If nitrogen be substituted for hydrogen in either of the above methods of preparation, _nitrides_ of the general formula RN are obtained; cerium nitride, however, cannot be obtained by heating the element in the gas.[154] These compounds are also obtained when the carbides are heated in ammonia. They are amorphous solids, which yield ammonia when acted upon by water. [154] Dafert and Miklanz, _Monats._ 1912, ~33~, 911. ~Hydroxides.~--The hydroxides are thrown down as gelatinous precipitates on the addition of alkalies to hot dilute solutions of the salts; precipitation in the cold, or in strong solution, usually gives a basic salt, or an hydroxide mixed with a large quantity of basic salt. The hydroxides are insoluble in excess of precipitant, but the precipitation is inhibited by the presence of some organic hydroxy-acids.[155] [155] For effect of tartaric acid, see p. 133. The hydroxides are insoluble in water, but dissolve very readily in acids. The most basic of them absorb carbon dioxide from the air; lanthanum hydroxide is exceptional in that it colours litmus blue. Whilst hydrogen peroxide in neutral solution does not react with rare earth salts,[156] alkalies in presence of this reagent precipitate gelatinous hydrated peroxides, which are very unstable, decomposing on standing, or on treatment with acids, with evolution of oxygen. The general formula R₄O₉ + _x_H₂O was proposed for these compounds by Cleve, but more recently the formula R(OOH)(OH)₂ has been advanced.[157] [156] Compare behaviour of thorium and zirconium, Ch. XVI. [157] Melikoff and Pissarjewski, _Zeitsch. anorg. Chem._ 1899, ~21~, 70; Melikoff and Klimento, _Chem. Zentr._ 1902, ~1~, 172. ~Oxides.~--In their most stable state of oxidation, the rare earth elements are generally trivalent. In the case of cerium, the dioxide, CeO₂, is more stable than the sesquioxide Ce₂O₃, but the ceric salts are unstable, and are very readily reduced to cerous compounds, corresponding to the oxide Ce₂O₃. Higher oxides are known with certainty among the other elements only in the cases of praseodymium and terbium, but these do not give rise to salts. The oxides R₂O₃ are fairly strong bases, being comparable in strength to the alkaline earths, and far more strongly basic than alumina and oxides of other trivalent elements; thus they liberate ammonia from ammonium compounds, whilst the salts they form with strong acids are not easily hydrolysed. Their relative strengths as bases are expressed in the following series, in which the elements are placed in order of diminishing electropositive character:[158] La, Ce´´, Pr, Nd, Yt, Eu, Gd, Sa, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Ce^{iv}.