nut
galls common ink is produced. With prussiate of potash Prussian blue is
formed. Any of these tests will decide between iron and gold.
Should
a lump of quartz be suspected to contain gold, the fact may easily be
established as follows:—Pound the quartz finely—the finer the better.
Boil this for a considerable time in an equal mixture of nitric and
muriatic acid, filter the solution through linen or cotton. It will
destroy these, but that is no matter; the experimenter must also be
careful not to get any of the acid on his clothes, or it will destroy
them. If he burn his fingers with the acid, he will not do so a second
time. Now add carbonate of soda to the solution when cool, and this
will precipitate all baser metals. Filter again, and add a solution of
oxalic acid till it ceases to effervesce. The gold will now be thrown
down in the form of a black powder, which may be converted into the
usual form by melting.
We
will now notice a new process for separating the precious metal from
black sand and quartz, which has been patented in America. When quartz
is stamped, it is found experience, that from the softness of gold, a
great portion of the laminated filaments are rubbed off, sometimes
amounting to from one-fourth to two-fifths of the metal; and this
filament can never be.detached from th« iron and sand by any plan of
amalgamation. The patentee, who has taken his idea from Dr. Percy—who
propounded much the same thing in a paper read before the London
Chemical Society—uses neither more nor less than a fresh and liquid
bleaching chloride of lime. The mode of its action will be readily
understood from what we have before stated, and the ingenious miner can
easily try the effect of this hypochlorous solvent for himself.
As
gold readily melts, it may be thought that, by heating a piece of
quartz beyond the melting point of gold, this metal will flow from the
quartz. This is not so; the gold will be melted, it is true, but it
will be in the matrix as before. The quartz is infusible, and in order
to get at the gold, it is necessary to render it fusible. If to
finely-powdered quartz we add several substances, this effect will be
produced. Mix carbonate of soda with the finely-powdered quartz, and
when it has arrived at a certain heat it is quartz no longer but melted
glass, through which the gold, if any, will sink to the bottom of the
crucible.
But
suppose the gold and quartz to be melted, the same difficulty presents
itself as to how to get gold from the quartz-glass which has been
formed. We must have recourse to something which will take the whole of
the gold from the glass, and which will readily give it back again in
its pure state. This condition is answered by mixing with it a quantity
of lead. This metal takes up all the gold, and may be readily separated
from the quartz-glass. We should here remark that lime and oxide of
iron, as well as some other substances, will convert the quartz into
glass as v. ell as carbonate of soda. Into the nature of these it is
not necessary to enter, as we are only showing the principle of gold
smelting, leaving the miner to apply it in practice.
Having
now got our mixture of gold and lead, the quartz-glass may be taken
from the crucible and thrown away. The remaining step is to separate
the gold from the lead. This is done by a process termed "
cupeltation." The miner would scarcely have time or experience to
effect this process, but it will not be uninteresting to him to know
its principle. When lead is heated to a high temperature, it rapidly
absorbs the oxygen of the atmosphere, and, if heated to redness, the
oxide thus formed, melts. But gold never oxidises, and cannot be
volatilised at any heat procurable in an ordinary furnace. This
distinction in the properties of the two metals causes their easy
separation. Many substances readily absorb melted oxide of lead,
amongst which is bone-ash, which substance, compressed into as solid a
state as possible, will take up all the lead and leave the gold behind;
the lead also, if in sufficient quantity, taking with it all baser
metals, leaving the gold pure, or alloyed with silver only, and we have
previously given the method of separating this. We may, however, mention, that when silver is to be "parted," as it is termed)
