1. Inflammable spirits, which revive gold from its solutions in form
of yellow films, have no such effect on solutions of platina. This
experiment affords not only a criterion for distinguishing with
certainty whether gold has been debased by platina, but likewise an
infallible means of recovering it perfectly pure from any admixture of
that mineral. If the compound be dissolved in aqua-regis, the solution
mingled with twice its quantity or more of the spirit, and the mixture
suffered to stand for some days in a glass slightly covered; the whole
of the gold arises to the surface, leaving the whole of the platina
dissolved. The golden pellicles may be collected, by pouring the matter
into a filter just large enough to contain it. The dissolved platina
passes thro’, leaving the gold upon the paper, which is to be washed
with fresh parcels of water till the liquor runs colourless. The paper
is then to be carefully squeezed together, and burnt in a crucible
previously lined with vitrefied borax: when fully sunk down, a little
fresh borax is to be injected, and the fire raised to melt the gold.
The use of lining the crucible with borax is to prevent any moleculæ of
the gold from lodging in its cavities.--This process is attended with
one inconvenience, the slowness of the separation of the gold from the
solution: this may be in some measure expedited by employing a spirit,
which has been distilled from vegetables, that give over an essential
oil.
2. As essential oils take up gold from aqua-regis, and keep it
dissolved for a time upon the surface of the acid; a pure colourless
oil, that of rosemary, was poured into about half its quantity of a
solution of platina, the mixture well shaken, and suffered to rest:
the oil quickly arose, without taking up any thing from the platina,
or receiving any colour: the acid liquor underneath remained coloured
as at first. Compositions of platina and gold being dissolved in
aqua-regis, and treated in the same manner, the whole of the gold was
imbibed by the oil, and the whole of the platina remained dissolved
in the acid. The oil, loaded with the gold, appeared of a fine yellow
colour, and, on standing for a few hours, threw off great part of its
contents, in bright yellow films, to the sides of the glass. Sundry
other distilled oils were made trial of, with the same event. The gold
is easily recovered, by setting the oil on fire; and, when thoroughly
burnt out, melting the residuum with borax, as in the preceding
experiment. After the separation of the oil employed at first, it may
be proper, for the greater security, to add a little more; which, if
any part of the gold should happen to have been left in the liquor,
will effectually take it up.
3. The experiment was repeated likewise with the subtile fluid,
prepared from vinous spirits with the vitriolic acid, called by the
chemists æther. The separation succeeded in the same manner as before;
the æther receiving nothing from pure platina, but instantly taking up
the gold from a mixture of the two. It is observable, that the gold
imbibed by this fluid is kept permanently dissolved by it; without
separating or reviving, as it does from the common essential oils and
vinous spirits.
4. The liquors remaining in these experiments, after the extraction of
the gold, appear on all trials the same with the common solutions of
platina; and readily betray their being impregnated with that mineral
by their colour, by the precipitation with tin, by their yielding
a sparkling red precipitate with volatile spirits, _&c._ A far more
minute proportion of platina, mixed with gold, is more distinguishable
by these processes, than by those with alkaline salts above-mentioned;
these exhibiting the whole of the platina dissolved by itself, those
only a part of it.