_Of_ SILVER.
Next to Gold, Silver is the most perfect metal. Like Gold it resists
the utmost violence of fire, even that in the focus of a burning-glass.
However, it holds only the second place among metals; because it is
lighter than Gold by almost one half; is also somewhat less ductile;
and, lastly, because it is acted upon by a greater number of solvents.
Yet Silver hath one advantage over gold, namely that of being a little
harder; which makes it also more sonorous.
This metal, like Gold, begins to flow when it is so thoroughly
penetrated by the fire as to appear ignited like a live coal.
While this metal is in fusion, the immediate contact of the vapour
of burning coals deprives it almost entirely of its malleability, in
the same manner as we observed happens to Gold: but both these metals
easily recover that property by being melted with nitre.
The nitrous acid is the true solvent of Silver, and being somewhat
dephlegmated will very readily and easily take up a quantity of Silver
equal in weight to itself.
Silver thus combined with the nitrous acid forms a metallic salt which
shoots into crystals, called by the name of _Lunar Crystals_, or
_Crystals of Silver_.
These crystals are most violently caustic: applied to the skin they
quickly affect it much as a live coal would; they produce a blackish
eschar, corroding and entirely destroying the parts they touch.
Surgeons use them to eat away the proud fungous flesh of ulcers. As
Silver united with the nitrous acid hath the property of blackening all
animal substances, a solution of this metallic salt is employed to dye
hair, or other animal matters, of a beautiful and durable black.
These crystals flow with a very moderate heat, and even before they
grow red. Being thus melted they form a blackish mass; and in this
form they are used by Surgeons, under the title of _Lapis Infernalis_,
_Infernal Stone_, or _Lunar Caustic._
Silver is also dissolved by the vitriolic acid: but then the acid must
be concentrated, and in quantity double the weight of the Silver; nor
will the solution succeed without a considerable degree of heat.
Spirit of salt and _aqua regis_, as well as the other acids, are
incapable of dissolving this metal; at least in the ordinary way.
Though Silver be not soluble in the acid of sea-salt, nor easily in the
acid of vitriol, as hath just been observed, it doth not follow that
it hath but a weak affinity with the latter, and none at all with the
former: on the contrary, it appears from experiment that it hath with
these two acids a much greater affinity than with the acid of nitre:
which is singular enough, considering the facility with which this
last acid dissolves it.
The experiment which proves the fact, is this. To a solution of Silver
in the nitrous acid, add the acid either of vitriol or of sea-salt, and
the Silver will instantly quit its nitrous solvent to join with the
superadded acid.
Silver thus united with the vitriolic or the marine acid is less
soluble in water than when combined with the nitrous acid; and for
this reason it is, that when either of these two acids is added to
a solution of Silver, the liquor immediately becomes white, and a
precipitate is formed, which is no other than the Silver united with
the precipitating acid. If the precipitation be effected by the
vitriolic acid, the precipitate will disappear upon adding a sufficient
quantity of water, because there will then be water enough to dissolve
it. But the case is not the same when the precipitation is made by the
marine acid; for Silver combined therewith is scarce soluble in water.
This Precipitate of Silver, procured by means of the marine acid,
is very easily fused, and when fused changes to a substance in some
measure transparent and flexible, which hath occasioned it to be called
by the name of _Luna Cornea_. If it be proposed to decompound this
_luna cornea_, that is, to separate the marine acid from the Silver
with which it is united, the _luna cornea_ must be melted along with
fatty and absorbent matters, with which the acid will unite, and leave
the metal exceeding pure.
It must be observed, that if, instead of the marine acid, sea-salt
in substance be added to a solution of Silver in the nitrous acid, a
Precipitate is also produced, which by fusion appears to be a true
_luna cornea_. The reason is, that the sea-salt is decomposed by the
nitrous acid, which seizes its basis, as having a greater affinity
therewith than its own acid hath; and this acid being consequently
disengaged and set at liberty unites with the Silver, which, as has
been shewn, has a greater affinity with it than with the nitrous acid.
This is an instance of decomposition effected by means of one of those
double affinities mentioned by us in our seventh proposition concerning
Affinities.
From what hath been already said it is clear, that all these
combinations of Silver with acids may be decompounded by absorbent
earths and by fixed alkalis; it being a general law with regard to all
metallic substances. We shall not therefore repeat this observation
when we come to treat of the other metals; unless some particular
occasion require it.
With regard to Silver I must take notice that, when separated by these
means from the acids in which it was dissolved, it requires nothing
but simple fusion to restore it to its usual form; because it does
not, any more than Gold, lose its Phlogiston by those solutions and
precipitations.
Silver unites with sulphur in fusion. If this metal be only made
red-hot in a crucible, and sulphur be then added, it immediately flows;
the sulphur acting as a flux to it. Silver thus united with sulphur
forms a mass that may be cut, is half malleable, and hath nearly the
colour and consistence of Lead. If this sulphurated Silver be kept a
long time in fusion, and in a great degree of heat, the sulphur flies
off and leaves the Silver pure. But if the sulphur be evaporated by a
violent heat, it carries off with it part of the Silver.
Silver unites and mixes perfectly with Gold in fusion. The two metals
thus mixed form a compound with properties partaking of both.
Metallurgists have hitherto sought in vain for a perfectly good and
easy method of separating these two metals by the _dry way_ only: (this
term is used to signify all operations performed by fusion): but they
are conveniently enough parted by the _moist way_, that is, by acid
solvents. This method is founded on the above-mentioned properties of
Gold and Silver with respect to acids. It hath been shewn that _aqua
regis_ only will dissolve Gold; that Silver, on the contrary, is not
soluble by _aqua regis_, and that its proper solvent is the acid of
nitre; consequently, when Gold and Silver are mixed together, if the
compound mass be put into _aqua fortis_, this acid will take up all the
Silver, without dissolving a particle of the Gold, which will therefore
remain pure; and by this means the desired separation is effected. This
method, which is commonly made use of by Goldsmiths, and in Mints, is
called the _Parting Assay_.
It is plain, that if _aqua regis_ were employed instead of _aqua
fortis_, the separation would be equally effected; and that the only
difference between this process and the former would consist in this,
that now the Gold would be dissolved, and the Silver remain pure. But
the operation by _aqua fortis_ is preferable; because _aqua regis_
does take up a little Silver, whereas _aqua fortis_ hath not the least
effect on Gold.
It must be observed, that, when Gold and Silver are mixed together in
equal parts, they cannot be parted by the means of _aqua fortis_. To
enable the _aqua fortis_ to act duly on the Silver, this metal must
be, at least, in a triple proportion to the Gold. If it be in a less
proportion, you must either employ _aqua regis_ to make the separation,
or, if you prefer the use of _aqua fortis_, melt the metalline mass,
and add as much Silver as is necessary to make up the proportion
above-mentioned: and hence this Process is called _Quartation_.
This effect, which is pretty singular, probably arises from hence, that
when the Gold exceeds or even equals the Silver in quantity, the parts
of both being intimately united, the former are capable of coating over
the latter, and covering them so as to defend them from the action of
the _aqua fortis_; which is not the case when there is thrice as much
Silver as Gold.
There is one thing more to be taken notice of with regard to this
process; which is, that perfectly pure _aqua fortis_ is rarely to be
met with, for two reasons; first, it is difficult in making it wholly
to prevent the rising of the medium employed to disengage the nitrous
acid; that is, a little of the vitriolic acid will mix with the vapours
of the _aqua fortis_: secondly, unless the salt-petre be very well
purified it will always hold some small portion of sea-salt, the acid
of which, we know, is very readily set loose by the vitriolic acid,
and consequently rises together with the vapours of the _aqua fortis_.
It is easy to see that _aqua fortis_ mixed either with the one or the
other is not proper for the Parting Process; because, as has just been
said, the vitriolic and the marine acid equally precipitate Silver
dissolved in the nitrous acid; by which means, when they are united
with that acid, they weaken its action upon the Silver, and hinder the
dissolution. Add, that _aqua fortis_ adulterated with a mixture of
spirit of salt becomes an _aqua regis_, and consequently is rendered
capable of dissolving Gold, in proportion as its action upon Silver is
diminished.
In order to remedy this inconvenience, and free _aqua fortis_ from
the vitriolic or marine acid with which it is tainted, Silver must
be dissolved therein: by degrees as the metal dissolves, those
heterogeneous acids lay hold of it, and precipitate with it in the form
of a white powder, as we observed before. This precipitate being wholly
fallen, the liquor grows clear; after which, if it be found capable of
dissolving more silver, without turning milky, it may be depended on as
a perfectly pure _aqua fortis_. Then filtre it, dissolve more Silver
in it, as long as it will take up any, and you will have a solution
of Silver in a very pure _aqua fortis_. By means of this solution may
other _aqua fortis_ be purified: for pour a few drops thereof into
a very impure _aqua fortis_, and immediately the vitriolic or marine
acid, with which that _aqua fortis_ is contaminated, will join the
Silver and fall therewith to the bottom. When the solution of Silver,
prepared as above, does not in the least affect the transparency of
the _aqua fortis_, it is then very pure, and fit for the purposes of
Quartation.
This operation of purifying _aqua fortis_ by a solution of Silver is
called the _Precipitation_ of _Aqua Fortis_, and _aqua fortis_ thus
purified is called _Precipitated Aqua Fortis_.
When Silver is dissolved in _aqua fortis_ it may be separated
therefrom, as hath been shewn, by absorbent earths and fixed alkalis.
We shall see by and by that there are other means of effecting this:
but whatever way it be separated from its solvent it recovers its
metalline form, as Gold does, by being simply fused without any
additament.