THE account of this extraordinary mineral, formerly read to this
illustrious Society, and honoured with their approbation, being
since published in the _Philosophical Transactions_, renders any
recapitulation of the discoveries hitherto made unnecessary.
The near and remarkable relation betwixt platina and gold, not only
in point of gravity, but in many less obvious properties, hitherto
supposed to belong to gold alone; and their as manifest disagreement in
others, particularly colour, ductility, and fusibility; induced me to
examine, what effects they might have in combination with one another
in different proportions; and whether there is reason to credit the
report of great frauds having been committed by mixing them together;
how far such abuses are practicable; and, what is of more importance,
the means by which they are discoverable.
_Experiments of the Mixture of Platina and Gold._
+EXPERIMENT+ I.
1. Twelve carats[56] of fine gold, and the same quantity of the purer
grains of platina, were urged in a blast-furnace, for near an hour,
with a fire so strong, that a slip of Windsor brick, with which the
crucible was covered, tho’ defended by a thin coating of pure white
clay, had begun to melt. Upon breaking the vessel, the metal was found
in one smooth lump or bead; which, after being nealed by the flame of a
lamp, and boiled in alum-water, appeared, both in the mass, and upon
the touchstone, of a pale bell-metal colour, without any resemblance
to gold. It bore several strokes, and stretched considerably under
the hammer, before it begun to crack about the edges. On viewing
the fracture with a magnifying glass, the gold and platina appeared
unequally mixed; and several small particles of the latter were seen
distinct: nor was the mixture intirely uniform after it had again and
again been returned to the fire, and suffered many hours of strong
fusion.
2. Eighteen carats of gold and six of platina (= 3:1) were melted
together as the foregoing, in an intense fire continued about an hour.
The bead, nealed and boiled, was less pale-coloured than the former,
but had nothing of the colour of gold. It forged tolerably well, like
coarse gold. To the naked eye it appeared uniform; but a good magnifier
discovered in this, as well as in the other, some inequality of
mixture, not withstanding the fusion was two or three times repeated,
with the strongest degrees of heat we were capable of exciting by large
bellows.
3. Twenty carats of gold and four of platina (= 5:1) were kept in
strong fusion for above an hour and a half. These united into an equal
mass, in which no granule of platina, or dissimilarity of parts, could
be distinguished. The colour was still so dull and pale, that the
compound could scarcely be judged by the eye to contain any gold. It
hammered well into a pretty thin plate; but we could not draw it into
wire of any considerable fineness.
4. Twenty-two carats of gold were melted in the same manner with two
of platina (= 11:1) the same that standard gold contains of alloy.
The mixture was uniform, of a tolerable golden colour, but easily
distinguishable from that of standard gold by a dingy bell-metal cast.
It worked well, was forged into a thin plate without cracking, and
drawn into moderately fine wire.
5. Twenty-two carats and a half of gold, and one and a half of platina
(= 15:1), melted into an uniform mass, which, after the usual nealing
and boiling, proved somewhat tougher than the preceding, and of a
better colour.
6. Twenty-three carats of gold were melted with one of platina; which
is nearly half the proportion, that standard gold contains of alloy.
The compound worked extremely well, but was distinguishable from gold
by a manifest dinginess, which it retained after repeated forgings,
fusions, nealings, and boilings.
7. Twenty-three carats and one-fourth of gold, and three-fourths of a
carat of platina (= 31:1), formed an equal mixture, very malleable,
ductile like the three foregoing whilst hot as well as cold, but not
intirely free from their peculiar dingy colour.
8. A mixture of twenty-three carats and a half of gold, with half
a carat of platina (= 47:1), was very soft and flexible, of a good
colour, without any thing of the disagreeable cast, by which all the
foregoing compositions were readily distinguishable, in the mass as
well as on the touchstone, from fine or standard gold.
9. A mixture of twenty-three carats and three-fourths of gold, with
one-fourth of a carat of platina (= 95:1), could not be distinguished
by the eye or hammer from the fine gold itself.
In all these processes, even where the proportion of platina was small,
the fusion was performed by a vehement fire, that the mineral might be
the more intimately dissolved, and equally diffused thro’ the gold.
The necessity of this precaution appeared from an experiment formerly
related; in which one of platina having been melted with four of gold,
the button appeared not much paler than standard gold with silver
alloy. On a second fusion it lost its yellow colour, which had at
first been only external, from an imperfect mixture, great part of the
platina being concealed in the internal part of the mass, and covered
as it were by a golden coat.
The crucibles were rubbed on the inside with chalk, to prevent any
particles of the metal from lodging in their cavities. A little borax
was employed in each as a flux; with the addition of nitre, by which
the colour of gold is somewhat heightened. On remelting some of the
mixtures with sundry other additions, powdered charcoal seemed to
improve the colour most.
+EXPERIMENT+ II.
The preceding compositions, after being gently hammered and boiled,
were weighed hydrostatically with great care, by a very tender balance,
in distilled water, wherein the gravity of standard gold turned out
17.788.
All the mixtures proved heavier than standard gold. Their gravities
were nearer to the medium of the gravities of the ingredients, than
those of the compositions of platina with any of the other metals
formerly given an account of; none falling considerably short of the
mean gravity, and some rather exceeding it.
+------------------------+
| Gravity. |
| By | By |
|Experiment.|Calculation.|Difference.
Platina | 17.000 | |
Platina 1 Gold 1 | 18.140 | 18.142 | 0. 02}
Platina 1 Gold 3 | 18.613 | 18.714 | 0.101}
Platina 1 Gold 5 | 18.812 | 18.904 | 0.092}
Platina 1 Gold 11 | 18.835 | 19.094 | 0.259} Diminution.
Platina 1 Gold 15 | 18.918 | 19.142 | 0.224}
Platina 1 Gold 23 | 19.089 | 19.189 | 0.100}
Platina 1 Gold 31 | 19.128 | 19.213 | 0.085}
Platina 1 Gold 47 | 19.262 | 19.237 | 0.025}
Platina 1 Gold 95 | 19.273 | 19.261 | 0.012} Increase.
Gold | 19.285 | |
+-----------+------------+
+EXPERIMENT+ III.
As a mixture of platina with an equal quantity of gold has been
reported to be specifically heavier than gold itself, but turned out
otherwise in the above experiments; some further trials were made on
that head.
1. Instead of the crude mineral, whose gravity is but 17, we took
platina, that had been cupelled with lead, one of the neatest of
the buttons formerly mentioned, which, tho’ retaining a portion of
the lead, was nearly as ponderous as fine gold, _viz._ 19.240. This
was melted with equal its weight of the gold, in a strong fire, and
continued in fusion for about an hour: the mass proved spongy, and
very light. We remelted it several times with vehement degrees of
fire, suffering it to cool leisurely in the crucible, and, in order
to separate as much as possible of the lead, to which its sponginess
seemed owing, boiled it in aqua-fortis, and repeatedly injected
corrosive sublimate upon it during fusion: the mass, nevertheless,
still turned out cavernulous and brittle, and specifically lighter than
either the gold or platina by themselves.
2. I likewise endeavoured to combine platina with small proportions of
gold. By vehemence of fire, it was made to unite, tho’ not perfectly,
with half its weight and less: but the mixtures were extremely spongy
and brittle; in specific gravity one scarce 16, another less than 15.
3. As a cast metalline body from the Spanish West Indies, of which some
account will be given hereafter, appears to have been confounded with
the mineral platina, this also was melted with an equal quantity of
gold. They united with great ease, by a moderate fire, into an uniform
compound, tolerably compact, but whose specific gravity was only 16½;
which is nearly the mean gravity of the two ingredients.
+EXPERIMENT+ IV.
As a small portion of copper somewhat heightens the colour of pale
gold, platina was melted with eight times its weight of standard gold
made with copper alloy. The fusion was performed, as in the preceding
experiments, in a close crucible, with a strong fire, but without any
flux, and continued for about an hour. The metal appeared covered with
a black scurf, and had lost about ⅟200. It was much duller coloured,
harder to the hammer, and cracked sooner about the edges, than mixtures
of fine gold with a larger quantity of platina. By repeated fusion, and
frequent nealing, it became a little softer and tougher, so as to be
drawn into pretty fine wire; but the colour was still exceeding dull,
more resembling that of bad copper than of gold.
The specific gravity of this compound was 17.915; a little less than
the medium of the three ingredients unmixed, and a little greater than
the mean gravity resulting from the platina by itself, and the copper
and gold mixed; for copper, in the standard proportion, appears to
diminish the gravity of gold more than it ought to do according to
calculation.
From the foregoing experiments it appears, that platina is miscible
with gold, in certain proportions, without injuring either its colour
or ductility, or occasioning any considerable alteration in the
gravity: experiments related in former papers have shewn, that it
stands aqua-fortis, and the other trials by which the purity of gold is
estimated. It is to be hoped, that the abuses manifestly practicable
by this mineral have hitherto been but rarely made use of. To guard
against them is the object of this paper; to detect them, of the next.
XX. _Experimental Examination of_ Platina. _By_ William Lewis, _M.B.
F.R.S._
+PAPER+ VI.
_Experiments of distinguishing and purifying Gold mixed with Platina._
[Read Mar. 31, 1757.]