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Rh  Palladious oxide, PdO, is a black powder formed by heating spongy palladium to a dull red heat in a current of oxygen or by gentle ignition of the nitrate. It is insoluble in acids, is easily reduced, and decomposes when heated. Palladic oxide, PdO₂, is obtained in the hydrated condition, PdO₂•nH₂O, by the action of ozone on palladious chloride; by the electrolytic oxidation of palladious nitrate in slightly acid solution (L. Wöhler); and by the action of caustic potash on potassium palladio-chloride, the liquid being neutralized with acetic acid (I. Bellucci, Zit. anorg. Clem., 1905, 47, p. 287). It is a dark red or brown coloured powder, which loses oxygen on heating. When boiled with water it passes into the lower oxide. It is an energetic oxidizing agent, and when freshly prepared is soluble in dilute mineral acids. A hydrated form of the monoxide, PdO•nH₂O, is obtained by hydrolyzing a faintly acid solution of the nitrate (L. Wöhler,Zeit. anorg. Chem., 1905, 46, p. 323), or by the action of a slight excess of caustic soda on the double chloride K₂PdCl₆. It is a dark brown powder which loses its water of hydration when dried in air, and in the dry condition is difficultly soluble in acids. By the electrolytic oxidation of palladious nitrate L. Wöhler and F. Martin (lb., 1908, 57, p. 398), obtained a hydrated oxide, Pd₂O₃•nH₂O, as a dark brown powder which dissolves in hydrochloric acid, forming an unstable chloride.

Palladious chloride, PdCl₂, is obtained as a deliquescent crystalline mass when spongy palladium is heated to dull redness in a current of dry chlorine. A hydrated form, of composition PdCl₂•nH₂0, results on dissolving palladium in aqua regia, containing only a small proportion of nitric acid. It crystallizes from water as a reddish-brown solid. It absorbs hydrogen and is easily reduced. It combines with carbon monoxide to form compounds of composition PdCl₂•₂CO; ₂PdCl₂•₃CO; PdCl₂•CO (E. Fink, Comptes Rendus, 1898, 126, p. 646), and can be used for the determination of the amount of carbon monoxide in air (Potain and R. Drouin, lb., 1898, 126, p. 938). On treatment with dry ammonia gas it yields pall ado diam mine chloride, Pd(NH₃)₄Cl₂. Palladious chloride combines with hydroxylamine to form the compounds Pd(NH30)4Cl2 and Pd(NH₃O)₂Cl₂. The first results from the action of hydroxylamine on the chloride in the presence of sodium carbonate, and may be isolated as the free base. The other is thrown down as a yellow granular precipitate when a small quantity of dilute hydrochloric acid is added to the base, Pd(NH₃0)₄(OH)₂ (S. Feisel and A. Nowak, Ann., 1907, 351, p. 439). The chloride PdCU is only known in acid solution, and is obtained when palladium is dissolved in aqua regia or when palladic oxide is dissolved in concentrated hydrochloric acid. The solution is brown in colour and gradually loses chlorine, being converted into palladious chloride. Both chlorides combine with many other metallic chlorides to form characteristic double salts, the double potassium salts having the formulae K₂PdCl₄ and K₂PdCl₆. The former may be prepared by adding an excess of potassium chloride to palladious chloride, or by boiling K₂PdCl₆ with a large excess of water. It crystallizes in prisms which are readily soluble in water but are practically insoluble in absolute alcohol. It is decomposed by direct heating, and also by heating in a current of hydrogen. The latter compound is formed when chlorine is passed into a warm aqueous solution of the former or by dissolving palladium in aqua regia and saturating the solution with potassium chloride. It crystallizes in scarlet octahedral which darken on heating, and decompose when strongly heated. It is slightly soluble in cold water, but dissolves in warm dilute hydrochloric acid. When boiled with alcohol it is reduced to the metallic condition.

The subsulphide, Pd₂S, is obtained as a hard, green coloured mass when palladosammine chloride is fused with sulphur or when the sulphide PdS is fused with sulphur and ammonium chloride. It loses sulphur slowly when heated and is insoluble in acids. Palladious sulphide, PdS, is obtained by precipitation of the corresponding salts with sulphuretted hydrogen, or by the action of dry sulphuretted hydrogen gas on palladosammine chloride. As prepared in the dry way it is a hard, blue coloured, insoluble mass, but if obtained by precipitation is of a brownish-black colour and is soluble in nitric acid. When heated in air it oxidizes to a basic sulphate. The disulphide, PdS₂, is a brownish-black crystalline powder which is formed when the double ammonium palladium chloride (NH₄)₂PdCl₆ is heated to redness with caustic soda and sulphur. It combines with the alkaline sulphides. It gradually loses sulphur on heating, and is easily soluble in aqua regia. A sulphide of composition Pd₃S₄ has been described (R. Schneider, Pogg. Ann., 1873, 148, p. 625).

Palladium sulphate, PdS0₄•₂H₂0, is obtained by dissolving the oxide in sulphuric acid, or by the action of nitric and sulphuric acids on the metal. It forms a reddish-brown, deliquescent, crystalline mass, and is easily soluble in water, but in the presence of a large excess of water yields a basic sulphate. Palladium nitrate, Pd(N0₃)₂, crystallizes in brownish-yellow deliquescent prisms and is obtained by dissolving the metal in nitric acid. It is very soluble in water, and its aqueous solution decomposes on boiling, with precipitation of a basic nitrate. Palladium cyanide, Pd(CN)₂, is obtained as a yellowish precipitate when palladiuin chloride is precipitated by mercuric cyanide. It is insoluble in water, and on heating decomposes into palladium and cyanogen. It is soluble in solutions of the alkaline cyanides, with formation

of double cyanides of the type K₂Pd(CN)₄. On account of its insolubility and its stability it is useful for the separation of palladium from the other metals of the platinum group.

The palladium salts combine with ammonia to form characteristic compounds, which may be grouped into two main divisions: (1) the palladammines (palladosammines) of type [Pd(NH₃)₂X₂], and (2) the palladodiammes [Pd(NH₃)₄]X₂. The palladosammines are obtained by adding a large excess of ammonia to the palladious salts, the resulting clear solution being then precipitated by the mineral acid corresponding to the salt used. This method of preparation serves well for the chloride, from which other salts may be obtained by double decomposition. These salts are fairly stable, and arc red, yellow or orange in colour. The palladodiammine salts are mostly colourless, and are not very stable; acids convert them into the palladosammines, and they lose two molecules of ammonia very easily. They are formed by the action of a large excess of ammonia on the palladious salts or on the corresponding palladosammine salts in the presence of water.

Numerous determinations of the atomic weight of palladium have been made, the values obtained varying from 105.7 to 107.249 (see Amer. Chem. Jour., 1899, 21, p. 943; Ann., 1905, 341, p. 235; Jour. Chem. Soc., 1894, 65, p. 20). The International Commission on Atomic Weights, 1909, recount several new determinations: Haas (Dissertation, Erlangen, 1908) from reduction of palladosammine bromide obtained the value 106.7; Kemmerer (Thisis, Pennsylvania, 1908), from reduction of the corresponding chloride and cyanide obtains a mean value of 106.434; whilst A. Gutbicr and his collaborators, from analyses of palladosammine chloride and bromide, obtained the values 106.64±0.03 and 106.65 ±0.02 from the chloride, and 106.655 from the bromide (Jour. pr. chem., 1909, ii. 79, pp. 235, 457).

 PALLADIUS, RUTILIUS TAURUS AEMILIANUS, a Roman author of the 4th century A.D. He wrote a poem on agriculture (De re rustica) in fourteen books, the material being derived from Columella and other earlier writers. The work is conveniently arranged, but far inferior in every other respect to that of Columella.

There is a modern German edition by Schmitt (Leipzig, 1898).

 PALLANZA, a small industrial town and summer and winter resort of the province of Novara, Piedmont, Italy. 659 ft. above sea-level. Pop. (1901), 4619 (town); 5247 (commune). It occupies a position of great natural beauty, on a promontory on the W. of Lago Maggiore, with a semicircle of mountains behind and the lake and Borromean Islands in front, 62 m. N. of Novara direct. The annual mean temperature is 55° Fahr.; January, 37.1°, July, 74°. There is a fine botanical garden.

 PALLAS, PETER SIMON (1741–1811), German naturalist and traveller, was born in Berlin on the 22nd of September 1741, the son of Simon Pallas, surgeon in the Prussian army and professor of surgery in Berlin. He was intended for the medical profession, arid studied at the universities of Berlin, Halle, Göttingen and Leiden. He early displayed a strong leaning towards natural history. In 1761 he went to England, where for a year he devoted himself to a thorough study of the collections and to a geological investigation of part of the coast; and at the age of twenty-three he was elected a foreign member of the Royal Society. He then spent some time in Holland, and the results of his investigations appeared at the Hague in 1766 in his Elenchus Zoophytorum and Miscellanea Zoologica, and in 1767–1804 in his Spicilegia Zoologica (Berlin). In 1768 he accepted the invitation of the empress Catharine II. to fill the professorship of natural history in the Imperial Academy of Science, St Petersburg, and in the same year he was appointed naturalist to a scientific expedition through Russia and Siberia, the immediate object of which was the observation of the transit of Venus in 1769. In this leisurely journey Pallas went by Kasan to the Caspian, spent some time among the Kalmucks, crossed the Urals to Tobolsk, visited the Altai mountains, traced the Irtish to Kolyvan, went on to Tomsk and the Yenisei, crossed Lake Baikal, and extended his journey to the frontiers of China. Few explorations have been so fruitful as this six years' journey. The leading results were given in his Reisen durch verschiedene Provinzen des rüssischcn Reichs (3 vols., St Petersburg, 1771–1776), richly illustrated with coloured plates. A French translation in 1788–1793, in 8 vols., with 9 vols, of plates, contained, in addition to the narrative, the natural history results of the expedition; and an English translation in three volumes appeared in 1812. As special results