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Lanthanum [symbol La, atomic weight 139.0 (O = 16)] one of the metals of the cerium group of rare earths. Its name is derived from the Gr. λανθάνειν, to lie hidden. It was first isolated in 1839 by C. G. Mosander from the “cerium” of J. Berzelius. It is found in the minerals gadolinite, cerite, samarskite and fergusonite, and is usually obtained from cerite. For details of the complex process for the separation of the lanthanum salts from cerite, see R. Bunsen (Pogg. Ann., 1875, 155, p. 377); P. T. Cleve (Bull. de la soc. chim., 1874, 21, p. 196); and A. v. Welsbach (Monats. f. Chem., 1884, 5, p. 508). The metal was obtained by Mosander on heating its chloride with potassium, and by W. F. Hillebrand and T. Norton (Pogg. Ann., 1875, 156, p. 466) on electrolysis of the fused chloride, while C. Winkler (Ber., 1890, 23, p. 78) prepared it by heating the oxide with a mixture of magnesium and magnesia. Muthmann and Weiss (Ann., 1904, 331, p. 1) obtained it by electrolysing the anhydrous chloride. It may be readily hammered, but cannot be drawn. Its specific gravity is 6.1545, and it melts at 810°. It decomposes cold water slowly, but hot water violently. It burns in air, and also in chlorine and bromine, and is readily oxidized by nitric acid.

Lanthanum oxide, La2O3, is a white powder obtained by burning the metal in oxygen, or by ignition of the carbonate, nitrate or sulphate. It combines with water with evolution of heat, and on heating with magnesium powder in an atmosphere of hydrogen forms a hydride of probable composition La2H3 (C. Winkler, Ber. 1891, 24, p. 890). Lanthanum hydroxide, La(OH)3, is a white amorphous powder formed by precipitating lanthanum salts by potassium hydroxide. It decomposes ammonium salts. Lanthanum chloride, LaCl3, is obtained in the anhydrous condition by heating lanthanum ammonium chloride or, according to C. Matignon (Compt. rend., 1905, 40, p. 1181), by the action of chlorine or hydrochloric acid on the residue obtained by evaporating the oxide with hydrochloric acid. It forms a deliquescent crystalline mass. By evaporation of a solution of lanthanum oxide in hydrochloric acid to the consistency of a syrup, and allowing the solution to stand, large colourless crystals of a hydrated chloride of the composition 2LaCl3·15H2O are obtained. Lanthanum sulphide, La2S3, is a yellow powder, obtained when the oxide is heated in the vapour of carbon bisulphide. It is decomposed by water, with evolution of sulphuretted hydrogen. Lanthanum sulphate, La2(SO4)3·9H2O, forms six-sided prisms, isomorphous with those of the corresponding cerium salt. By careful heating it may be made to yield the anhydrous salt. Lanthanum nitrate, La(NO3)3·6H2O, is obtained by dissolving the oxide in nitric acid. It crystallizes in plates, and is soluble in water and alcohol. Lanthanum carbide, LaC2, is prepared by heating the oxide with carbon in the electric furnace (H. Moissan, Compt. rend., 1896, 123, p. 148). It is decomposed by water with the formation of acetylene, methane, ethylene, &c. Lanthanum carbonate, La2CO3·8H2O, occurs as the rare mineral lanthanite, forming greyish-white, pink or yellowish rhombic prisms. The atomic weight of lanthanum has been determined by B. Brauner (Proc. Chem. Soc., 1901, 17, p. 63) by ignition of lanthanum sulphate at 500° C., the value obtained being 139 (O = 16).