The _oxide_ is colourless, and forms colourless salts with those acids in which the anion is not coloured. The oxide is distinguished from the other rare earth oxides in that it turns moistened litmus paper blue; it resembles lime, in hissing when slaked, absorbing carbon dioxide from the air, and liberating ammonia from ammonium salts. By fusion with alkali carbonates, and by digestion with concentrated alkali hydroxides, Baskerville and Catlett[247] claim to have obtained lanthanates and metalanthanates, but their work has not yet been confirmed. [247] _J. Amer. Chem. Soc._ 1904, ~26~, 75. The _sulphate_, La₂(SO₄)₃,9H₂O, is the least soluble of all the rare earth sulphates. The enneahydrate is the only form stable at ordinary temperatures,[248] though under special conditions, hydrates with 6 and with 16 molecules of water of crystallisation have been obtained. It separates in needles belonging to the hexagonal system; 100 parts of water dissolve at 0°, 3·01, and at 100°, 0·69 parts of the salt. The _acetylacetone compound_ melts at 185°. [248] Muthmann and Rölig, _Ber._ 1898, ~31~, 1718. A large number of other lanthanum compounds have been prepared, but these are so typical of the rare earth salts generally that no detailed treatment is required; for a full account of them, the reader is referred to Abegg’s classical handbook. ~Atomic Weight.~--A large number of determinations of this constant have been made, but the results even of recent investigations do not agree so closely as might be desired. The value adopted by the International Committee, 139·0, is based on the work of Brauner and Pavliček,[249] carried out in 1902. These authors give an account of all the determinations made up to that date, with critical discussion of the methods employed and the possible sources of error. The more important investigations have been based on the ratio La₂O₃ : La₂(SO₄)₃, for the determination of which the most stringent precautions must be taken. The synthetic method has generally been employed, on account of the tenacity with which the oxide clings to traces of sulphuric anhydride. In this method, the total decomposition of the acid sulphate, and the protection of the very hygroscopic sulphate, La₂(SO₄)₃, from atmospheric moisture, constitute the chief difficulties. By this method, H. C. Jones[250] in 1902 obtained a result (138·76) considerably lower than the value found by Brauner and Pavliček (_loc. cit._) A later research by Brill,[251] who carried out a synthetic sulphate determination on a minute scale, using a Nernst microbalance, gave the value 139·5, which, whilst considerably higher than either of the other figures, shows that Brauner and Pavliček’s number can hardly be too high. [249] _Trans. Chem. Soc._ 1902, ~81~, 1243. [250] _Amer. Chem. J._ 1902, ~28~, 23. [251] _Zeitsch. anorg. Chem._ 1906, ~47~, 464.