The _bromate_, Sa(BrO₃)₃,9H₂O, melts at 75°, and closely resembles the corresponding compounds of the didymium metals. The _sulphate_ crystallises with 8, and the _nitrate_ with 6 molecules of water. The _carbonate_, Sa₂(CO₃)₃,3H₂O, can be obtained only by passing carbon dioxide through an aqueous suspension of the hydroxide; addition of alkali carbonate to a solution of a samarium salt precipitates hydrated double carbonates. The _acetylacetone compound_ melts at 146°-147°C. Many organic salts have been prepared by James, Hoben, and Robinson (_loc. cit._). ~Atomic Weight.~--The earlier determinations of this constant were carried out with material not entirely free from europium. Demarçay[296] carried out a synthetic sulphate operation with the material which he obtained free from europium in 1900, and found values between the limits 147·2 and 148·0. The International Committee has adopted the value 150·4, which is based on the work of Urbain and Lacombe[297] in 1904. These authors made determinations of three series of ratios, obtained by (_a_) conversion of sulphate octohydrate to anhydrous sulphate, (_b_) conversion of anhydrous sulphate to oxide, and (_c_) conversion of sulphate octohydrate to oxide; these gave the values 150·314, 150·533, and 150·484 respectively, from which the mean atomic weight is 150·44.[298] [296] _Loc. cit._ [297] _Compt. rend._ 1904, ~138~, 1166. [298] These numbers are calculated by Brauner (Abegg’s _Handbuch_, III. i. p. 285) on the basis O = 16, S = 32·06, H = 1·0076, and are somewhat higher than those given by Urbain and Lacombe, who used the round numbers O = 16, S = 32, and H = 1. ~Detection.~--The absorption spectrum of samarium compounds is only visible in fairly concentrated solutions, so that the element cannot usually be detected in a mixture by this means. The position of the maxima of the strongest bands (Demarçay, _loc. cit._) are: 476 463 417 402 These are all in the blue and violet regions; the first and second are in the neighbourhood of neodymium and europium bands (_q.v._), and in concentrated solutions the bands would partially coincide. Since these are the two elements from which the separation is most difficult, and are moreover the most constant in their occurrence with samarium, the absorption spectrum is of very little use as a test. The arc spectrum is very rich in lines,[299] of which the most intense are: 3739·30 4152·38 4203·18 4225·48 4229·83 4236·88 4256·54 4319·12 4329·21 4334·32 4347·95 4391·03 4420·72 4421·32 4424·55 4434·07 4434·52 4452·92 4454·84 4458·70 4467·50 4519·80 4524·08 4544·12 4566·38 4577·88 4642·41 4674·79 [299] Exner and Haschek; Eder and Valenta; Rütten and Mersch, _Zeitsch. wiss. Photochem._ 1905, ~3~, 181. CHAPTER XIII THE TERBIUM GROUP