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.
Detection.
—Pure lanthanum compounds show no absorption in the visible region, and the pure oxide gives no cathode luminescence. The emission spectra show very characteristic lines in the violet and ultraviolet. The chief lines are:
| 3949·27 | 4238·55 | 6250·14 | 6394·46 |
| 3988·69 | 4333·98 | 6262·52 |
For arc spectra see Exner and Haschek; Eder and Valenta.[252]