SeeE. Bellierdela Chavignerie, Recherches sur le peintre Lantara, Paris, 1852.

In an abandoned copper mine at Riddarhyttan, Westmanland, in Sweden, there occurs a heavy compact mineral, which, though pretty abundant there, is hardly met with anywhere else. This mineral was long mistaken for tungsten (syn. scheelite), until Klaproth of Berlin in 1803 found in it a peculiar earth, which he called ochroite earth, as it becomes yellow when heated in air. About the same time Berzelius and Hisinger made the same discovery; and, (rightly) presuming the new earth to be an oxide of a new metal, they called the latter cerium (after the planet Ceres, the then latest discovery in astronomy) and the mineral cerite, which names have been retained to this day. Only the name “cerium” now has a more specific meaning, it having been shown by Mosander (in 1839-41) that Berzelius’s cerium is a mixture of three metallic radicles, namely, cerium proper, lanthanum (from ∖ aνfjaνevν, “to be concealed”), and didymium (from δίδυμος, “twin”). These metals are very closely related to one another in their chemical character, and may be conveniently trea·ted together. The extraction from cerite, of the oxide group, offers no difficulty. According to Marignac {Ann. Chim. Phys. [3], vol. xxvii.), the powdered mineral is made into a thick paste with oil of vitriol, and the reaction which sets in allowed to accomplish itself. The remaining dry white powder is placed in a crucible, and kept there for a long time at a temperature below redness, but sufficient to chase away the bulk of the free sulphuric acid. The residue is added in small instalments to a quantity of cold water, and the gangue (ferruginous silica) filtered off. The solution is boiled, when the greater part of the cerite-oxides comes down in the form of sulphate almost free from foreign oxides. The sulphates can be purified by redissolving them in the least quantity of water at 5° to 6° C., filtering, and reprecipitating by boiling. What remains in the mother-liquors is recovered by precipitation with sulphate of potash (which must be added as a solid and in sufficient quantity to saturate the solution) as an alum-like double sulphate. The purified sulphates are dissolved in cold water, precipitated as oxalates by means of oxalate of ammonia, and the washed oxalates ignited, when the pure cerite-oxide mixture remains. The separation of the three oxides from one another offers very great difficulties. Comparatively easy is the extraction of approximately pure oxide of cerium—by Berzelius’s method. Dissolve the mixed oxide (which must be free of sulphate if the method is to succeed) in nitric acid, evaporate to dryness, ignite the residue, and treat it with nitric acid diluted with one hundred times its weight of water. Only lanthanum and didymium dissolve, impure binoxide of cerium (CeO 2 ) remaining, which can be further purified by treatment with more concentrated nitric acid, which, however, besides the lanthanum and didymium, dissolves a good deal of the cerium itself. This method (like any of the rest) is founded upon the fact that salts of sesquioxide of cerium (Ce 2 O 3 ) are readily oxidized into salts of the feebly basic binoxide CeO 2 under circumstances which effect no higher oxidation in La 2 O 3 or Di 2 O 3 .

For the preparation of the oxides of lanthanum and didymium we may utilize the nitric mother-liquors obtained in the extraction of cerium-oxide. These are evaporated to dryness, the residue is ignited, and treated with very dilute nitric acid, which dissolves the lanthanum and didymium with only little cerium (Mosander, Marignac). A more complete elimination of the cerium is effected (Bunsen) by converting the nitrates into sulphates (by evaporation with sulphuric acid to dryness, and igniting the residue), dissolving these in sulphuric acid water, and boiling with powdered magnesite (MgCO 3 ). From the filtrate the lanthanum and didymium are precipitated (after acidulation by muriatic) with oxalic acid, and the oxalates filtered off, washed, and ignited. By repeating the magnesia and oxalic acid process two or three times, the oxides are obtained cerium-free. They are then made into anhydrous, neutral sulphates; these are dissolved in a minimum of water at 0° to 5° O., and the solution is heated to 30° to 35° C., when lanthanum sulphate chiefly separates out in small crystals, which are filtered off with the help of a filter-pump. A relatively lanthanum-free didymium sulphate remains dissolved (Mosander).

The metals were known only in a powdery form up to 1876, when Hillebrand and Norton succeeded in preparing them in a compact form by the electrolysis of the fused chlorides. The three metals are very similar to one another; they are steel-grey ductile true metals, melting at a somewhat lower temperature than silver. Specific gravities range from 6 ’ 1 to 6 ’ 6. They are more readily inflammable than magnesium.

The atomic weights of the three elements are now (1882) quoted as Ce = 141, La =139, Di =147.

Oxides and Salts.—Cerium has long been known to form two oxides, namely, a lower (“cerous ”) oxide, which is a pretty strong, and a higher (“ceric”) oxide, which is a feeble base. 16 ( = 0)parts of oxygen are combined, in the former with 92 (= “F”) parts, in the latter with ∣F parts of metal. Formerly F was looked upon as the atomic weight, and the oxides formulated as FO and F.O. respectively. But Mendelejeff on theoretical grounds insisted that the lower oxide must be looked upon as a sesquioxide Ce 2 O 3 , where Ce 2 = 3F or Ce = ∣F, and the higher (accordingly) as a binoxide, Ce 2 O 4 or rather CeO 2 . And he asserted the same in regard to the reputed monoxides of lanthanum and didymium. These remarks were little heeded even by the specialists until Hillebrand (partly in conjunction with Norton) succeeded in obtaining the three metals as such, and in a truly metallic condition, which admitted of an exact determination of the specific heats. The specific heats were found to be— for Ce Ό4479, for La ’ 04485, for Di Ό4563; and these numbers must be multiplied with Mendelejeffs atomic weight to produce “atomic heats” (6 - 6; 6'23; 6 ’ 6), approximating to values obtaining for other metals of established atomic weight. Hence Mendelejeff was right.

Cerous oxide, Ce 2 O 3 , is obtained by heating the oxalate in an atmosphere of hydrogen, as a bluish-grey powder. The higher oxide CeO 2 is obtained when any certain salt of a volatile acid {e.g., nitric) is ignited in air; it is a white powder, which when cold has only a slight touch of yellow in it, but at high temperatures assumes a deep orange-red colour. Cerous chloride, CeCl 3 , is obtained by heating the metal, or a mixture of either oxide and charcoal, in dry chlorine gas, as a yellowish-white sublimate, uniting with water into indistinct crystals Ce 2 Cl 6 + 5H 2 O. The cerous sulphate, Ce 2 (SO 4 ) 3 , separates out when its solution is heated. It is soluble in 6 parts of cold, and in about 60 of hot water, and forms a difficultly soluble double salt with sulphate of potash. To obtain the ceric sulphate, when the dioxide is treated with cold concentrated sulphuric acid, .and the solution formed by addition of water allowed to evaporate, the salt Ce 2 (SO 4 ) 3 (ous) + 2Ce(SO 4 ) 2 (ic) + 25H 2 O separates out in red. crystals looking like bichromate of potash. The mother-liquor yields yellow crystals of =Ce(SO 4 ) 2 +4H a O.

The most characteristic test for cerium salts is that the colourless cerous solutions, on addition of hypochlorite of soda, give a red precipitate of a ceric hydrate.

Didymium (Di 2 O 3 ) solutions have an astringent sweetish taste and a rose-red or violet colour. But their most characteristic property is their beautiful absorption-spectrum, which cφnes out distinctly with as little as half an inch deep of a solution containing τ ⅛ σ th of a per cent, of the oxide (Gladstone).

Lanthanum (La 2 O 3 ) solutions have a similar taste to those of didymium salts. They are colourless. The chloride when volatilized between the poles of an induction coil yields a highly characteristic rich line-spectrum, by means of which the least traces of the metal can be detected (Bunsen).

Of higher oxides of lanthanum or didymium we had hitherto only indications; but quite lately Dr Brauner {Chcm. News for 1881, December 23), in Roscoe’s laboratory, succeeded in preparing a definite pentoxide, Di 2 O 8 , of didymium, and also a hydrate of it, Di 2 O 5 .3H 2 O.

Sources.—Cerite, though the most abundant, is not the only native source of cerium, lanthanum, and didymium. A. Cossa has found traces of the metals in the ashes of numerous plants, and even in the human body. But it is more important to state that there are a number of rare minerals, of which the chief are known by the names of gadolinite, euxenite, samarskite, which, along with more or less of cerite-metals, contain other rarer earth-metals similar to these. Until lately the handbooks of chemistry quoted only three such rarer members of the family under the names of yttrium, erbium, and terbium; but these reputed individual elements have, during the last few years, been searchingly analysed by Marignac, Μ. Delafontaine, L. F. Nilson, P. T. Clève, J. L. Smith, and others, and under their hands resolved themselves into about a dozen separate elements. The rare earth-metals in fact bid fair to multiply like the little planets in astronomy; and, although in chemistry no firmly established fact can justly be called unimportant, the minor rare earths, in the meantime, are of no general interest, even to the general chemist.

See Würtz, Dictionnaire de Chimie, 1876; Roscoe and Schorlemmer, Handbook of Chemistry, 1879; Marignac's and Delafontaine's Memoirs in the Archives des Sciences physiques et naturelles; Liebig's Annalen der Chemie, 1858-64: Poggendorff’s Annalen, 1875; Journal f. prakt. Chemie, 1858-62; Zeitschrift f. Chemie, 1862. The Jahresbericht der Chemie is the surest guide to all the literature^