Palladium (sym. Pd, atom. wt. 106.2, sp. gr. 11.4) is one of the so-called noble metals, which in its colour and ductility closely resembles platinum. It is not fusible in an ordinary wind-furnace, but melts at a somewhat lower temperature than the last-named metal; and, when heated beyond its fusing-point, it volatilises in the form of a green vapour. It undergoes no change in the open air at ordinary temperatures; but at a low red heat it becomes covered with a purple film, owing to superficial oxidation. It is soluble in nitric and iodic acids, and in aqua regia. It combines readily with gold, which it has the property of rendering brittle and white. (When it forms 20 per cent. of the mass the alloy is perfectly white.) When alloyed with twice its weight of silver it forms a ductile compound, which has been employed for the construction of small weights; but for this purpose aluminium is superior. Professor Miller states that it 'has been applied in a few cases to the construction of graduated scales for astronomical instruments, for which, by its whiteness, hardness, and unalterability in the air, it is well adapted;' its scarcity must, however, prevent its general use for this purpose.
It was discovered in 1803 by Wollaston in the ore of platinum, of which it seldom forms so much as 1 per cent. Another source of this metal is the native alloy (termed ouro poudre) which it forms with gold in certain mines in Brazil; it is from this alloy that the metal is chiefly obtained.
Palladium forms with oxygen a protoxide, , which is the base of the salts of the metal; a binoxide, ; and according to some chemists, a suboxide, . On exposure to sufficient heat these compounds give off their oxygen and yield the metal. The salts of the protoxide are of a brown or red colour.