3082·15 3100·66 3422·62 3545·94 3549·52 3585·12 3646·36 3671·39 3719·63 3743·68 3768·60 3796·62 3814·18 3852·65 3916·70 4037·49 4050·05 4063·62 4070·51 4073·99 4085·73 4098·80 4130·59 4184·48 4251·90 4262·24 4325·83 4327·29 4342·35 6114·26 The spark spectra have been examined by Demarçay,[334] Baur and Marc,[335] Urbain[336] and Crookes.[337] [334] _Compt. rend._ 1900, ~131~, 343. [335] _Ber._ 1901, ~34~, 2460. [336] _Loc. cit._ [337] _Proc. Roy. Soc._ 1905, ~74~, 420. ~Terbium~, Tb = 159·2 Terbia occurs among the rare earth oxides in exceedingly small quantities, and its separation has in consequence presented such great difficulties that only within the last few years have terbium compounds been completely freed from gadolinium and neighbouring elements. In 1886 Lecoq de Boisbaudran,[338] by fractional precipitation of the hydroxides with ammonia, and subsequent fractional crystallisation of the double sulphates, obtained an oxide much richer in terbia than any specimen previously prepared; it was dark yellow in colour. In 1902 Marc[339] obtained from monazite a very dark oxide containing about 15 per cent. of terbia, whilst Feit[340] in 1905 obtained a dark brown oxide consisting of gadolinia with about 13 per cent. of terbia. Pure terbium compounds were obtained by Urbain in 1904,[341] by fractional crystallisation of the nitrate from nitric acid, in presence of bismuth nitrate, and by crystallisation of the double nickel nitrates, and precipitation with ammonia; he showed that the element was identical with the Z_{δ} and Z_{β} of de Boisbaudran,[342] with the Γ of Demarçay,[343] and with the G_{β} and possibly the G_{ζ} of Crookes[344] (see p. 193). [338] _Compt. rend._ 1886, ~102~, 395, 483. [339] _Ber._ 1902, ~35~, 2382. [340] _Zeitsch. anorg. Chem._ 1905, ~43~, 267. [341] _Compt. rend._ 1904, ~139~, 736; 1905, ~141~, 521; 1909, ~149~, 37. [342] _Ibid._ 1895, ~121~, 709; 1904, ~139~, 1015. [343] _Ibid._ 1900, ~131~, 343. [344] _Trans. Chem. Soc._ 1889, ~55~, 258.