The _double ceric nitrates_[219] are a large and very important class of compounds; they are the most stable of the ceric salts. With nitrates of the monovalent metals, ceric nitrate forms double nitrates of the type R₂Ce(NO₃)₆; these are deep red hygroscopic substances, crystallising in the monoclinic system, readily soluble in water and alcohol, but dissolving only sparingly in nitric acid. The ammonium salt is important for the separation of cerium. A series of double nitrates with the nitrates of manganese, magnesium, zinc, nickel, and cobalt has the general formula RCe(NO₃)₆,8H₂O, but these are much less stable in solution than the alkali double salts. [219] Meyer and Jacoby, _loc. cit._ ATOMIC WEIGHT OF CERIUM No less than twenty-eight separate determinations of the atomic weight of cerium have been carried out. The earlier determinations are rendered unreliable by the almost certain presence of other elements, and Brauner[220] has shown that some of the methods employed in later work give erroneous results. [220] _Trans. Chem. Soc._ 1885, ~47~, 879; also _Zeitsch. anorg. Chem._ 1903, ~34~, 207. A very careful determination was made by Robinson in 1884.[221] Cerium oxalate was heated in a stream of dry hydrogen chloride, mixed with carbon dioxide, and the anhydrous chloride freed from traces of acid in a vacuum over chalk. The weighed chloride was then dissolved in water, and titrated with silver nitrate. He obtained the value 140·26; recalculation from his data with the modern values for silver and chlorine give 140·19. Brauner points out that this result is too low, since no account was taken of the solubility of silver chloride in water. In the following year, Brauner[222] determined the ratio Ce₂(SO₄)₃ : 2CeO₂, and obtained the atomic weight 140·22. Wyrouboff and Verneuil[223] in 1897 disputed Brauner’s work, and as a result of several determinations gave the values 139·21, 139·43, and 139·50; their determinations, however, varied very considerably, and the work has been severely criticised by Brauner. In 1903, the latter author and Batěk[224] obtained the values 140·21 and 140·27 by the sulphate and oxalate methods respectively; whilst in the same year, using the same methods, Brauner[225] obtained from three independent series of determinations the values 140·25, 140·24, and 140·25. [221] _Proc. Roy. Soc._ 1884, ~37~, 150. [222] _Loc. cit._ [223] _Compt. rend._ 1897, ~124~, 1300. [224] _Zeitsch. anorg. Chem._ 1903, ~34~, 103. [225] _Zeitsch. anorg. Chem._ 1903, ~34~, 207. The International Atomic Weight Committee have accepted the value 140·25 since 1904. DETECTION AND ESTIMATION OF CERIUM