PLATT A, or PLAvixtrar, a metal, which in most of its properties is equal to gold, but in others it is very superior. It was first ascertained to be a distinct me tal by Scheffer, a Swedish chemist, in the year 1752. By him it was named white gold, because it resembled this metal in many of its properties. It immediately became subject to the experiments of all the chemists in Europe, and obtained, from its colour, the name of platina, sig nifying little silver, from the word plata, which is Spanish for silver. Platina has been found among the gold ores of South America, and more particularly in the mine of Santa Fe near Carthagena, and in the district of Choc° in Peru. Platina, in the state in which it reaches this coon try, is contaminated by the presence of several other metals, as iridium, osmium, rhodium, and palladium, and, in fact, it is merely an ore of platina. It is in the form of small grains or scales, of a whiter colour than iron, and extremely heavy. Various processes have been contrived for its purification ; but the one, which is the most simple and practicable, is de scribed in the ninth volume of Nichol son's Journal. Platina has the following properties. It is a white metal, resem bling silver in colour, but greatly ex ceeding it, and indeed all other metals, in specific gravity, being, when it is ham mered, twenty-three or twenty-four times heavier than water. It is not oxydized by the long continued and concurrent ac tion of beat and air. It has the property of welding, which belongs to no other metal but this and iron. It is not acted on by any other acid than the nitro-mu riatic and oxygenized muriatic. The for mer is best adapted to effect this solu tion. Sixteen parts of the compound acid are to be poured on one of the lami nated metal, and exposed to heat in a glass vessel ; nitrous gas is disengaged, and a reddish coloured solution is obtain ed, which gives a brown stain to the skin. The muriate of platina has the characteristic property of being precipi tated by a solution of muriate of ammonia. By this character, platina is distinguished from all other metals, and may be sepa rated when mingled with them in solu tion. The precipitate, thus obtained, is decomposed by a strong heat, and leaves pure platina. When pure potash is pour ed into the muriatic solution, a precipi tate ensues, which is not an oxide of pla tina, but a triple compound of that oxide with the alkali and acid. With soda, also, it forms a triple combination.
Patina is acted upon by fusion with nitrate of potash, and also with pure fix ed alkalies. The most delicate test of the presence of platina is muriate of tin. A solution of platina, so dilute as to be scarcely distinguishable from water, as sumes a bright red colour, on the addi tion of a single drop of the recent solu tion of tin.
Platina has been discovered by Dr. Wollaston to be a remarkably slow con ductor of caloric. When equal pieces of silver, copper, and platina, were covered with wax, and heated at one end, the wax was melted 31,-. inches on the silver ; on the copper ; and one inch only on the platina. Its expansion by heat is consi derably less than that of steel; which, between the temperatures of 32° and 212° is expanded about 12 parts in 10,000, while the expansion of platina is only about 10.
Platina combines with many of the me tals, and forms with them alloys, some of which are of considerable 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 platina is susceptible of being formed into different 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 exposed to a red heat, and hammered into bars. By the heating and hammering, the arsenic is driven off; and the metal is purified and becomes infusi ble, but retains its ductility, so that it may he wrought like iron. It has been found extremely difficult 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 might be effected by the following process. He placed a very thin plate of pure platina at the bot tom of a matrass containing a quantity of mercury. The matrass was put upon a sand bath, and heat applied, till the mer cury 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 different from the other combinations of mercury with the metals, for the platina did not lose its solid form. M. Chenevix, in the course of experiments and researches respecting a supposed new metal called palladium, succeeded in forming an amalgam with platina and mercury. He heated purified platina in the form of fine powder, with ten times its weight of mercury, and rub bed 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 origi nal weight of the platina remained. Pla tina combines with copper by means of fusion, and gives it hardness. When the proportion of copper is three or four tunes greater than that of platina, the alloy is ductile, susceptible of a fine polish, and is not altered by exposure to the air.
This alloy has been employed in the fa brication of mirrors for telescopes. Gold combines readily with platina, but it re quires a very powerful heat for the fusion ofthese two metals. Platina diminishes the colour of gold, unless it be in very small quantity. When the proportion of plati na is above the colour of the gold be gins to be altered. There is no percep tible change in the specific gravity 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 suffi 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 peculiarly appropriate, as there are few chemical agents whose effects it cannot resist. There is indeed little doubt but it might be employed with equal advan tage in the construction of instruments and utenils, in various arts and manufac tures.