P L A

P L A

Plate, in geography, a town of Upper Saxony, in Pomera-nia , on the ilega ; 17 miles E. S. E. of Laminin, -and 22 S. ofColberg.

Plate, a town of Hispaniola , or S't. Domingp, on (he S. sideof tiie North Peninsula. Lon. 75. 40. W, of Paris . Lat. 10.16. N.

PLATEN, among printers, the flat part of the press wherebythe impression is made.

PLATFORM, in architecture, is a row of beams which sup-ports the timber-work of a roof, and lie on the top of a wall wherethe entablature ought to be raised. This term is also used for akind of terrace, or broad smooth open walk, at the top of a build-ing, from whence a fair prospect may be taken of the adjacentcountry.

Platform, in the military art, is an elevation of earth, on whichcannon are placed to fire on the enemy; such are the mounts inthe middle of curtins.

Platform, or Orlop, in a man of war, is a place on the lowerdeck, abaft the main-mast, between it and the cockpit, and roundabout the main-capstan, where provision is made for the woundedmen in lime of action.

PLAT1C ASPECT, in astrology, a rav cast from one planetto another, not exactly, but withii; the orbit of its own light.

PLAT1NA, or PLATINUM , the most precious of all themetals excepting gold, and by some reckoned superior even to it.It has hitherto been found only in America, in Clioco in Peru ,and in the mine of Santa near Cartiiagena. It was unknown inEurope tii! Mr. Wood brought some of it from Jamaica , in J741.Platina , when pure, is of a white colour like silver, but not sobright. It has no taste nor smell. Its hardness is S. Its specificgravity, alter being hammered, is 23-000 ; so that it is by far theheaviest body known. It is exceedingly ductile and malleable ; itmay lie hammered out into very thin plates, and drawn unto wiresnot exceeding oiie-l<)40th of an inch in diameter. In these pro-perties it is probably inferior to gold, but it seems to surpass allthe other metals. Its tenacity is such, that a wire of platina one-120th of an inch in diameter is capable of supporting a weight of497 pounds without breaking. It is the most infusible of all themetals, and cannot be melted, in any quantity at least, by thestrongest artificial heat, which can be produced. Macquer andBeaume melted small particles of it by a blow-pipe, and Lavoisier by exposing them on red hot charcoal to a stream of oxygen gas.It may, indeed, be melted without difficulty when combined ormixed with other bodies ; but then it is not in a state of purity.Pieces of platina, when heated to whiteness, may be welded to-gether by hammering in the same manner as hot iron. This metalis not in the smallest degree altered by the action of air or water.It cannot be combined with oxygen, and converted into an oxide,bv the strongest artificial heat of our furnaces. It cannot bedoubted, however, that if we could subject it to a sufficient, heat,platina would burn and be oxidated like other metals: for whenVan Marum exposed a wire of platina to the action of his power-ful electrical machine, it burnt with a faint white flame, and wasdissipated into a species of dust, which proved to be the oxide ofplatina. By putting a plalina-wirc into the flame produced by thecombustion of hydrogen gas mixed with oxygen, it was made toburn with all the brilliancy of iron-wire, and to emit sparks inabundance. This metal may be oxidated in any quantity bv boil-ing it in sixteen times its weight of nitro-muriatic acid (aqua regia.)The acid dissolves it, and assumes first a yellow, and afterwards adeep red or rather brown colour. Oil the addition of lime to thesolution, a yellow powder falls to the bottom. This powder is theoxide ot platina. Its properties have not been examined with suf-ficient acruracv. It seems to contain but a small proportion of oxy-gen ; probably not more than 0.07 : this oxide may be decomposed,and the oxygen driven off, by exposing it to violent heat. Neithercarbon nor hydrogen can be combined with platina; but M. Proustlias found it combined with sulphur in native platina, and it uniteswithout difficulty to phosphorus. By mixing together an ounce ofplatina, an ounce of phosphoric glass, and a drachm of pow-dered charcoal, and applying a heat of about 32° Wedgewood, M.Pelletier formed a phospnuret of platina weighing more than anounce. It was partly in the form of a button, and partly in cubiccrystals. It was covered above bv a blackish glass. It was of asalver white colour, very brittle, and bard enough to strike fire with

VOL. IV.—NO, 178.

steel. Platina is acted upon by fusion with nitrate of potass, andalso with pure fixed alkalies. The most delicate test of the pre-sence of platina is muriate of tin. A solution of platinr, so diluteas to be scarcely distinguishable from water, assumes a bright redcolour, on the addition of a single drop of the recent solution oftin. Platina has been discovered by Dr. Wollaston to be a re-markably slow conductor of caloric. Platina , on account of itspeculiar properties, its infusibidly, densi v, and indestructibility,could it be obtained in sufficient quantity, and at a moderateprice, would undoubtedly prove one of the most useful and mostimportant of the metals yet known. The importance and utilityof platina, on account of its scarcity, have been hitherto limited tochemical purposes ; and for different chemical instruments andutensils, it has been found peculiarly appropriate, as there are fewchemical agents whose effects it cannot resist. There is indeedlittle doubt but it might be employed with equal advantage in theconstruction of instrun ants and utensils, in various arts and manu-factures. Dr. Lewis found that copper was much improved by-allaying it with platina in certain proportions ; and that equal partsof platina and brass formed a compound not subject to tarnish, amiwhich might be employed with great advantage for the speculum*of telescopes.

PLATING, is the art of covering baser metals with a thinplate of silver either for use or for oniament. The silver-plate isgenerally made to adhere to the baser metal by means of solder ;which is of two kinds, the soft and the hard, or tire tin and silversolders. The former of these consists of tin alone, the latter gene-rally 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 gentryhammered upon it, the hammer employed being always coveredwith a piece of cloth. The silver now forms, as it were, a mouldto the ring, and whatever of it is not intended to be used is cut oil’.'Phis mould is fastened to the ring of the buckle by two or threecramps of small iron-wire ; after which the buckle, with the platedside undermost, is laid upon a plate of iron sufficiently hot to meltthe tin, but not the silver. The buckle is then covered with pow-dered resin or anointed with turpentine ; and lest there should bea deficiency of tin, a small portion of rolled tin is likewise meltedon it. The buckle is now taken off with tongs, and commonlylaid on a bed of sand, where the plate and the ring, while thesolder is yet in a state of fusion, are more closely compressed by asmart stroke with a block of wood. The buckle is afterwards bentand finished. Sometimes the melted tin is poured into the silver-mould, which has been previously rubbed over with some flux.The buckle-ring is then put among.the melted tin, and the platingfinished. This is called by the workmen filling up. When thehard solder is employed, the process is in many respects different.Before the plate is fitted to the iron or other metal, it is rubbedover with a solution of borax. Stripes of silver are placed alongthe joinings of the plate ; and instead of two or three cramps, as inthe former case, the whole is wrapped round with small wire ; thesolder and joinings are again rubbed with the borax, and the wholeput into a charcoal fire till the solder be in fusion. When takenout, the wire is instantly removed, the plate is cleaned by the ap-plication of some acid, and afterwards made smooth by the strokesof a hammer

Plating, French , is when silver-leaf is burnished on a pieceof metal in a certain degree of heat. When silver is dissolved in.aquafortis , and precipitated upon another metal, the process iscalled silvering.

Plating, Metal , is when a bar of silver and copper are takenoff at least one equal side. 1 be equal sides are made smooth, andthe two bars fastened together by wire wrapped round them.These bars are then sweated in a charcoal-fire; and after sweating,they adhere as closely together as if they were soldered. Afterthis thev are flattened into a plate between two rollers, when thecopper appeals on one side and the silver on the other. This sortof plate is named plated metal.

PLATO, an illustrious philosopher of antiquity, was by de-scent an Athenian, though the place of his birth was the island offEgina. His descent by his father was from Codrus the last kingof Athens, and by bis mother from Solon the celebrated legislator.The time of his birth is placed in the beginning of the 88th Olyvn-niad ; but Dr. Enfield thinks it mav be more accurately fixed m4Q ' A- the