vitriolic taint, for the reasons already assigned.
Part of the Acid of Sea-salt may be separated from its Alkaline basis
by the force of fire alone, without the intervention of any other body.
With this view the Salt must be put into the retort without being
dried. At first an insipid water rises; but it gradually becomes acid,
and hath all the properties of Spirit of Salt. When the Salt in the
retort is grown perfectly dry, nothing more can be forced over by any
degree of heat whatever. If you would obtain more Acid from the same
Salt, you must take it out of the retort, where you will find it in a
lump, reduce it to powder, and expose it to the air for some time, that
it may attract the moisture thereof; or else wet it at once with some
rain water, and distil as before. You will again have an insipid water,
and a little Spirit of Salt; which will in like manner cease to rise
when the Salt in the retort becomes dry. This operation may be repeated
as often as shall be thought proper: and perhaps it may be possible to
decompose Sea-salt entirely by means thereof, without the interposition
of any other body. The Spirit of Salt thus obtained is exceeding weak,
in small quantity, and loaded with much water.
This experiment proves, that moisture greatly facilitates the
separation of the Acid of Sea-salt from the matters with which it is
united: and this is the reason that, in distilling Spirit of Salt with
the additament of an earth, the operation requires much less fire
at the beginning, while the earth and salt retain a great deal of
humidity, than towards the end, when they begin to grow dry.
After the operation there remains in the retort a saline and earthy
mass, which contains, 1. Some entire Sea-salt that has suffered no
decomposition; 2. A Glauber's Salt which is, as we said before, a
Neutral Salt consisting of the Vitriolic Acid united with the Alkaline
basis of the Sea-salt, from which it hath expelled its proper Acid; 3.
Part of the earth used as an intermedium, still retaining a portion of
its original Vitriolic Acid, which happening not to lie near enough to
any particles of Sea-salt, could not exert its power in decomposing
them, and so remains united with its earthy basis; 4. Another part
of the same earth, impregnated with some of the Marine Acid, which
combined therewith upon being expelled from its Alkaline basis by the
Vitriolic Acid, and which the force of fire was unable to separate
from it when the matters were grown perfectly dry. In consequence
of what remains in this _caput mortuum_, if the whole mass be
triturated, moistened with a little water, and distilled a second time,
considerably more Spirit of Salt will be obtained from it: and the same
is to be said of all distillations of this sort.
Spirit of Salt obtained by the means of any other additament than
concentrated Oil of Vitriol is generally very weak: but it may be
dephlegmated and concentrated, if required, much in the same manner as
Oil of Vitriol. For this purpose you must put it into a glass cucurbit,
set it in a _balneum mariæ_, fit thereto a head and a receiver, and
with a moderate degree of heat draw off one third or one half of the
liquor. What comes over into the receiver will be the most aqueous
part, which being the lightest will rise first, impregnated however
with a little acid: in the cucurbit will be left a concentrated Spirit
of Salt, or the most acid part, which being the heaviest will not rise
with the degree of heat that is capable of carrying up the phlegm.
Spirit of Salt thus concentrated, called also _Oil of Salt_, does not
smoke: it is of a yellow colour inclining to green, and an agreeable
smell, not unlike that of saffron.
PROCESS IV.
_To decompose Sea-salt by means of the Nitrous Acid._ Aqua regis.
_Quadrangular Nitre._
Take dried Sea-salt; bruise it to powder; put it into a glass retort,
leaving one half of the vessel empty. Pour upon it a third of its
weight of good Spirit of Nitre. Place your retort in the sand-bath of a
reverberating furnace; put on the dome; lute to the retort a receiver
having a small hole in it, and heat the vessels very slowly. There will
come over into the receiver some vapours, and an acid liquor. Increase
the fire gradually till nothing more rises. Then unlute the vessels,
and pour the liquor out of the receiver into a crystal bottle, stopped
like others containing Acid Spirits.
_OBSERVATIONS._
The Nitrous Acid hath a greater affinity than the Marine Acid with
fixed Alkalis. When therefore Spirit of Nitre and Sea-salt are mixed
together, the same consequences, in some measure, will follow, as when
the Vitriolic Acid is mixed with that salt; that is, the Nitrous Acid
will, like the Vitriolic, decompose it, by dislodging its Acid from
its Alkaline basis, and assuming its place. But as the Nitrous Acid is
considerably weaker, and much lighter, than the Vitriolic Acid, a good
deal of it rises along with the Acid of Sea-salt during the operation.
The liquor found in the receiver is therefore a true _Aqua regis_.
If decrepitated Salt, and a right smoking Spirit of Nitre, be employed
in this process, the _Aqua regis_ obtained will be very strong; and,
during the operation, very elastic vapours will rush out and burst the
vessels, if those precautions be not taken which we pointed out as
necessary in distilling the Spirit of Nitre, and the smoking Spirit of
Salt.
The operation being finished, there is left in the retort a saline
mass, containing Sea-salt not decomposed, and a new species of Nitre,
which having for its basis the Alkali of Sea-salt, that is, as we have
several times observed, an Alkali of a peculiar nature, differs from
the common Nitre, 1. In the figure of its crystals; which are solids of
four sides, formed like lozenges: 2. In that it crystallizes with more
difficulty, retains more water in its crystals, attracts the moisture
of the air, and dissolves in water with the same circumstances as
Sea-salt.
CHAP. IV.
_Of_ BORAX.
PROCESS.
_To decompose Borax by the means of Acids, and to separate from it the
Sedative Salt by sublimation and by crystallization._
Reduce to a fine powder the Borax from which you intend to extract
the Sedative Salt. Put this powder into a wide-necked glass retort.
Pour upon it an eighth part of its weight of common water, to moisten
the powder; and then add concentrated Oil of Vitriol to the weight of
somewhat more than a fourth part of the weight of Borax. Set the retort
in a reverberatory, make a moderate fire at first, and augment it
gradually till the retort become red-hot.
A little phlegm will first come over, and then with the last moisture
that the heat expels the Sedative Salt will rise; by which means some
of it will be dissolved in this last phlegm, and pass therewith into
the receiver; but most of it will adhere in the form of saline flowers
to the fore-part of the neck of the retort, just where it is clear of
the groove of the furnace. There they collect into a heap, which the
succeeding flowers push insensibly forward till they slightly stop the
passage. Those which rise after the neck is thus stopped stick to the
after-part of it which is hot, vitrify in some measure, and form a
circle of fused Salt. In this state the flowers of the Sedative Salt
seem to issue out of the circle, as from their basis: they appear
like very thin, light, shining scales, and must be brushed off with a
feather.
At the bottom of the retort will be left a saline mass: dissolve this
in a sufficient quantity of hot water; filter the solution in order
to free it from a brown earth which it deposites; set the liquor to
evaporate, and crystals of Sedative Salt will form in it.
_OBSERVATIONS._
Though Borax is of great use in many chymical operations, especially in
the fusion of metals, as we shall have occasion to see, yet, till of
late years, Chymists were quite ignorant of its nature, as they still
are of its origin; concerning which we know nothing with certainty, but
that it comes rough from the East Indies, and is purified by the Dutch.
M. Homberg was one of the first that attempted to analyse this Salt. He
shewed, that on mixing it with the Vitriolic Acid, and distilling the
mixture, a salt sublimes in little fine needles. This product of Borax
he called by the name of _Sedative Salt_, because he found it had the
property of moderating the great tumult and heat of the blood in fevers.
After M. Homberg, other Chymists also exercised themselves on Borax. M.
Lemery discovered that the Vitriolic is not the only Acid by means of
which the Sedative Salt may be obtained from Borax; but that either of
the other two Mineral Acids, the Nitrous or the Marine, may be used in
its stead.
M. Geoffroy hath greatly facilitated the means of obtaining the
Sedative Salt from Borax; having shewn that it may be extracted by
crystallization as well as by sublimation; and that the Sedative Salt
so obtained is in no respect inferior to that which was procured before
by sublimation only. To him also we are indebted for the discovery,
that in the composition of Borax there is an Alkaline Salt of the same
nature as the basis of Sea-salt. This he found by observing that he got
a Glauber's Salt from a solution of Borax into which he had poured some
Vitriolic Acid with a view to obtain its Sedative Salt.
Lastly, M. Baron, whom we mentioned before on occasion of this Salt,
hath proved, by a great number of experiments, that a Sedative Salt
may be procured from Borax by the help of Vegetable Acids, which was
never done by any body before him; that the Sedative Salt is not
a combination of an Alkaline matter with the Acid made use of in
extracting it, as some of its properties seemed to indicate; but that
it exists previously and completely formed in the Borax; that the Acid
employed to extract it only helps to disengage it from the Alkali with
which it is united; that this Alkali is actually of the same nature as
the basis of Sea-salt, because that after extracting the Sedative Salt,
which by its union therewith forms the Borax, a Neutral Salt is found,
of the same sort with that which would be produced by combining the
basis of Sea-salt with the particular Acid made use of; that is, if
with the Vitriolic Acid, a Glauber's Salt; if with the Nitrous Acid,
a quadrangular Nitre; and if with the Marine Acid, a true Sea-salt;
and, lastly, that the Sedative Salt may be re-united to its Alkali, and
reproduce a Borax.
Nothing therefore now remains, to give us all the insight we can desire
into the nature of Borax, but to know what the Sedative Salt is. M.
Baron hath already given us certain negative notices concerning it,
by shewing what it is not; that is, that the Acid employed in its
extraction doth not enter into its composition. We have great reason to
hope, that he will carry his inquiries still further, and clear up all
our doubts on this subject.
The Sedative Salt may be extracted from Borax, not only by the means
of pure and simple Acids, but also by the same Acids combined with a
metallic basis. Thus Vitriols, for instance, may be employed for this
purpose with good success. It is easy to see, that the Vitriol must
be decomposed on this occasion, and that its Acid cannot unite with
the Alkali in which the Sedative Salt is lodged, without quitting its
metallic basis, which must of course precipitate.
The Sedative Salt actually sublimes, when a liquid containing it is
distilled; but it does not therefore follow, that it is naturally
volatile. It rises only by the aid of the water with which it is mixed.
The proof of this assertion is, that, when all the humidity of the
mixture containing this Salt is dissipated, no more Salt will rise,
be the fire ever so violent; and that by adding more water to moisten
the dried mass containing it, more Salt will every time be obtained,
through many repeated distillations. In the same manner, if some
Sedative Salt be moistened, and exposed to a proper degree of heat, a
small quantity thereof will rise at first by the help of the water; but
as soon as it grows dry it remains exceedingly fixed. This observation
we owe to M. Rouelle.
The Sedative Salt hath the appearance and the taste of a Neutral Salt:
it does not change the colour of the juice of violets; nor does it
easily dissolve in water; for it requires a quart of boiling water to
dissolve two ounces of it: yet, with regard to Alkalis, it has the
properties of an Acid; it unites with those salts, forms therewith a
saline compound which crystallizes, and even expels the Acids that
happen to be combined with them; so that it decomposes the same Neutral
Salts that are decomposed by the Vitriolic Acid.
The Sedative Salt, when suddenly exposed to the violent heat of a
naked fire, loses near half its weight, melts, puts on and retains the
appearance of glass; but its nature still remains unchanged. This glass
dissolves in water, and shoots anew into crystals of Sedative Salt.
This Salt communicates to the Alkaline salt with which it is united,
when in the form of Borax, the property of melting with a moderate
heat, and forming a kind of glass; and it is this great fusibility that
recommends the frequent use of Borax as a flux for assaying ores. It is
also employed sometimes as an ingredient in the composition of glass;
but, in time, it always communicates thereto the fault which its own
glass hath, namely that of tarnishing with the air. The Sedative Salt
hath, moreover, the singular property of dissolving in Spirit of Wine,
and of giving to its flame, when set on fire, a beautiful green colour.
All these observations we owe to Mess. Geoffroy and Baron.
M. Geoffroy prepares the Sedative Salt by crystallization only, in
the following manner. "He dissolves four ounces of refined Borax in a
sufficient quantity of warm water, and then pours into the solution
one ounce and two drams of highly concentrated Oil of Vitriol, which
makes a crackling noise as it falls in. When this mixture has stood
evaporating for some time, the Sedative Salt begins to make its
appearance in little, fine, shining plates floating on the surface of
the liquor. The evaporation is then to be stopped, and the plates will
by little and little increase in thickness and breadth. They unite
together into little tufts, forming with each other sundry different
groups. If the vessel be ever so little stirred, the regularity of
the crystals will be disturbed; so that it must not be touched till
the crystallization appears to be finished. The crystalline clusters,
being grown too bulky and too heavy, will then fall of themselves to
the bottom of the vessel. This being observed, the saline liquor must
be gently decanted from those little crystals, which, as they are not
easily dissolved, must be washed clean, by pouring cold water slowly
on the sides of the pan, three or four times successively, in order
to rinse out all remains of the saline liquor, and then set first to
drain, and afterwards to dry in the sun. This Salt, in the form of
light flakes of snow, is now soft to the touch, cool in the mouth,
slightly bitter, crackling a little between the teeth, and leaving a
small impression of acidity on the tongue. It will keep long without
giving or calcining, if managed according to the preceding directions;
that is, if it be exactly freed from its saline liquor.
"It differs from the Sedative Salt obtained by sublimation in this
respect only, that notwithstanding its seeming lightness it is a
little heavier than the other. M. Geoffroy supposes the cause of this
weight to be, that, as several of the thin plates adhere together in
crystallizing, they retain between them some small matter of humidity;
or, if you will, that, as they form larger crystals, they present
less surface to the air which elevates light bodies: whereas, on the
contrary, the other Sedative Salt, being driven up by the force of
fire, rises into the head of the cucurbit in a more subtile form,
having its particles much more expanded and divided.
"M. Geoffroy, having put his Sedative Salt made by crystallization to
all the same trials with that made by sublimation, satisfied himself
that there is no other difference between the two. If the Sedative Salt
made by crystallizations happens to calcine in the sun; that is, if its
lustre tarnishes, and its surface grows mealy, it is a sign that it
still contains either a little Borax or some Glauber's Salt: for these
two Salts are apt to calcine in this manner, and pure Sedative Salt
should not be subject to this inconvenience. In order to purify it, and
free it entirely from those Salts, it must be dissolved once more in
boiling water. As soon as the water cools, the Sedative Salt reappears
in light, shining, crystalline plates, swimming in the liquor. After
standing four and twenty hours, the liquor must be decanted, and the
salt washed with fresh water; by which means it will be very pure and
beautiful."
Glauber's Salt and Borax dissolve in water with vastly more ease than
the Sedative Salt, and consequently do not crystallize so readily by
much: so that the small portion of those salts which may have been left
on the surface of the Sedative Salt, being diffused through a large
quantity of water, continues in a state of solution, while the Sedative
Salt crystallizes; which being also washed afterwards with fair water,
it is impossible that the smallest particle of those other Salts
should remain adhering to it; and consequently this must be deemed an
excellent way of purifying it.
SECTION II.
_Of Operations on_ METALS.
CHAP. I.
_Of_ GOLD.
PROCESS I.
_To separate Gold, by Amalgamation with Mercury, from the Earth and
Stones with which it is found mixed._
Pulverize the earths and stones containing Gold. Put the powder into a
little wooden tray; dip this tray in water, gently shaking it and its
contents. The water will grow muddy, by taking up the earthy parts of
the ore. Continue washing it in this manner till the water cease to
appear turbid. Upon the ore thus washed pour strong vinegar, having
first dissolved therein, by the help of heat, about a tenth part of its
weight of alum. The powder must be quite drenched and covered with this
liquor, and so left to stand for twice twenty-four hours.
Decant the vinegar, and wash your powder with warm water, till the
last that comes off hath no taste: then dry it, and put it into an
iron mortar, with four times its weight of Quick-silver: triturate the
whole with a heavy wooden pestle, till all the powder be of a blackish
colour: then pour in a little water, and continue rubbing for some time
longer. More earthy and heterogeneous particles will be separated from
the metalline parts by means of this water, which will look dirty: it
must then be decanted, and more fair water added. Repeat this several
times; then dry what remains in the mortar with a sponge, and by the
help of a gentle heat: you will find it an Amalgam of the Mercury with
the Gold.
Put this Amalgam into a chamoy bag: tie a knot on its neck, and squeeze
it hard between your fingers, over some wide-mouthed vessel; there
will issue through the pores of the leather numberless little jets
of Mercury, forming a sort of shower, that will collect into large
globules in the vessel placed underneath. When you can force out no
more Mercury by this means, open the bag, and in it you will find the
Amalgam freed from the superfluous Mercury; the Gold retaining only
about as much thereof as nearly equals itself in weight.
Put this Amalgam into a glass retort; set this retort in the sand-bath
of a reverberating furnace; cover it quite over with sand; apply a
glass receiver half full of water, so that the nose of the retort may
be under the water. The receiver need not be luted to the retort. Give
a gradual heat, and raise the fire till drops of the sublimed Mercury
appear in the neck of the retort, and fall into the water with a
hissing noise. If you hear any noise in the retort slacken your fire a
little. Lastly, when you observe, that, though you raise the fire still
higher than before, nothing more will come over, take out your retort,
break it, and there you will find the Gold, which must be melted in a
crucible with Borax.
_OBSERVATIONS._
Gold is a perfect metal, which can by no means be deprived of its
Phlogiston, and on which few, even of the most powerful chymical
solvents, have any effect: and therefore it almost always hath its
metalline form when found in the earth; from which it may sometimes
be separated by simple lotion. The Gold dust found in the sands of
certain rivers is of this kind. When it resides in stones, or tenacious
earths, it may be extracted by the process here delivered; to wit, by
Amalgamation, or combination of Mercury with Gold. Mercury is incapable
of uniting with any earthy substances, not even with the metallic
earths, when they are deprived of their Phlogiston, and consequently
have not the metalline form.
Hence it follows, that when Mercury is triturated with particles of
Gold, of earth, and of stone, mingled together, it unites with the
Gold, and separates it from those heterogeneous matters. Yet, if there
be along with the Gold any other metal, in its metalline form, except
Iron, the Mercury will amalgamate with that also. This often happens
to Silver, which being a perfect metal as well as Gold, is for that
reason sometimes dug up in its metalline form, and even incorporated
with Gold. When this is the case, the mass that remains in the retort,
after abstracting the Mercury of the Amalgama, is a compound of Gold
and Silver, which are to be separated from each other by the methods
we shall give for that purpose. The present process is therefore
applicable to Silver as well as Gold.
Sometimes Gold is intimately combined with such mineral matters as
hinder the Mercury from acting upon it. In that case the mixed mass
must be roasted before you proceed to Amalgamation: for if the matters
be volatile, such, for instance, as antimony or arsenic, the fire
will carry them off; so that, after roasting, the Amalgamation will
succeed. But sometimes these matters are fixed, and require fusion; if
so, recourse must be had to some particular methods, which we shall
describe when we come to treat of Silver, as these two perfect metals
are to be treated in the same manner.
Ores containing Gold must be washed before an Amalgam is attempted;
that the metalline parts, being freed from the numerous particles of
earth with which they are encompassed, may the more readily incorporate
with the Mercury. Besides, it is the property of Mercury to take the
form of a dark unmetallic powder, after being long rubbed with other
matters, so that it cannot be easily distinguished from the particles
of earth. And hence, if you still continue to grind the matters
together, after the Amalgamation is completed, and wash them again
and again, the water that comes off will always look turbid, being
impregnated with some particles of the Amalgam. This is easily proved:
for if you let the turbid water settle, and distil the sediment, you
will obtain Quick-silver from it.
The ore is to be steeped in vinegar charged with alum, in order to
cleanse the surface of the Gold, which is often covered with a thin
coat of earth that obstructs the Amalgamation.
Great care must be taken that the Mercury employed in this operation be
very pure. If it be adulterated with any metallic substance, it must be
freed therefrom by the methods which we shall propose in their proper
place.
The way of separating Mercury from Gold is founded on the different
properties of these two metallic substances; the one being exceedingly
fixed, and the other very volatile. The union which Mercury contracts
with the metals is not intimate enough, to give the new compound which
results therefrom all the properties of either of the two united
substances; at least so far as concerns their degrees of fixity
and volatility. Hence it comes, that, in our Amalgam, the Gold
communicates but very little of its fixity to the Mercury, and the
Mercury communicates to the Gold but very little of its volatility. Yet
if the Mercury be distilled off with a much greater degree of heat than
is necessary to elevate it, a pretty considerable quantity of Gold will
most certainly be carried up along with it.
It is also of consequence, on another account, that the fire be duly
governed on this occasion. For if too great a degree of heat be
applied, and the fire afterwards lowered, the water in the receiver,
which covers the nose of the retort, will rise into its body, break it
to pieces, and spoil the operation.
The cause of this phenomenon depends on the property which air
possesses of rarefying with heat and condensing with cold, joined to
its weight. As soon as the retort is acted on by a less degree of heat
than acted on it the instant before, the air contained therein is
condensed, and leaves a _vacuum_, which the external air, by virtue of
its weight, tends to occupy; but, the orifice of the retort being under
water, the external air can no way gain admittance, but by pushing in
before it the water which intercepts its passage. This caution, as we
observed above, must be applied to all distillations, where the vessels
are disposed as they are in this.
Care must also be taken that the nose of the retort be not placed too
deep under water: for as the neck grows very warm during the operation,
because the degree of heat required to raise Mercury is about three
times greater than that which raises water, it may easily be broken by
the contact of the cold water in the receiver.
This method of extracting Gold and Silver from their ores, by
Amalgamation with Mercury, is not to be absolutely depended on as a
sure proof of the quantity of those metals that may be contained in
the earth assayed by this means: for some small part of the Amalgam is
always lost in washing it; and, moreover, the Mercury, when squeezed
through chamoy, always carries with it a small portion of Gold. So that
if you desire to know more exactly, by this method, the quantity of
Gold or Silver contained in any earth, the Amalgam must not be squeezed
through chamoy, but distilled altogether. Much the surest method of
making an accurate assay is that by fusion and scorification, which we
shall describe under the head of Silver.
In some countries, and especially in America, the method of
Amalgamation is used for extracting Gold and Silver in large
quantities, from the matrices which contain them in their metalline
form. Agricola and other metallurgists have described the machines by
means whereof such Amalgamations are managed.
PROCESS II.
_To dissolve Gold in_ Aqua regis, _and by that means to separate it
from Silver_. Aurum Fulminans. Aurum Fulminans _reduced_.
Take Gold that is perfectly pure, or alloyed with Silver only. Reduce
it to little thin plates, by hammering it on an anvil. If it be not
sufficiently tough, neal it till it be red in a moderate, clear fire,
quite free from smoking coals, and then let it cool gradually, which
will restore its ductility.
When the plates are thin enough, make them red hot once more, and cut
them into small bits with a pair of sheers. Put these bits into a tall,
narrow-mouthed cucurbit, and pour on them twice their weight of good
_Aqua regis_, made of one part Sal Ammoniac, or Spirit of Salt, and
four parts Spirit of Nitre. Set the cucurbit in a sand-bath moderately
heated, stopping its orifice slightly with a paper coffin, to prevent
any dirt from falling in. The _Aqua regis_ will presently begin to
smoke. Round the little bits of Gold will be formed an infinite number
of small bubbles, which will rise to the surface of the liquor. The
Gold will totally dissolve, if it be pure, and the solution will be of
a beautiful yellow colour: if the Gold be alloyed with a small quantity
of Silver, the latter will remain at the bottom of the vessel in the
form of a white powder. If the Gold be alloyed with much Silver, when
the Gold is dissolved the Silver will retain the form of the little
metalline plates put into the vessel.
When the dissolution is completed, gently pour off the liquor into
another low, wide-mouthed, glass cucurbit, taking care that none of the
Silver, which lies at the bottom in the form of a powder, escape with
the liquor. On this powder of Silver pour as much fresh _Aqua regis_ as
will cover it entirely; and repeat this till you are sure that nothing
more can be taken up by it. Lastly, having decanted the _Aqua regis_
from the Silver, wash the Silver with a little Spirit of Salt weakened
with water, and add this Spirit of Salt to the _Aqua regis_ in which
your Gold is dissolved. Then to the body containing these liquors fit
a head and a receiver, and distil with a gentle heat, till the matter
contained in the cucurbit become dry.
_OBSERVATIONS._
It is certain that _aqua regis_ is the true solvent of Gold, and that
it does not touch Silver: so that if the Gold dissolved in it were
alloyed with Silver, which is often the case, the two metals would by
this means be pretty accurately separated from each other. But if you
desire to obtain from this solution a Gold absolutely pure, you must
free it, before you dissolve it, from every other metallic substance
but Silver; because _aqua regis_ acts upon most of the other metals
and the semi-metals. We shall shew under the head of Silver, as we
promised before, how to purify a mass of Gold and Silver from every
other metallic alloy. Thither also we refer the common parting assay
performed by means of _aqua fortis_: because in that operation the
Silver is dissolved, and not the Gold.
If the Gold put to dissolve in _aqua regis_ be pure, the dissolution
is easily and readily effected. But if, on the contrary, it be alloyed
with Silver, the _aqua regis_ finds more difficulty in dissolving it.
Nay, if the Silver exceed the Gold in quantity, the dissolution will
not take place at all, for the reasons adduced in our Theoretical
Elements; of which we shall speak more fully when we come to treat of
the Parting Assay.
In the process we directed the Gold to be dissolved in a tall body.
This precaution is necessary to prevent the loss of some part thereof:
for it is the property of _aqua regis_ to carry off along with it
some of the Gold, especially when there is any Sal Ammoniac in its
composition, if the vessel be heated while the dissolution is going on,
or if the _aqua regis_ be very strong. Yet it is proper to make use of
_aqua regis_ that is too strong rather than too weak: for if it prove
too strong, and be observed not to act upon the metal for that reason,
it is easy to weaken it, by gradually adding small quantities of pure
water, till you perceive it begin to act with vigour. This is a general
rule regarding all metallic dissolutions in Acids.
When the solution of Gold is evaporated to dryness, if you desire to
reduce into a mass the Gold dust left at the bottom of the cucurbit,
you must put it into a crucible, and cover it with pulverized borax,
mixed with a little nitre and calcined wine-lees; then cover the
crucible close, heat it with a moderate fire, which must be afterwards
increased so as to melt the contents. At the bottom of the crucible you
will find a lump of Gold, over which the salts you added will be as it
were vitrified. These salts are added chiefly to promote the fusion of
the metal.
The Gold may, if you will, be separated from its solvent without
evaporating the solution as above directed. You need only mix with the
solution a fixed or volatile Alkali by little and little, till you see
no more precipitate fall, and then let the liquor stand to settle, at
the bottom of which you will find a sediment: filter the whole, and dry
what is left on the filter.
Both fixed and volatile Alkalis possessing, as hath been frequently
repeated, a greater affinity with Acids than metallic substances have,
they precipitate the Gold, and separate it from the Acids in which it
is dissolved: but it is of great consequence to take notice, that,
if you attempt to melt this precipitated Gold in a crucible, it will
fulminate as soon as it feels the heat, with such a terrible explosion,
that, if the quantity be at all considerable, it may prove fatal to
the operator: even rubbing it a little hard will make it blow up. This
preparation is therefore called _Aurum Fulminans_.
Hitherto no satisfactory explanation hath been given of this
phenomenon. Some Chymists considering, that, in the precipitation of
the Gold, a Nitre is regenerated by the union of the Alkali with the
Nitrous Acid which enters into the composition of the _aqua regis_,
imagine that some of this regenerated Nitre, combining with the
precipitated Gold, takes fire and detonates, either by means of some
small portion of Phlogiston that may be contained in the Alkali, or
by means of that which constitutes the Gold itself. But, in the first
place, it is well known that Fixed Alkalis do not contain Phlogiston
enough to make Nitre detonate. Indeed, if a Volatile Alkali be employed
in the precipitation, a Nitrous Ammoniacal Salt will be formed,
containing Phlogiston enough to be capable of detonating without the
concourse of any additional Phlogiston: but this detonation of the
Nitrous Ammoniacal Salt is not to be compared, as to the violence of
its effects, with the fulmination of Gold. Besides, we do not find that
Gold precipitated by a Volatile Alkali explodes with greater force than
that precipitated by a Fixed Alkali. As for the Gold, it is certain
that it suffers no decomposition at all by fulminating. When fulminated
under a glass bell, in such small quantities as not to endanger the
operator, the Gold is found scattered about under the bell in very fine
particles, without having undergone any alteration.
Others have fancied this fulmination of the Gold to be nothing but the
decrepitation of the Sea-salt that is regenerated, in the precipitation
of the metal, by the Fixed Alkali uniting with the Acid of Sea-salt
which makes part of the _aqua regis_. But to this it may be said,
that Gold precipitated by a Volatile Alkali fulminates as violently
as that precipitated by a Fixed Alkali; and yet no sea-salt can be
formed in the liquor by the addition of a Volatile Alkali, but only a
Sal Ammoniac which has not the property of decrepitating. Moreover,
there is no comparison, as to the effects, between the decrepitation of
Sea-salt and the fulmination of Gold.
Nor, lastly, can this fulmination be attributed, as it is by some, to
the effort made by the Salts to escape from amidst the particles of
Gold, in which they are supposed by them to be imprisoned: for then we
might deprive this Gold entirely of its fulminating quality by only
boiling it in water, and so washing off all the saline particles,
which probably adhere to its surface only. It is plain there is great
room for very beautiful discoveries on this subject. In Walerius's
Mineralogy we find some observations that may throw a little light on
the point before us.
"The quantity," says he, "of fulminating Gold precipitated exceeds that
of the Gold dissolved: if the _aqua regis_ be made with Sal Ammoniac
the explosion will be stronger; it will also be more violent if the
solution be precipitated with a Volatile Alkali, than if a Fixed Alkali
be used for that purpose."
One of the speediest and easiest methods to deprive this Gold of its
fulminating quality, is to grind in a mortar twice as much flowers
of Sulphur as you have Gold to reduce, mixing your fulminating Gold
therewith by little and little, as you grind them together; then to
put the mixture into a crucible, and heat it just enough to melt the
Sulphur. Part of the Sulphur will be dissipated in vapours, and the
rest will burn away. When it is quite consumed, increase the fire so
as to make the crucible red-hot. When you perceive no more smell of
Sulphur, pour on the Gold a little Borax, previously melted in another
crucible with a Fixed Alkali, as calcined Wine-lees, or Nitre fixed
with Tartar; and then raise the fire sufficiently to make the whole
flow. After the fusion is completed, you will find a button of Gold at
the bottom of the crucible under the Salts.
Fulminating Gold may also be reduced by pouring on it a sufficient
quantity of Fixed Alkali reduced to a liquor, or of oil of Vitriol,
evaporating all the moisture, and gradually throwing what remains,
mixed up with some pinguinous matter, into a crucible kept red-hot in
a furnace. The reason why these substances deprive the Gold of its
fulminating quality, depends on the causes that produce the fulmination.
Gold may also be separated from _aqua regis_, and precipitated by the
means of several metallic substances that have a greater affinity,
either with _aqua regis_, or with one of the two Acids that compose it.
Mercury is one of the fittest for this purpose. On dropping a solution
of Mercury in the Nitrous Acid by little and little into a solution of
Gold, the mixture becomes turbid, and a precipitate is formed. Continue
dropping in more of the solution of Mercury till no more precipitate
falls; then let the liquor stand to settle, and at the bottom of it
you will find a sediment, which is the precipitated Gold: pour off the
liquor by inclination, and wash the precipitate with fair water.
Mercury hath a greater affinity with the Marine than with the Nitrous
Acid. The affinity which Mercury hath with the Marine Acid is also
greater than that of Gold with the Marine Acid; for unless this Acid
be associated either with the Nitrous Acid, or at least with a certain
proportion of Phlogiston, it will not dissolve Gold. Hence it comes,
that when a solution of Mercury in the Nitrous Acid is dropped into a
solution of Gold in _aqua regis_, the Mercury unites with the Acid of
Sea-salt, which is an ingredient in the _aqua regis_: but the Marine
Acid cannot on this occasion join the Mercury, without deserting the
Gold and the Nitrous Acid with which it was united; and then the Gold,
which cannot be kept in solution by the Nitrous Acid alone, is forced
to quit its solvent and precipitate. The liquor, therefore, that now
floats over the Gold thus precipitated, must contain Mercury united
with the Acid of Sea-salt: and in fact it yields a true Corrosive
Sublimate, which is known to be a combination of Mercury with the
Marine Acid.
Mercury dissolved in Spirit of Nitre is employed to procure the
precipitation we are speaking of; because metallic substances, when so
comminuted by an Acid, are much fitter for such experiments than when
they are in a concrete form.
Gold precipitated in this manner by a metallic substance doth not
fulminate.
PROCESS III.
_To dissolve Gold by Liver of Sulphur._
Mix together equal parts of common Brimstone, and a very strong Fixed
Alkali; for instance, Nitre fixed by Charcoal. Put them in a crucible,
and melt the mixture, stirring it from time to time with a small rod.
There is no occasion to make the fire very brisk; because the Sulphur
facilitates the fusion of the Fixed Alkali. Some sulphureous vapours
will rise from the crucible; the two substances will mix intimately
together, and form a reddish compound. Then throw into the crucible
some little pieces of Gold beat into thin plates, so that the whole do
not exceed in weight one third part of the Liver of Sulphur: raise the
fire a little. As soon as the Liver of Sulphur is perfectly melted, it
will begin to dissolve the Gold with ebullition; and will even emit
some flashes of fire. In the space of a few minutes the Gold will be
entirely dissolved, especially if it was cut and flatted into small
thin leaves.
_OBSERVATIONS._
The process here delivered is taken from M. Stahl. The design of that
ingenious Chymist's inquiry was to discover how Moses could burn the
golden calf, which the Israelites had set up and worshipped while he
was on the mount; how he could afterwards reduce that calf to powder,
throw it into the water which the people used, and make all who had
apostatized drink thereof, as related in the Book of Exodus.
M. Stahl, having first observed that Gold is absolutely unalterable
and indestructible by the force of fire alone, be it ever so violent,
concludes, that without a miracle Moses could not possibly perform the
above-mentioned operations on the golden calf any way but by mixing
with the Gold some matter qualified to alter and dissolve it. He then
takes notice, that pure Sulphur does not act upon Gold at all, and
that many other substances, which are thought capable of dividing and
dissolving it, cannot however do it so completely as is necessary to
render that metal susceptible of the effects related. He then gives the
method of dissolving it by Liver of Sulphur, described in the process.
Liver of Sulphur dissolves likewise all the other metals: but M.
Stahl observes, that it attenuates Gold more than any other metallic
substance, and unites with it much more intimately than with the rest.
This appears from what happens, on attempting to dissolve in water
any of the mixts resulting from the union of another metal with the
Liver of Sulphur: for then the metal separates, and appears in the
form of a powder or fine calx; whereas, when Gold is united with Liver
of Sulphur, the whole compound dissolves in water so perfectly, that
the Gold even passes with the Liver of Sulphur through the pores of
filtering paper.
If an Acid be poured into a solution of this combination of Gold with
Liver of Sulphur, the Acid unites with the Alkali of the _Hepar_, and
the Gold falls to the bottom of the liquor along with the Sulphur,
which doth not quit it. The Sulphur thus precipitated with the Gold
is easily carried off by a slight torrefaction, after which the
Gold remains exceedingly comminuted. The Sulphur of this compound
may also be destroyed by torrefaction, without the trouble of a
previous solution and precipitation, and then also the Gold remains so
attenuated as to be miscible with liquors, and floats on them, or swims
in them, in such a manner that it may easily be swallowed with them in
drinking. From all this M. Stahl concludes there is great reason to
believe it was by means of the Liver of Sulphur that Moses divided, and
in a manner calcined, the golden calf, so that he could mingle it with
water, and make the Israelites drink it.
PROCESS IV.
_To separate Gold from all other metallic Substances by means of
Antimony._
Having put the Gold you intend to purify into a crucible, set it in
a melting furnace, cover it, and make the Gold flow. When the metal
is in fusion cast upon it, by a little at a time, twice its weight
of pure crude Antimony in powder, and after each projection cover
the crucible again immediately: this done keep the matter in fusion
for a few minutes. When you perceive that the metallic mixture is
perfectly melted, and that its surface begins to sparkle, pour it out
into a hollow iron cone, previously heated, and smeared on the inside
with tallow. Immediately strike with a hammer the floor on which the
cone stands; and when all is cold, or at least sufficiently fixed,
invert the cone and strike it: the whole metallic mass will fall out,
and the under part thereof, which was at the point of the cone, will
be a Regulus more or less yellow as the Gold was more or less pure.
On striking the metallic mass the Regulus will freely part from the
sulphureous crust at top.
Return this Regulus into the crucible, and melt it. Less fire will do
now than was required before. Add the same quantity of Antimony, and
proceed as at first. Repeat the same operation a third time, if your
Gold be very impure.
Then put your Regulus into a good crucible, much larger than is
necessary to hold it. Set your crucible in a melting furnace, and heat
the matter but just enough to make it flow, with a smooth, brilliant
surface. When you find it thus conditioned, point towards it the nose
of a long-snouted pair of bellows, and therewith keep gently and
constantly blowing. There will arise from the crucible a considerable
smoke, which will abate greatly when you cease to blow, and increase
as soon as you begin again. You must raise the fire gradually as you
approach towards the end of the operation. If the surface of the metal
lose its brilliant polish, and seem covered with a hard crust, it is
a sign the fire is too weak; in which case it must be increased, till
the surface recover its shining appearance. At last, when no more smoke
rises, and the surface of the Gold looks neat and greenish, cast on
it, by little and little, some pulverized Nitre, or a mixture of Nitre
and Borax. The matter will swell up. Continue thus adding more Nitre
gradually, till no commotion is thereby produced in the crucible; and
then let the whole cool. If you find, when the Gold is cold, that it is
not tough enough, melt it over again; when it begins to melt, cast in
the same Salts as before; and repeat this till it be perfectly ductile.
_OBSERVATIONS._
Antimony is a compound, consisting of a semi-metallic part united with
about a fourth part of its weight of common Sulphur. It appears, in the
ninth column of the Table of Affinities, that all the metals, Mercury
and Gold excepted, have a greater affinity than the reguline part of
Antimony with Sulphur. If therefore Gold, adulterated with a mixture
of Copper, Silver, or any other metal, be melted with Antimony, those
metals will unite with the Sulphur of the Antimony, and separate it
from the reguline part, which being thus set free will combine and be
blended with the Gold. These two metallic substances, forming a mass
far heavier than the other metals mixed with the sulphur, fall together
to the bottom of the crucible in the form of a Regulus, while the
others float over them like a sort of scoria or flag: and thus the Gold
is freed from all alloy but the reguline part of the Antimony.
As all the other metals have a great affinity with Sulphur, and Gold
is the only one that is capable of resisting its action, one would
think Sulphur alone might be sufficient to free it from the metals
combined with it, and that it would therefore be better to employ pure
Sulphur, in this operation, than to make use of Antimony; the reguline
part of which remaining united with the Gold requires another long and
laborious operation to get rid of it.
Indeed, strictly speaking, Sulphur alone would be sufficient to
produce the desired separation: but it is proper to observe, that, as
Sulphur alone is very combustible, most of it would be consumed in
the operation before it could have an opportunity to unite with the
metallic substances; whereas, when it is combined with the Regulus
of Antimony, it is thereby enabled to bear the action of the fire
much longer without burning, and consequently is much fitter for the
purpose in question. Besides, if we were to make use of pure Sulphur,
a great part of the Gold, which is kept in perfect fusion, and its
precipitation facilitated, by the Regulus of Antimony, would remain
confounded with the Sulphureous scoria.
Nevertheless, seeing the metals with which Gold is alloyed cannot
be separated from it by Antimony, but that a quantity of Regulus
proportioned to the quantity of the metals so separated will unite
with the Gold, and that the more Regulus combines with the Gold, the
more tedious, chargeable, and laborious will the operation prove, this
consideration ought to have some influence in directing our process.
Therefore, if the Gold be very impure, and worse than sixteen carats,
we must not mix it with crude Antimony alone, but add two drams of
pure Sulphur for every carat the Gold wants of sixteen, and lessen the
quantity of Antimony in proportion to that of the real Gold.
It is necessary to keep the crucible close covered, after mixing the
Antimony with the Gold, to prevent any coals from falling into it: for,
if that should happen, the melted mass would puff up considerably, and
might perhaps run over.
The inside of the cone, into which you pour the melted metallic mass,
must be greased with tallow, to prevent its sticking thereto, and that
it may come easily out. Striking the floor, on which the cone with the
melted metal stands, helps the precipitation and descent of the Regulus
of Gold and Antimony to the bottom of the cone.
Less fire is requisite to melt this compound Regulus, in order to add
fresh Antimony, than was necessary before the Gold was mixed with the
reguline part of the Antimony; because this metallic substance, being
much more fusible than Gold, promotes its melting. The Antimony is
mixed with the Gold by repeated projections, that the separation of
the metals may be accomplished with the greater ease and accuracy. Yet
the operation might be successfully performed, by putting in all the
Antimony at once, and with one melting only.
The metalline mass found at the bottom of the cone after all these
operations, is a mixture of Gold with the reguline part of the
Antimony. All the rest of the process consists only in separating this
reguline part from the Gold. As Gold is the most fixed of all metals,
and as the Regulus of Antimony cannot bear the violence of fire without
flying off in vapours, nothing more is necessary for this purpose but
to expose the compound, as directed in the process, to a heat strong
enough and long enough continued, to dissipate all the Regulus of
Antimony. This semi-metal exhales in the form of a very thick white
smoke. It is proper to blow gently into the crucible during the whole
operation; because the immediate contact of the fresh air incessantly
thrown in promotes and considerably increases the evaporation: and this
is a general rule applicable to all evaporations.
The fire must be gradually raised as the Regulus of Antimony is
dissipated, and the operation draws towards an end; because the mixed
mass of Regulus of Antimony and Gold becomes so much the less fusible
as the proportion of the Regulus is lessened. Though the Regulus of
Antimony be separated from the Gold in this operation, because the
latter is of such a fixed nature that it cannot be volatilized by the
degree of fire which dissipates the Regulus; yet, as the Regulus is
very volatile, it will undoubtedly carry up some of the Gold along with
it, especially if you hurry on the evaporation too fast, by applying
too great a degree of fire, by blowing too briskly into the crucible,
and still more if you evaporate your mixture in a broad flat vessel
instead of a crucible. All these things must therefore be avoided, if
you would lose no more Gold than you needs must.
However, unless the evaporation be carried to the utmost, by the means
above pointed out, a small portion of the Regulus of Antimony will
always remain combined with the Gold, which defends it from the action
of the fire. This small portion of Regulus hinders the Gold from being
perfectly pure and ductile. In order therefore to consume and scorify
it, we cast Nitre into the crucible when we perceive it to emit no more
white vapours.
We know that Nitre has the property of reducing all metallic substances
to a calx, Gold and Silver excepted; because it deflagrates with
the phlogiston to which their metalline form is owing: but as this
accension of the Nitre occasions a tumid effervescence, care must be
taken to throw it in but by little and little at a time; for if too
much be projected at once the melted matter will run over.
This operation might be considerably abridged by taking advantage of
the property which Nitre possesses of thus consuming the phlogiston
of metallic substances; as by means thereof we might destroy all the
Regulus of Antimony incorporated with the Gold, without having recourse
to a long and tedious evaporation. But then we should at the same time
lose a much greater quantity of Gold, by reason of the tumult and
ebullition which are inseparable from the detonation of Nitre. On the
whole, therefore, if Nitre be made use of to purify Gold, great care
must be taken to apply but very little of it at a time.
All the Silver that was mixed with the Gold, and indeed a little of
the Gold itself, remains confounded with the sulphureous scoria, which
floats upon the Golden Regulus after the addition of the Antimony: we
shall shew in the Chapter on Silver how these two metals are to be
separated from the Sulphur.
CHAP. II.
_Of_ SILVER.
PROCESS I.
_To separate Silver from its Ore, by means of Scorification with Lead._
Beat to powder in an iron mortar the ore from which you mean to
separate the Silver, having first roasted it well in order to free it
from all the Sulphur and Arsenic that it may contain. Weigh it exactly:
then weigh out by itself eight times as much granulated Lead. Put one
half of this Lead into a test, and spread it equally thereon: upon this
Lead lay your ore, and cover it quite over with the remaining half of
the Lead.
Place the test thus loaded under the further end of the muffle in a
cupelling furnace. Light your fire, and increase it by degrees. If you
look through one of the apertures in the door of the furnace you will
perceive the ore, covered with calcined Lead, swim upon the melted
Lead. Presently afterwards it will grow soft, melt, and be thrown
towards the sides of the vessel, the surface of the Lead appearing in
the midst thereof bright and shining like a luminous disc: the Lead
will then begin to boil, and emit fumes. As soon as this happens, the
fire must be a little checked, so that the ebullition of the Lead may
almost entirely cease, for about a quarter of an hour. After this it
must be excited to the degree it was at before, so that the Lead may
begin again to boil and smoke. Its shining surface will gradually
lessen, and be covered with _scoriæ_. Stir the whole with an iron hook,
and draw in towards the middle what you observe towards the sides of
the vessel; to the end that, if any part of the ore should still remain
undissolved by the Lead, it may be mixed therewith.
When you perceive that the matter is in perfect fusion, that the
greatest part of what sticks to the iron hook, when you dip it in the
melted matter, separates from it again, and drops back into the vessel;
and that the extremity of this instrument, when grown cold, appears
varnished over with a thin, smooth, shining crust; you may look on
these as marks that the business is done, and the more uniform and
evenly the colour of the crust is, the more perfect may you judge the
scorification to be.
Matters being brought to this pass, take the test with a pair of tongs
from under the muffle, and pour its whole contents into an iron cone,
first heated and greased with tallow. This whole operation lasts about
three quarters of an hour. When all is cold, the blow of a hammer
will part the Regulus from the scoria; and as it is not possible, how
perfect soever the scorification be, to avoid leaving a little Lead
containing Silver in the scoria, it is proper to pulverize this scoria,
and separate therefrom whatever extends under the hammer, in order to
add it to the Regulus.
_OBSERVATIONS._
Silver, as well as Gold, is often found quite pure, and under its
metalline form, in the bowels of the earth; and in that case it may
be separated from the stones or sand in which it is lodged by simple
washing, or by Amalgamation with Mercury, in the same manner as before
directed for Gold. But it also happens frequently, that Silver is
combined in the ore with other metallic substances and minerals, which
will not admit of this process, but force us to employ other methods of
separating it from them.
Sulphur and Arsenic are the substances to which Silver and the other
metals usually owe their mineral state. These two matters are never
very closely united with Silver; but may be pretty easily separated
from it by the action of fire, and the addition of Lead. If Arsenic be
predominant in a Silver ore, it will unite with the Lead by the help
of a pretty moderate heat, and quickly convert a considerable quantity
thereof into a penetrating fusible glass, which has the property of
scorifying with ease all substances that are capable of scorification.
When Sulphur predominates, the scorification proceeds more slowly,
and doth not always succeed; because that mineral combined with Lead
lessens its fusibility, and retards its vitrification. In this case,
part of the Sulphur must be dissipated by roasting: the other part
unites with the Lead; and that, being rendered lighter by this union,
floats on the rest of the mixture, which chiefly contains the Silver.
At last, the joint action of the air and of the fire dissipates the
portion of Sulphur that had united with the Lead: the Lead vitrifies
and reduces to a scoria whatever is not either Silver or Gold: and
thus the Silver being disentangled from the heterogeneous matters with
which it was united, one part thereof being dissipated and the other
vitrified, combines with the portion of Lead which is not vitrified,
and falls through the scoria, which, to favour its descent, must be in
perfect fusion.
The whole process, therefore, consists of three distinct operations.
The first is Roasting, which dissipates some of the volatile substances
found united with the Silver: the second is Scorification, or the
Vitrification of the fixed matters also united with the Silver, such
as sand, stones, metals, _&c._ and the third is precipitation, or the
separation of the Silver from the scoria. The two first are, as hath
been shewn, preparatives for the last, and indeed produce it.
As every thing we said concerning Gold, when we treated of the process
of Amalgamation, is to be applied to Silver, which may be extracted
by the same method when it is in its metalline form; in the same
manner, all we now advance touching the method of extracting Silver
by Scorification, when it is depraved with a mixture of heterogenous
matters, is equally applicable to Gold in the same circumstances: and
indeed Silver almost always contains more or less Gold naturally.
In the process we directed, that the ore should be pulverized before
it be exposed to the fire, with a view to enlarge its surface, and by
that means facilitate the action of the Lead upon it, as well as the
evaporation of its volatile parts.
We recommended the precaution of slackening the fire a little at the
beginning of the operation, only to prevent the Lead from being too
hastily converted into litharge, lest it should penetrate and corrode
the test before it had wholly dissolved the ore: but if we were
perfectly certain of the vessel's being so good as to be in no danger
of penetration by the Lead, this precaution would be needless.
It is proper to add eight parts of Lead for one of ore; though so
much is not always absolutely necessary, especially when the ore is
very fusible. The success of this operation depends chiefly on the
completeness of the Scorification; and therefore the addition of more
Lead than enough is attended with no inconvenience: for, as it always
promotes the Scorification, it can never do any harm.
If the ore be mixed with such earthy and stony parts as cannot be
separated from it by washing, it is the more difficult of fusion,
even though the stones should be such as are most disposed to vitrify;
because the most fusible earths and stones are always less so than most
metallic substances. In that case it will be necessary, for effecting
the Scorification, to mix thoroughly with the pulverized ore an equal
quantity of Glass of Lead, to add twelve times as much granulated Lead,
and then to proceed as directed for a fusible ore; exposing the mixture
to a degree of fire strong enough, and long enough kept up, to give
the scoria all the properties above required as signs of a perfect
scorification.
Silver ore is sometimes mixed with Pyrites, and the ore of Arsenic, or
Cobalt, which also make it refractory. As the Pyrites contain a large
quantity of Sulphur, which is very volatile as well as Arsenic; in
this case it is proper to begin with freeing the ore from these two
extraneous substances. This is easily done by roasting: only be sure,
when you first expose the ore to the heat, to cover the vessel in which
you roast it, for some minutes, with an inverted vessel of the same
width; because such sorts of ore are very apt to fly when they first
feel the heat.
After this uncover it, and leave it exposed to the fire till no more
sulphureous or arsenical matters rise. Then mix it with the same
quantity of Glass of Lead as we ordered for ores rendered refractory by
the admixture of earths or stones, and proceed in the same manner.
It is the more necessary to roast Silver ore infected with Sulphur and
Arsenic, because, as Sulphur obstructs the fusion of Lead, it cannot
but do hurt, and protract the operation; and Arsenic does mischief, on
the other hand, by scorifying a very great quantity of Lead too hastily.
When the Sulphur and Arsenic are dissipated by roasting, the ore must
be treated like that which is rendered refractory by stony and earthy
matters; for as the pyrites contain much iron, there remains, after the
Sulphur is evaporated, a considerable quantity of martial earth, which
is difficult to scorify. The pyrites, as well as the cobalts, contain
moreover an unmetallic earth, which is hard to fuse.
The general rule therefore is, when the ore is rendered refractory
by any cause whatever, to mix it with Glass of Lead, and to add a
larger quantity of granulated Lead. Yet some ores are so refractory
that Lead alone will not do the business, and recourse must be had to
some other flux. That which is fittest for the present purpose is the
_Black Flux_, composed of one part of Nitre and two parts of Tartar
deflagrated together. The Phlogiston contained in this quantity of
Tartar is more than sufficient to alkalizate the Nitre. This Flux,
therefore, is nothing more than Nitre alkalizated by Tartar, mixed with
some of the same Tartar that hath not lost its Phlogiston, and is only
reduced to a sort of coal.
The _White Flux_ is also very fit to promote fusion; but on this
occasion the Black Flux is preferable, because the Phlogiston of the
Black Flux prevents the Lead from being too soon converted to litharge,
and so gives it time to dissolve the metallic matters. The White Flux,
which is the result of equal parts of Tartar and Nitre alkalizated
together, being no more than an Alkali destitute of Phlogiston, or
containing but very little, doth not possess this advantage.
If Silver should be combined in the ore with Iron in its metalline
state, which however does not commonly happen, then, in order to
separate them, the Iron must be deprived of its Phlogiston, and
converted to a _crocus_ before the mixed mass be melted with Lead;
which may be done by dissolving it in the Vitriolic Acid, and then
evaporating the Acid.
We are necessitated to make use of this contrivance, because Iron in
its metalline form cannot be dissolved either by Lead or by the Glass
of Lead; but when it is reduced to a calx, litharge unites with it and
scorifies it.
If you have not at hand the utensils necessary for performing the
operation we have been describing in a test, and under the muffle; or
if you have a mind to work on a greater quantity of ore at a time, you
may make use of a crucible for the purpose, and perform the operation
in a melting furnace.
In this case the ore must be prepared, as above directed, according to
its nature, and mixed with a proper quantity of Lead and Glass of Lead;
the whole put into a good crucible, leaving two thirds thereof empty,
and covered with a mixture of Sea-salt and a little Borax, both very
dry, to the thickness of a full half inch.
This being done, set the crucible in the midst of a melting furnace,
raise the coals quite to the lip of the crucible; light the fire;
cover the furnace with its dome; but do not urge the fire more than
is necessary to bring the mixture to perfect fusion: leave it thus in
fusion for a good quarter of an hour; stir the whole with a bit of
strong iron wire; then let it cool; break the crucible, and separate
the Regulus from the scoria.
The Salts added on this occasion are fluxes, and their use is to
procure a perfect fusion of the scoria.
If the melted matters be left exposed to the fire, either in a test
or in a crucible, longer than is above prescribed, the portion of
Lead, that hath united and precipitated with the Silver, will at last
vitrify, and at the same time scorify all the alloy with which that
metal may be mixed. But as there are no vessels that can long endure
the action of litharge, without being pierced like a sieve, some of the
Silver may escape through the holes or fissures of the vessel, and so
be lost. It is better, therefore, to complete the purification of your
Silver by the operation of the Cupel, the description of which follows.
PROCESS II.
_The refining of Silver by the Cupel._
Take a cupel capable of containing one third more matter than you
have to put into it: set it under the muffle of a furnace, like that
described in our Theoretical Elements, as peculiarly appropriated to
this sort of operation. Fill the furnace with charcoal; light it; make
the cupel red-hot, and keep it so till all its moisture be evaporated;
that is, for about a good quarter of an hour, if the cupel be made
wholly of the ashes of burnt bones; and for a whole hour, if there be
any washed wood-ash in its composition.
Reduce the Regulus which remained after the preceding operation to
little thin plates, flatting them with a small hammer, and separating
them carefully from all the adherent scoria. Wrap these in a bit of
paper, and with a small pair of tongs put them gently into the cupel.
When the paper is consumed the Regulus will soon melt, and the scoria,
which will be gradually produced by the Lead as it turns to litharge,
will be driven to the sides of the cupel, and immediately absorbed
thereby. At the same time the cupel will assume a yellow, brown, or
blackish colour, according to the quantity and nature of the scoria
imbibed by it.
When you see the matter in the cupel in a violent ebullition, and
emitting much smoke, lower the fire by the methods formerly prescribed.
Keep up such a degree of heat only that the smoke which ascends
from the matter may not rise very high, and that you may be able to
distinguish the colour which the cupel acquires from the scoria.
Increase the fire by degrees, as more and more litharge is formed and
absorbed. If the Regulus examined by this assay contain no Silver,
you will see it turn wholly into scoria, and at last disappear. When
it contains Silver, and the quantity of Lead is much diminished, you
will perceive little vivid irises, or beautiful rain-bow colours,
shooting swiftly along its surface, and crossing each other in many
different directions. At last, when all the Lead is destroyed, the
thin dark skin, that is continually protruded by the Lead while it is
turning into litharge, and which hitherto covered the Silver, suddenly
disappears; and, if at this moment the fire happen not to be strong
enough to keep the Silver in fusion, the surface of that metal will at
once dart out a dazzling splendour: but, if the fire be strong enough
to keep the Silver in fusion, though freed from all mixture of Lead,
this change of colour, which is called its _fulguration_, will not be
so perceptible, and the Silver will appear like a bead of fire.
These phenomena shew that the operation is finished. But the cupel
must still be left a minute or two under the muffle, and then drawn
slowly out with the iron hook towards the door of the furnace. When the
Silver is so cooled as to be but moderately red, you may take the cupel
from under the muffle with your little tongs, and in the middle of its
cavity you will find an exceeding white bead of Silver, the lower part
whereof will be unequal, and full of little pits.
_OBSERVATIONS._
The Regulus obtained by the former process consists altogether of
the Silver contained in the ore, alloyed with the other metals that
happened to be mixed therewith in its mineral state, and a good deal of
the Lead that was added to precipitate the Silver. The operation of the
cupel may be considered as the sequel of that process, being intended
only to reduce into a scoria whatever is not Gold or Silver. Lead being
of all metals that which vitrifies the most easily, which most promotes
the vitrification of the rest, and the only one which, when vitrified,
penetrates the cupel, and carries along with it the other metals which
it hath vitrified, is consequently the fittest for that purpose. We
shall see in its place, that Bismuth hath the same properties with
Lead, and may be substituted for it in this operation.
Care must be taken to chuse a cupel of a proper capacity. Indeed it
should rather be too big than too little: because the operation is
no way prejudiced by an excess in its size; whereas, if it be too
small, it will be over-dosed with Lead, and at last the litharge, which
destroys every thing, will corrode its cavity, and eat holes through
the very body of the vessel. Add, that the ashes, of which the cupel
is made, being once glutted with litharge, absorb it afterwards but
slowly, and that the quantity of this vitrified litharge, becoming too
great to be contained in the substance of the vessel, exsudes through
it, and drops on the floor of the muffle, which it corrodes and renders
unequal; and moreover solders to it the vessels set thereon. It may be
laid down as a general rule for determining the size of a cupel, that
it weigh, at least, half as much as the metallic mass to be refined in
it.
It is also of the utmost consequence that the cupel be well dried
before the metal be put into it. In order to make sure of this point,
it must be kept red-hot for a certain time, as is above directed:
for though to the sight and to the touch it may appear very dry, it
nevertheless obstinately retains a small matter of moisture, sufficient
to occasion the loss of some of the metal; which, when it comes to
melt, will be thereby spirited up, in the form of little globules, to
the very roof of the muffle. The cupels that stand most in need of
an intense heat to dry them, are those chiefly in whose composition
wood-ashes are employed: for whatever care be taken to lixiviate those
ashes before they are used, they will still retain a little alkaline
salt; and that, we know, is very greedy of moisture, will not part
entirely with it, but by the means of a violent calcination, and
presently re-imbibes it when exposed to the air.
A little Phlogiston also may still be left in the ashes of which the
cupels are made; and that is another reason for calcining them before
they are used. By this means the remaining Phlogiston is dissipated,
which might otherwise combine with the litharge during the operation,
reduce it, and occasion such a ferment in the matter as to make some of
it run over; to these inconveniencies, which any remainder of moisture
or Phlogiston may produce, we must add the cracks and flaws, which are
very incident to cupels not perfectly freed from both those matters.
It is of no less importance to the success of this operation, that a
due degree of heat be kept up. In the process we have described the
marks which shew the heat to be neither too strong nor too weak; when
it exceeds in either of these respects it may be known by the following
signs.
If the fume emitted by the Lead rise like a spout to the roof of
the muffle; if the surface of the melted metal be extremely convex,
considering the quantity of the mass: if the cupel appear of such a
white heat, that the colour communicated thereto by the imbibed scoria
cannot be distinguished: all these shew that the heat is too great, and
that it ought to be diminished. If, on the contrary, the vapours only
hover, as it were, over the surface of the metal; if the melted mass
be very flat, considering its quantity; if its ebullition appear but
faint; if the _scoriæ_, that appear like little fiery drops of rain,
have but a languid motion; if the scoria gather in heaps, and do not
penetrate the cupel; if the metal be covered with it as with a glassy
coat; and, lastly, if the cupel look dull; these are proofs that the
heat is too weak, and ought to be increased.
The design of this operation being to convert the Lead into litharge,
and to give it sufficient time and opportunity to scorify and carry
off with it whatever is not Gold or Silver; the fire must be kept up
to such a degree that the Lead may easily be turned into litharge, and
yet that litharge not be absorbed too hastily by the cupel, but that
a small quantity thereof may all along remain, like a ring, round the
melted metal.
The fire is to be gradually increased as the operation draws nearer
to its end: for, as the proportion of the Lead to the Silver is
continually lessening, the metallic mass gradually becomes less
fusible; while the Silver defends the Lead mixed with it from the
action of the fire, and prevents its being easily converted into
litharge.
When the operation is finished, the cupel must still be left under
the muffle, till it has imbibed all the litharge, to the end that the
bead of Silver may be easily taken out: for, without this precaution,
it would stick so fast as not to be removed, but by breaking off part
of the cupel along with it. Care must also be taken to let this bead
of Silver cool gradually, and be perfectly fixed, before you draw it
from under the muffle; for if you expose it at once to the cold air,
before it be fixed, it will swell, shoot into sprigs, and even dart out
several little grains to a considerable distance, which will be lost.
If the bead appear to have a yellowish tinge, it is a sign that it
contains a great deal of Gold, which must be separated from it by the
methods to be hereafter shewn.
It is proper to observe, that there is scarce any Lead that does not
contain some Silver; too little perhaps to defray the charges necessary
to separate it, yet considerable enough to lead us into an error, by
mixing with the Silver obtained from an ore, and increasing its weight.
And therefore, when the operations above described are applied to the
assaying of an ore, in order to know how much Silver it yields, it is
previously necessary to examine the Lead to be used, and to ascertain
the quantity of Silver it contains, which must be deducted from the
total weight of the bead of Silver obtained by purifying it in this
manner.
Silver may be separated from its ore, and at the same time refined, by
the single operation of the cupel, without any previous scorification
with Lead. In order to do this, you must pound the ore; roast it, to
dissipate all its volatile parts; mix it with an equal quantity of
litharge, if it be refractory; divide it into five or six parcels,
wrapping each in a bit of paper; weigh out eight parts of granulated
Lead for one of ore, if it be fusible, and from twelve to sixteen,
if it be refractory; put one half of the Lead into a very large
cupel under the muffle; add thereto one of the little parcels of
ore, when the Lead begins to smoke and boil; immediately slacken the
fire a little; continue the same degree of heat till you perceive
that the litharge formed round the metal, and on its surface, begins
to look bright; then raise the fire; add a fresh parcel of ore;
continue proceeding in the same manner till you have put in all the
ore; then add the remaining half of the granulated Lead, and conduct
the succeeding part of the operation in the same manner as that of
cupelling.
In this operation it is necessary that the fire be not too strongly
urged, and that it be diminished every time you add a fresh
parcel of ore; that so the Lead and the litharge may have time
to dissolve, scorify, and carry off into the pores of the cupel,
all the adventitious matters with which your Silver may be mixed.
Notwithstanding this precaution, when the ore is refractory, there
often gathers in the cupel a great quantity of scoria, together also
with some of the ore that could not be dissolved and scorified. It is
with a view to remedy this inconvenience that the second moiety of the
Lead is added towards the end, which completes the dissolution and
scorification of the whole; so that by means thereof no scoria, or very
little, is left in the cupel at the end of the operation.
The operation of the cupel is chiefly used to purify Silver from the
alloy of Copper; because this metal, being more fixed and harder to
calcine than other metallic substances, is the only one that remains
united with Silver and Lead, after roasting and scorification with
Lead. It requires no less than sixteen parts of the Lead to destroy
it in the cupel, and separate it from Silver. It melts into one mass
with the Lead; and the glass produced by these two metals, deprived
of their phlogiston, inclines to a brown or a black colour; by which
appearance chiefly we know that our Silver was alloyed with Copper.
PROCESS III.
_To purify Silver by Nitre._
Granulate the Silver you intend to purify, or reduce it to thin plates;
put it into a good crucible; add thereto a fourth part in weight of
very dry pulverized Nitre, mixed with half the weight of the Nitre of
calcined Wine-lees, and about a sixth part of the same weight of common
glass in powder. Cover this crucible with another crucible inverted;
which must be of such a size that its mouth may enter a little way into
that of the lower one, and have its bottom pierced with a hole of about
two lines in diameter. Lute the two crucibles together with clay and
Windsor-loam. When the lute is dry, place the crucibles in a melting
furnace. Fill the furnace with charcoal, taking care however that the
fuel do not rise above the upper crucible.
Kindle the fire, and make your vessels of a middling-red heat. When
they are so, take up with the tongs a live-coal, and hold it over
the hole of the upper crucible. If you immediately perceive a vivid
splendour round the coal, and at the same time hear a gentle hissing
noise, it is a sign that the fire is of a proper strength; and it must
be kept up at the same degree till this phenomenon cease.
Then increase the fire to the degree requisite to keep pure Silver in
fusion; and immediately after take your vessels out of the furnace.
You will find the Silver at the bottom of the lower crucible, covered
with a mass of Alkaline scoria of a greenish colour. If the metal be
not rendered perfectly pure and ductile by this operation, it must be
repeated a second time.
_OBSERVATIONS._
The purification of Silver by Nitre, as well as the process for
refining it on the cupel, is founded on the property which this metal
possesses of resisting the force of the strongest fire, and the
power of the most active solvents, without losing its phlogiston.
The difference between these two operations consists wholly in the
substances made use of to procure the scorification of the imperfect
metals, or semi-metals, that may be combined with the Silver. In the
former this was obtained by Lead, and here it is effected by Nitre.
This Salt, as we have shewn, hath the property of calcining and quickly
destroying all metallic substances, by consuming their phlogiston,
except the perfect metals, Gold and Silver, which alone are able to
resist its force. This method may therefore be employed to purify Gold
as well as Silver, or indeed both the two mixed together.
In this operation the Nitre is gradually alkalizated, as its Acid is
consumed with the phlogiston of the metallic substances. The Alkaline
Salt and pounded glass are added, with a view to promote the fusion of
the metalline calces, as fast as they are formed, and to fix and retain
the Nitre, which, as we shall presently see, is apt to fly off in a
certain degree of heat.
The precaution of covering the crucible with another crucible inverted,
which hath only a small hole in its bottom, is designed to prevent any
of the Silver from being lost in the operation: for when the Nitre
comes to be acted on by a certain degree of heat, and especially when
it deflagrates with any inflammable matter, part of it flies off, and
so rapidly too as to be capable of carrying off with it a good deal of
the Silver. The little hole left in the covering crucible is necessary
for giving vent to the vapours, which rise during the deflagration
of the Nitre, as they would otherwise open themselves a passage by
bursting the vessels. After the operation this vent-hole is found beset
with many little particles of Silver, which would have been lost if the
crucible had not been covered.
If you should observe, during the detonation of the Nitre, that a great
many vapours issue through the vent-hole with a considerable hissing
noise, even without applying the coal, you must take it for a sign that
the fire is too brisk, and accordingly check it; else a great deal of
the Nitre will be dissipated, and with it much Silver.
You must observe to take the Silver out of the fire as soon as it is in
fusion: for if you neglect this, the Nitre being entirely dissipated
or alkalizated, the calces of the metals destroyed by it may possibly
recover a little phlogiston, communicated either by the vapours of
the charcoal, or by little bits of coal accidentally falling into the
crucible; by which means some portion of those metals being reduced
will mix again with the Silver, prevent its having the desired degree
of purity and ductility, and oblige you to begin the operation afresh.
PROCESS IV.
_To dissolve Silver in_ Aqua Fortis, _and thereby separate it from
every other metalline substance. The Purification of_ Aqua Fortis.
_Silver precipitated by Copper._
The Silver you intend to dissolve being beaten into thin plates, put it
into a glass cucurbit; pour on it twice its weight of good precipitated
_aqua fortis_; cover the cucurbit with a piece of paper, and set it on
a sand-bath moderately heated. The _aqua fortis_ will begin to dissolve
the Silver as soon as it comes to be a little warm. Red vapours will
rise; and from the upper surfaces of the Silver there will seem to
issue streams of little bubbles, ascending to the top of the liquor,
between which and the Silver they will form, as it were, a number of
fine chains: this is a sign that the dissolution proceeds duly, and
that the degree of heat is such as it ought to be. If the liquor appear
to boil and be agitated, a great many red vapours rising at the same
time, it is a sign that the heat is too great, and should be lessened
till it be reduced to the proper degree indicated above: having
obtained that, keep it equally up till no more bubbles or red vapours
appear.
If your Silver be alloyed with Gold, the Gold will be found, when the
dissolution is finished, at the bottom of the vessel in the form of a
powder. This solution must now be decanted while it is yet warm; on
the powder pour half as much fresh _aqua fortis_ as before, and make
it boil; again decant this second _aqua fortis_, and repeat the same
a third time; then with fair water wash the remaining powder well: it
will be of a brown colour inclining to red. In the observations we
shall show how the Silver is to be separated from the _aqua fortis_.
_OBSERVATIONS._
All the processes on Silver already delivered, whether for extracting
it from its ores, or for refining it, either by the Cupel or by Nitre,
are applicable to Gold also. And if Silver be alloyed with Gold before
it undergo those several operations, it will still remain alloyed
therewith after them, in the same manner, and in the same quantity;
because both metals bear them equally. All therefore that can be
expected from those several assays, is the separation of every thing
that is neither Silver nor Gold from these two metals. But in order to
separate these two from each other, recourse must be had either to the
process laid down under the head of Gold, or to that here described,
which is the most commodious, the most usual, and known by the names of
_Quartation_ and the _Parting Assay_.
_Aqua fortis_ is the true solvent of Silver, and is utterly incapable
of dissolving the least atom of Gold. If therefore a mass consisting of
Gold and Silver be exposed to the action of _aqua fortis_, that Acid
will dissolve the Silver contained in the compound, without touching
the Gold, and the two metals will be separated from each other. This
method of parting them is just the reverse of that described before
under the head of Gold, which is effected by the means of _aqua regis_.
To the success of this separation, by means of _aqua fortis_, several
conditions are essentially necessary. The first is, that the Gold and
Silver be in due proportion to each other; that is, there must be at
least twice as much Silver as Gold in the metalline mass, otherwise the
_aqua fortis_ will not be able to dissolve it, for the reason formerly
given. If therefore the mass contain too little Silver, it must either
be melted down again, and a proper quantity of Silver added; or else,
if the Gold be in a sufficient proportion to the Silver, they may be
parted by means of _aqua regis_.
Secondly, it is necessary that the _aqua fortis_ employed in this
operation be absolutely pure, and free from any taint of the Vitriolic
or Marine Acid: for, if it be adulterated with the Vitriolic Acid,
the Silver will precipitate as fast as it dissolves, and so the
precipitated Silver will again mix with the Gold. If the _aqua fortis_
contain any of the Marine Acid, the Silver will be precipitated in
that case also; and this inconvenience will be attended with another,
namely, that the menstruum, being partly an _aqua regis_, will dissolve
some of the Gold. You must therefore be very sure that your _aqua
fortis_ is pure, before you set about the operation. In order to
discover its quality, you must try it by dissolving, in a small portion
thereof, as much Silver as it will take up: if the _aqua fortis_ grow
opaque and milky as it dissolves the Silver, it is a sign it contains
some foreign Acid, from which it must be purified.
In order to effect this, let the portion of _aqua fortis_ used for the
above trial stand to settle: the white milky part will gradually fall
to the bottom of the vessel. When it is all fallen, gently decant the
clear liquor, and pour a few drops of this decanted solution of Silver
into the _aqua fortis_ which you want to purify. It will instantly
become milky. Let the white particles precipitate as before, and then
add a few more drops of your solution of Silver. If the _aqua fortis_
still become milky, let it precipitate again, and repeat this till you
find that a drop of your solution of Silver, let fall into this _aqua
fortis_, does not make it in the least turbid. Then filter it through
brown paper, and you will have an _aqua fortis_ perfectly fit for the
Parting Assay.
The white particles that appear and settle to the bottom, on dissolving
silver in an _aqua fortis_ adulterated with a mixture of some foreign
Acid, are no other than that very Silver, which is no sooner dissolved
by the Nitrous Acid than it deserts that solvent to unite with the
Vitriolic or Marine Acid, wherewith it has a greater affinity, and
falls to the bottom with them. And this happens as long as there
remains in the _aqua fortis_ a single atom of either of those two Acids.
When therefore your _aqua fortis_ hath dissolved as much Silver as it
is capable of taking up, and when all the white particles formed during
the dissolution are settled to the bottom, you may be assured that
the portion which remains clear and limpid is a solution of Silver in
an exceeding pure _aqua fortis_. But if the solution of Silver thus
depurated be mixed with an _aqua fortis_ adulterated with the Vitriolic
or Marine Acid, a like precipitation will immediately ensue, for the
reasons above given, till the very last particle of the heterogeneous
Acid contained in the _aqua fortis_ be precipitated.
_Aqua fortis_ purified by this method contains no extraneous substance
whatever, except a small portion of Silver; so that it is very fit for
the parting process. But if it be intended for other chymical purposes,
it must be rectified in a glass retort with a moderate heat, in order
to separate it from the small portion of Silver it contains, which will
remain at the bottom of the retort.
The third condition necessary to the success of this operation is, that
your _aqua fortis_ be neither too aqueous, nor too highly concentrated.
If too weak, it will not act upon the Silver: and the consequence will
be the same if it be too strong. Both these inconveniencies are easily
remedied: for in the former case part of the superfluous phlegm may be
drawn off by distillation; or a sufficient quantity of much stronger
_aqua fortis_ may be mixed with that which is too weak: and, in the
latter case, very pure rain water, or a weaker _aqua fortis_, may be
mixed with that which is too strong.
You may satisfy yourself whether or no your _aqua fortis_ hath the
requisite degree of strength, by dissolving therein a thin plate
consisting of one part Gold and two or three parts Silver; which plate
must be rolled up in form of a paper coffin. If, when all the Silver
contained in the plate is dissolved, the Gold remains in the form
of the coffin, it is a sign that your solvent has a due degree of
strength. If, on the contrary, the Gold be reduced to a powder, it is a
proof that your _aqua fortis_ is too strong, and ought to be weakened.
The Gold remaining after the dissolution of the Silver must be melted
in a crucible with Nitre and Borax, as hath already been said under the
process for parting Gold and Silver by means of _aqua regis_. As to the
Silver which remains dissolved in the _aqua fortis_, there are several
ways to recover it.
The most usual is to precipitate it by the interposition of Copper,
which hath a greater affinity than Silver with the Nitrous Acid[7].
For this purpose the solution is weakened by adding twice or thrice as
much very pure rain water. The cucurbit containing the solution is set
on a sand-bath gently heated, and very clean plates of copper put into
it. The surfaces of these plates are soon covered with little white
scales, which gradually fall to the bottom of the vessel, as they come
to be collected in quantities. It is even proper to strike the cucurbit
gently now and then, in order to shake the scales of Silver from the
copper plates, and so make room for a new crop.
[7] See the Table of Affinities, Column IV.
The _aqua fortis_ parts with the Silver by degrees only, as it
dissolves the Copper; and therefore the liquor gradually acquires a
blueish green colour as the precipitation advances. This precipitation
of the Silver is to be continued as long as any remains dissolved in
the _aqua fortis_: you may be sure that your liquor contains no more
Silver, if the surface of a fresh plate of Copper laid therein remain
clean and free from ash-coloured or greyish particles: or if one drop
of a solution of Sea-salt let fall into it produce no white or milky
cloud.
The precipitation being finished, the liquor is to be gently poured off
from the precipitated Silver, which must be rinsed in several waters,
and even made to boil therewith, in order to free it wholly from the
dissolved Copper. The Silver thus well washed must be thoroughly dried,
mixed with a fourth part of its weight of a flux compounded of equal
parts of Nitre and calcined Borax, and then melted in a crucible.
On this occasion care must be taken to raise the fire gently and
gradually, till the Silver be brought to fusion.
With what accuracy soever the precipitated Silver be washed, in order
to free it from the solution of Copper, yet the Silver will always be
found alloyed with a small portion of the Copper: but then this Copper
is easily destroyed by the Nitre, with which the Silver is afterwards
melted; so that the latter metal remains perfectly pure after the
operation.
Though the Silver be not previously cupelled, but be alloyed with
other metallic substances at the time it is thus dissolved, yet
the dissolving, precipitating, and fusing it with Nitre, would be
sufficient to separate it accurately from them all, and refine it to a
degree of purity equal to that obtained by the cupel.