PLATINUM is a white metal extremely difficult of fusion, and unaltered by the joint action of heat and air. It varies in density from 21 to 21.5, according to the degree of mechanical compression which it has sustained; it is extremely ductile, but cannot be beaten into such thin leaves as gold and silver. (Brande, 4th Ed. p. 822.)

The particles of the generality of the metals, when separated from the foreign matters with which they are combined, are joined into solid masses by simple fusion; but platinum being nearly infusible when pure, requires a very different treatment, which was introduced by Dr. Wollaston, and is now conducted in the following manner by Messrs. Johnson and Cock, of London, the celebrated metallurgists.

The platinum is first dissolved chemically, and it is then thrown down in the state of a precipitate; next it is partly agglutinated in the crucible into a spongy mass, and is then compressed whilst cold in a rectangular mould by means of a powerful fly-press or other means, which in operating upon 500 ounces, converts the platinum into a dense block about 5 inches by 4, and 2 1/2 inches thick. This block is heated in a smith's forge, with two tuyeres meeting at an angle, at which spot the platinum is placed amidst the charcoal fire; when it has reached the welding point, or almost a blue heat, it receives one blow under a heavy drop, or a vertical hammer somewhat like a pile-driving engine; it then requires to be reheated, and it thus receives a fresh blow about every 20 minutes, and in a week or ten days, it is sufficiently welded or consolidated on all sides to admit of being forged into bars, and converted into sheets, rods, or wires by the ordinary means.

The motive for operating upon so great a quantity is for making the large pans for concentrating sulphuric acid, in only two or three pieces, which are soldered together with fine gold In France, 2,000 ounces are sometimes welded into one mass, so that the vessels may be absolutely entire; a practice which is considered in this country to be unnecessarily costly. For small quantities the treatment is the same, but in place of the drop, the ordinary flatter and sledge hammer are used.

Platinum is exceedingly tough and tenacious, and "hangs to the file worse than copper," on which account, when it is used for the graduated limbs of mathematical instruments, the divisions should be cut with a diamond point, and which is the best instrument for fine graduations of all kinds, and for ruling grounds, or the lined surfaces for etching. See p. 180.

Platinum is employed in Russia for coin. This valuable metal is also used for the touch-holes of fowling-pieces, and in various chemical and philosophical apparatus, in which resistance to fusion or to the acids is essential.

The alloys of platinum are scarcely used in the arts; that with a small quantity of copper is employed in Paris for dental surgery. For alloys of platinum and steel, see Quarterly Journal of the Royal Inst vol. ix. p. 328. The alloy of equal parts of steel and platinum is therein highly spoken of as a mirror.

" Dr. Von Eckart's alloy, contains platinum 2.40, silver 3.53, and copper 11.71. It is highly elastic, of the same specific gravity as silver, and not subject to tarnish, it can be drawn to the finest wire from 1/2 of an inch diameter without annealing, and does not lose its elasticity by annealing. It is highly sonorous, and bears hammering red-hot, rolling and polishing."

Mr. Ross added to silver, one-fourth of its weight of platinum, and he considers that it took up one-tenth its weight The alloy became much harder than silver, capable of resisting the tarnishing influences of sulphur and hydrogen, and was fit for graduations.

An alloy of platinum with ten parts of arsenic is fusible at a heat a little above redness, and may therefore be cast in moulds. On exposing the alloy to a gradually-increasing temperature in open vessels, the arsenic is oxidized and expelled, and the platinum recovers its purity and infusibility. - Turner's

Tin also so greatly increases the fusibility of platinum, that it is hazardous to solder the latter metal with tin-solder, although gold is so used.

Platinum, as well as gold, silver and copper, are deposited by the electrotype process; and silver plates thus platinized are employed in Smee's Galvanic Battery.