PALLADIUM PALLADIUS in Brazilian ore, but usually as an alloy. It exists in platinum ore from the Ural and Santo Domingo, and it is also found, mixed with gold and selenide of lead, in the Hartz, and in aurif- erous ore from Zacotinga and Coudonga in Bra- zil, mixed with specular iron. It is also alloyed with gold and silver in the oro pudre of Por- pez, Brazil, often amounting to 10 per cent. It is extracted from platinum ore by digesting this in nitro-muriatic acid, precipitating the platinum from the decanted liquor by chloride of ammonium, and the palladium from the fil- trate by cyanide of mercury, and then calcining the cyanide thus obtained. From the pallado- auriferous ore of Brazil it is extracted by fu- sing this with an equal weight of silver and some nitre, which reduces the baser metals and earthy parts to slag. The alloy is cast into bars and again fused in black-lead crucibles with an equal weight of silver, so that the gold shall amount to one fourth of the mixture. This alloy is then granulated by pouring it into water through a sieve, when it is heated with twice its weight of equal quantities of nitric acid and water, the liquor decanted, and the residue boiled with pure nitric acid in quantity equal to two thirds the weight of granules used. From these nitric acid solutions the sil- ver is precipitated by common salt, and the palladium and copper from the filtrate by zinc, in wooden vessels. The resulting black pow- der is dissolved in nitric acid, the solution supersaturated with ammonia, and the filtrate from this saturated with hydrochloric acid, which precipitates the greater part of the palladium as a yellow ammonio-protochloride, which is then washed in cold water and re- duced to a metallic state by ignition. The re- mainder of the palladium and the whole of the copper may be precipitated from the hydro- chloric acid solution by iron. The symbol of palladium is Pd: its atomic weight, 106*5; sp. gr., 11-4 to 11-8. It is the most fusible of all the metals of the platinum group, beginning to fuse in the forge, and easily melting before the oxyhydrogen blowpipe at 2,480. Its col- or is intermediate between silver and platinum. When obtained from the cyanide, or from the ammonio-protochloride by ignition, it has the form of a spongy gray mass, which when fine- ly divided floats on water, and has a blood-red color by transmitted light. It is dimorphous, having the form of cubes and octahedrons, and also of six-sided tables, with cleavage parallel to the terminal faces. It is about as hard as platinum, but somewhat less ductile. "When heated on lime to the melting point of iridium, it volatilizes in green vapors, which condense to a bistre-colored dust of metal and oxide. It oxidizes at a lower temperature than silver, and is easily oxidized by hydrated alkalies. Its al- loys with iron, tin, lead, arsenic, and bismuth are very fusible and brittle. With twice its weight of silver it forms a ductile alloy not lia- ble to tarnish, and well adapted for the con- struction of small weights. Palladium is also used for the construction of graduated scales for astronomical instruments. Its alloy with gold is hard, and remarkable for its whiteness. With mercury it forms a fluid amalgam. Pal- ladium has the remarkable property of absorb- ing many times its volume of hydrogen, yield- ing it again at a high temperature, and was employed by Graham in experiments on the occlusion of hydrogen. Palladium foil heated for three hours between 195 and 106 F. ab- sorbed 643 volumes of hydrogen; and if the metal after having been heated to redness was allowed to cool in vacua, it absorbed at com- mon temperatures 376 volumes of the" gas. No alteration was produced in the metallic appear- ance of the foil. Spongy palladium absorbed 686 volumes of hydrogen, but no oxygen or nitrogen. When a wire of the metal is made the negative pole of a voltaic cell decompo- sing water acidulated with sulphuric acid, a still greater quantity of hydrogen can be ab- sorbed, as much as 936 volumes to one of pal- ladium, the metal increasing in bulk from 100 to nearly 105 volumes, or 16 times as much as if heated from 32 to 212. When the galvanic current is reversed, and the piece of palladium becomes the positive pole, the hydrogen is rap- idly converted into water by union with the nascent oxygen ; and by applying a clamp with a movable index, the expansion and contrac- tion of the metal on changing the current can be easily observed. Palladium, like platinum, forms two classes of compounds: the palla- dious compounds, in which it is bivalent, and the palladia, in which it is quadrivalent. The dichloride, or palladious chloride, Pd01 2 , is ob- tained by the action of nitro-muriatic acid. The tetrachloride or palladic chloride exists only in solution and in combination with alkaline chlo- rides. It is formed by digesting the dichloride in nitro-muriatic acid, has an intense brown color, and is decomposed by evaporation. Pal- ladious iodide is precipitated from the chloride or nitrate, as a black mass, by soluble iodides. Palladium salts are employed for the quantita- tive analysis of iodine, as chlorine and bromine are not precipitated by them. The oxides of palladium are the monoxide, or palladious oxide, PdO, and the dioxide or palladic oxide, PdO 2 . The latter is not obtainable in a separate con- dition, but exists as a hydrated palladic oxide, which obstinately retains a portion of alkali when precipitated from solutions of palladic chloride by the action of alkalies. There are three sulphides, PdS, PdS 2 , and Pd 2 S. Palla- dious nitrate has the form of rhombic prisms, soluble in a small quantity of water, but decom- posing and forming a basic nitrate in a large quantity. The other salts are of little interest. PALLADIFS. I. Surnamed SopMsta or latro- sophista, a Greek medical writer, of whose life nothing is known except that he must have flourished between the 2d and 9th centuries. He wrote commentaries on the works of Hip- pocrates " On Fractures " and " On Epidem- ics," and a treatise " On Fevers," all of which
