PLA 
•which are of consideiahle importance in 
the working of this metal. Platina forms an 
alloy with arsenic, which is brittle and very 
fusible. It is in this state of alloy that pla- 
tina is susceptible of being formed into dif- 
ferent utensils and instruments for which it 
is peculiarly fitted. It is first fused with this 
metal, and then cast into moulds, at first in 
the form of square plates. It is then ex- 
posed to a red heat, and hammered into 
bars. By the heating and hammering, the 
arsenic is driven off, and the metal is puri- 
fied and becomes infusible, but retains its 
ductility, so that it may be wrought like 
iron. It has been found extremely dif- 
ficult to combine platina and mercury. 
Guyton had observed that the adhesive 
force of platina and mercury is greater than 
that of metals which do not combine with 
it ; and that it is not inferior even to those 
which readily form alloys ; from which he 
conjectured that the alloy of platina and 
mercury mifflit be effected by the following 
process. He placed a very thin plate of 
pure platina at the bottom of a matrass con- 
taining a quantity of mercury. The matrass 
was put upon a sand bath, and heat ap- 
plied, till the mercury boiled and the 
matrass became red-hot. When the platina 
was taken out, it was found to have acquired 
additional weight, and to have become 
very brittle. But this combination is dif- 
ferent from the other combinations of mer- 
cury with the metals, for the platina did not 
lose its solid form. M. Chenevix, in the 
course of experiments and researches re- 
specting a supposed new metal called palla- 
dium, succeeded in forming an amalgam 
with platina and mercury. He heated pu- 
rified platina in the form of fine powder, 
with ten times its weight of mercury, and 
rubbed them together for a long time. The 
result was an amalgam of platina, which 
being exposed to a violent heat, lost all the 
mercury it contained, and the original 
weight of the platina remained. Platina 
combines with copper by means of fusion, 
and gives it hardness. When the proportion 
of copper is three or four times greater 
than that of platina, the alloy is ductile, sus- 
ceptible of a fine polish, and is not altered 
by exposure to the air. This alloy has 
been employed in the fabrication of mirrors 
for telescopes. Gold combines readily with 
platina, but it requires a very powerful 
heat for the fusion of these two metals. 
Platina diminishes the colour of gold, unless 
it be in very small quantity. When the 
PLA 
proportion of platina is above jj, the colour 
of the gold begins to be altered. There is 
no perceptible change in the specific gra- 
vity or the ductility of gold from this alloy. 
Platina, on account of its peculiar pro- 
perties, its infusibility, density, and indes- 
tructibility, could it be obtained in suffix 
cient quantity, and at a moderate price, 
would undoubtedly prove one of the most 
useful and most important of the metals 
yet known. The importance and utility of 
platina, on account of its scarcity, have 
been hitherto limited to chemical pur- 
poses ; and for different chemical instru- 
ments and utensils, it has been found pecu- 
liarly appropriate, as there are few chemi- 
cal agents whose effects it cannot resist. 
There is indeed little doubt but it might be 
employed with equal advantage in the con- 
struction of instruments and utensils, in 
various arts and manufactures. 
PLATING, is the art of covering baser 
metals with a thin plate of silver either for 
use or for ornament. It is said to have 
been invented by a spur-maker, not for 
show but for real utility. Till then the 
more elegant spurs in common use were 
made of solid silver ; and from the flexibi- 
lity of that metal, they were liable to be 
bent into inconvenient forms by the 
slightest accident. To remedy this, defect, 
a workman at Birmingham contrived to 
make the branches of a pair of spurs hollow, 
and to fill that hollow with a slender rod of 
steel or iron. Finding this a great improve- 
ment, and being desirous to add cheapness 
to utility, he continued to make the hollow 
larger, and of course the iron thicker and 
thicker, till at last he discovered the means 
of coating an iron spur with silver in such a 
manner as to make it equally elegant with 
those which were made wholly of that me- 
tal, The invention was quickly applied to 
other purposes ; and to numberless utensils 
which were formerly made of brass or iron 
are now given the strength of these metals, 
and the elegance of silver, for a small addi- 
tional expense. The silver plate was for- 
merly made to adhere to the baser metal 
by means of solder ; which is of two kinds, 
the soft and the hard, or the tin and silver 
solders. The former of these consists of tin 
alone, the latter generally of three parts of 
silver and one of brass. AVlien a buckle, for 
instance, is to be plated by means of the soft 
solder, the ring, before it is bent, is first 
tinned, and then the silver plate is gently 
haqunered upon it, the hammer employed 
