The mother-liquors, fraction 4, are treated with bismuth magnesium nitrate,[242] which is intermediate in solubility between the analogous compounds of samarium and europium, and the crystallisation continued. The less soluble fraction contains the samarium compound, in which bismuth is the only impurity; this is easily removed by treatment with sulphuretted hydrogen. The remaining fractions are used as a source of the terbium elements (see p. 186). [242] See Urbain and Lacombe, _Compt. rend._ 1903, ~137~, 792; _ibid._ 1904, ~138~, 84 and 1136. The double carbonate method[243] is very suitable for the preparation of pure lanthanum compounds after the removal of cerium. The mixture of salts is added to a warm 50% solution of potassium carbonate, and to the clear liquid, water is added gradually, with constant stirring. The double carbonates of the most positive elements are the least soluble, and are first thrown down, so that the precipitate is rich in lanthanum; it is collected and washed with a 25% potassium carbonate solution, and the process repeated. A few repetitions suffice to separate lanthanum completely from the other members of the group. The method may also be used for the purification of praseodymium salts. [243] Meyer, _Zeitsch. anorg. Chem._ 1904, ~41~, 94. ~Lanthanum~, La = 139·0 As the most electropositive element of the rare earth group, lanthanum is the most similar in its chemical properties to the metals of the alkaline earths. The _metal_ itself (see p. 115) oxidises even in dry air, and in moist air rapidly becomes coated with a white layer of hydroxide; it attacks water, and burns vigorously when heated in the air. An alloy with aluminium, of the formula LaAl₄, has been prepared by Muthmann and Beck[244]; it forms lustrous white crystals, very stable in the air and very resistant towards acids. [244] _Annalen_, 1904, ~331~, 46. The _hydroxide_ is of interest from the fact that, if precipitated under suitable conditions, it has the power of taking up solid iodine to form a deep blue adsorption compound[245]; colloidal solutions of basic lanthanum acetate are also coloured blue by addition of a few drops of iodine solution. If precipitation with alkali be carried out in presence of hydrogen peroxide, an hydrated _peroxide_ of the composition La₂O₅,_n_H₂O is obtained.[246] This compound partially decomposes with evolution of oxygen at ordinary temperatures; towards carbon dioxide and acids it acts as a true peroxide, with formation of hydrogen peroxide. [245] Damour, _Compt. rend._ 1857, ~43~, 976; see also Biltz, _Ber._ 1904, ~37~, 719 [246] Melikoff and Pissarjewski, _Zeitsch. anorg. Chem._ 1899, ~21~, 70.