PLA 
P L A 
PLA 
451 
its stem all of an equal girt for a considerable 
length : we have an account of some trees 
being eight or nine yards in circumference, 
and which, when felled, afforded twenty loads 
of wood. 
The varieties of these two species are the 
Spanish or middle plane-tree, having remark- 
ably large leaves of three or five, narrower 
segments ; and the maple-leaved plane-tree, 
having smaller leaves, somewhat lobated into 
1 five segments, resembling the maple-tree 
, leaf. 
All these elegant trees are of hardy tempe- 
j nature, so as to prosper here in any common 
i soil and exposure in our open plantations, 
&c. and are some of the most desirable trees 
j of the deciduous tribe. Their propagation is 
; by seed, layers, and cuttings. All the sorts 
! will take tolerably by cutting off the strong 
young shoots ; but the platanus occidental^ 
i more freely than the oriental kind. Autumn 
is the best season : as soon as the leaf falls, 
choose strong young shoots, and plant them 
in a moist soil; many of them will grow, and 
make tolerable plants by next autumn. 
PLATBAND of a door or ivindozv, is 
used for the lintel, where that is made square, 
or not much arched: these platbands are usu- 
j ally crossed with .bars of iron when they have 
a great bearing, but it is much better to ease 
them by arches of discharge built over them. 
PLATFORM, in the military art, an ele- 
vation of earth, on which cannon is placed to 
fire on the enemy: such are the mounts in 
the middle of curtins. On the ramparts there 
1 is always a platform, where the cannon are 
mounted. It is made by the heaping up. of 
earth on the rampart ; or by an arrangement 
j of madriers, rising insensibly, for the cannon 
to roll on, either in a casement or on attack 
in the outworks. All practitioners are agreed 
I that no shot can be depended on, unless the 
piece can be placed on a solid platform ; for 
if the platform shakes with the lirst impulse 
of the powder, the piece must likewise shake, 
which will alter its direction, and render the 
shot uncertain. 
Platform, or Orlop, in a ship of war, a 
place on the lower deck, abaft the main-mast, 
between it and the cockpit, and round about 
the main capstan, where provision is made 
for the wounded men in time of action. 
PLAT IN A. See Platinum. 
PLATING, is the art of covering baser 
I 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 
j for real utility. Till then' the more elegant 
j spurs in common use were made of solid sil- 
• ver ; and from the flexibility of that metal, 
they were liable to be bent into inconvenient 
forms by the slightest accident. To remedy 
I this defect, a workman at Birmingham con- 
I trived 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 im- 
I provement, and being desirous to add cheap- 
I ness to utility, he continued to make the hol- 
low 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 
i manner as to make it equally elegant with 
| those which were made wholly of that metal. 
| 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 additional ex- 
pence. 
The silver plate was formerly made to ad- 
here 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. 
When 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 hammered upon it, the ham- 
mer employed being always covered with a 
piece ot doth. The silver now forms, as it 
were, a mould to the ring, and whatever of it 
is not intended to be used is cut olf . This 
mould is fastened to the ring of the buckle by 
two or three cramps of iron wire ; after which 
the buckle, with the plated side undermost, 
is laid upon a plate of iron sufficiently hot to 
melt the tin, but not the silver. The buckle 
is then covered with powdered resin, or 
anointed with turpentine ; and lest there 
should be a deficiency of tin, a small portion 
of rolled tin is likewise melted on it. The 
buckle is now taken off’ with tongs, and 
commonly laid on a bed of sand ; where the 
plate and the ring, while the solder is yet in 
a state of fusion, are more closely compressed 
by a smart stroke with a block of wood. The 
buckle is afterwards bent and finished. 
The mode of plating at present is, to fasten 
plates of silver upon thicker plates of copper, 
and then rolling them together into thin 
plates. The copper is twelve times thicker 
than the silver, and one ounce of silver is 
rolled to a surface of three feet or more. 
The plates being thus made, they are then 
stamped by a single stroke into the size and 
form of buckles," buttons, spoons, Ac. 
PLATINUM, one of the perfect metals, 
has hitherto been found only in Peru, and in 
the mine Santa Fe, near Carthagena. The 
workmen of these mines must no doubt have 
been early acquainted with it ; but they seem 
to have paid very little attention to it. It 
was unknown in Europe till Mr. Wood 
brought some of it from Jamaica in 1741. In 
1748 it was noticed by don Antonio de Ul- 
loa, a Spanish mathematician, who had ac- 
companied the French academicians to Peru, 
in their voyage to measure a degree of the 
meridian. Several papers on it were pub- 
lished by Dr. Watson in the 46th volume of 
the Philosophical Transactions. These im- 
mediately attracted the attention of the most 
eminent chemist’s. In 1752, Mr. Scheffer ot 
Sweden published the first accurate examina- 
tion of its properties. He proved it to be a 
new metal, approaching very much to the 
nature of gold, and therefore gave it the 
name of aurum album, white gold. 
1. 1. Platinum, w hen pure, is of a white co- 
lour like silver, but not so bright. It has no 
taste nor smell. 
2. Its hardness is 8. Its specific gravity, 
after being hammered, is 23.000: so that it is 
by far the heaviest body known 
3. It is exceedingly ductile and malleable: 
it may be hammered out into very thin plates, 
and drawn into wires not exceeding 
inch in diameter. In these properties it is 
probably inferior to gold, but it seems to sur- 
pass ah the other metals. 
4. Its tenacity is such, that a wire of pla- 
tinum 0.078 inch in diameter, is capable of 
3 L 2 
supporting a weight of 274.31 lbs. avoirdu- 
pois without breaking. 
5. It is the most infusible of all metals, and 
cannot be melted, in any quantity at least* 
by the strongest artificial heat which can be 
produced. Macquer and Baume melted small 
particles of it by means of a blow pipe, and 
Lavoisier by exposing them on red-hot char- 
coal to a stream of oxygen gas. I t may in- 
deed be melted without difficulty when com- 
bined or mixed with other bodies-; but then 
it is not in a state of purify. Pieces or plati- 
num, when heated to whiteness, may bo- 
welded together by hammering in the same 
manner as hot ii on. 
6. This metal is not in the smallest degree 
altered by the action of air or water. 
II. It cannot be combined with oxygen 
and converted into an oxide by the strongest 
artificial heat of our furnaces. Platinum, in- 
deed, in the state in which it is brought from 
America, may be partially oxidated by ex- 
posure to a violent heat, as numerous expe- 
riments have proved; but in that state it is 
not pure, but combined with a quantity of 
iron. It cannot be doubted, however, that if 
w’e could subject it to a sufficient heat, pla- 
tinum would burn and be oxidated like other 
metals: for when Van Martrm exposed a 
wire of platinum to the action of his powerful 
electrical machine, it burnt w ith a faint white 
ilame, and was dissipated into a species of 
dust, which proved to be the oxide of plati- 
num. By putting a platinum wire into the 
flame produced by the combustion of hydro- 
gen gas mixed with oxygen, it was made to 
burn with all the brilliancy of iron wire, and 
to emit sparks in abundance. 1 his metal 
may be oxidated in any quantity by boiling 
it in 16 times its weight of nitro-muriatic acid 
(aqua regia). The acid dissolves it, and as- 
sumes first a yellow, and afterwards a deep 
red or rather brown colour. On the addition 
of lime to the solution, a yellow pow'der falls 
to the bottom. This powder is the oxide of 
platinum. Its properties have not been ex- 
amined with sufficient accuracy. It seems to 
contain but a small proportion of oxygen ; 
probably not more than 0.07 : yet it is in all 
probability a peroxide. 
This oxide may be decomposed, and the ox- 
ygen driven off, by exposing it to violent heat. 
III. Neither carbon nor hydrogen can be 
combined with platinum ; but M. Proust has 
found it combined with sulphur in native pla- 
tinum, and it unites without difficulty to phos- 
phorus. By mixing together an ounce of 
platinum, an ounce, of phosphoric glass, and 
a dram of powdered charcoal, and applying, a 
heat of about 32° Wedgeworth, M. Pelletier 
formed a phosphuret of platinum weighing 
more than an ounce. It was partly in the 
form of a button, and partly in cubic cry- 
stals. It w'as covered above by a blackish 
glass. It was of a silver-white colour, very 
brittle, and hard enough to strike fire with 
steel. When exposed to a fire strong enough 
to melt it, the phosphorus was disengaged, 
and burnt on the surface-. 
He found also, that when phosphorus was 
projected on red-hot platinum, the metal in- 
stantly fused, and formed a phosphuret. As 
heat expels the phosphorus, Mr. Pelletier has 
proposed this as an easy method of purifying 
platinum. 
IV. Platinum, as far as is known, does no! 
combine with the simple incombustibles. 
