IN an experiment related in the fourth paper, an amalgam of one part
of platina and two of gold with a suitable quantity of mercury, having
been triturated with water for a considerable time, and occasionally
washed over, the platina was gradually thrown out, and the gold
retained by the quicksilver.
Repetitions of this experiment have shewn, that tho’ the separation
succeeds in some cases, it does not perfectly in all: that if there
is any particle of the platina imperfectly dissolved in the gold
(which will generally be the case, unless the quantity of gold is
three or four times greater than that of the platina), this part will
be retained, after long trituration, undissolved by the mercury,
uncomminuted by the pestle, and too ponderous to be washed off in its
gross form. A variety of mixtures of platina and gold were treated in
the manner above described; and the gold, recovered from the amalgams,
submitted to further examinations. Where the proportion of platina was
large, the microscope almost always discovered still some granules of
it on the fracture of the ingot: where the proportion was small, the
recovered gold was frequently, but not constantly, found to be pure.
From these experiments it appears, that mercury has a greater affinity
with gold than platina, and that platina is capable of being totally
separated by elutriation; but that the process is too vague and
undetermined to be applicable in the way of assay, as we have no mark
of the precise time for discontinuing it, and as we can never be
certain, without making another assay, whether the whole of the platina
is separated or not. As a preparatory examination, where the quantities
of platina and gold to be separated are large, it is nevertheless of
good use, as greatest part of the platina may by this means be washed
over with little trouble, and the gold brought into a less compass, so
as to be commodiously submitted to a perfect purification by the means
hereafter pointed out. This process has a similar effect on platina and
gold to that of stamping and washing on metallic ores; which could not
be reduced into pure metal in the furnace to advantage, without the
previous separation of great part of the earthy and stony matter by
water.