~Atomic Weights.~--The values determined by Urbain (_loc. cit._) for the fractions obtained by the nitrate method gave the number 170·1 for the least soluble fraction free from terbium, and 173·4 for the most soluble fraction. Auer von Welsbach (_loc. cit._) obtained the values 172·9 and 174·2 for the least soluble and most soluble fractions from the double oxalate crystallisation respectively. More recently[393] he has determined these constants with highly purified material, employing a modified method. The weighed anhydrous sulphates are transformed into the oxalates, which are then ignited to the oxides. He obtained the values Yb = 173·00, Lu = 175·00. [393] _Monats._ 1913, ~34~, 1713. The values adopted by the International Committee are Yb = 172·0 and Lu = 174·0. ~Spectra.~--The spark spectra are of more use in distinguishing the two elements than the arc spectra. The spark spectrum of the old ytterbium was mapped by Exner and Haschek,[394] and of the two compounds by both discoverers (_loc. cit._). See also Eder and Valenta.[395] [394] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1899, ~108~, II_a_, 1123. [395] _Ibid._ 1910, ~119~, II_a_, 3. The arc spectra have been mapped by Eder and Valenta (loc. cit.) and by Exner and Haschek; the latter authors give as the most intense lines the following: Yb Lu ┌────────────┴──────────────┐ 3031·26 2615·50 3397·21 4124·87 3107·99 2911·53 3472·65 4184·40 3289·50 3077·75 3507·57 4518·74 3464·47 3198·27 3508·55 5476·88 3988·16 3254·45 3554·58 5983·92 5556·67 3281·89 3568·00 5984·32 3312·30 3624·10 6222·10 3359·74 3636·41 6463·40 3376·69 3876·80 ~Celtium~ The separation of Marignac’s ytterbium into the two elements described above was accomplished by Urbain with the yttria earths extracted from xenotime. In carrying out the same process with the ytterbia earths from gadolinite, that author[396] obtained from the mother-liquor an earth for which the coefficient of magnetisation was found to be 4·1 × 10⁻⁶; lutecia has a coefficient three to four times as great. A spectroscopic examination revealed the presence of lines which did not correspond with those of any known body, and Urbain considered that a new element, for which he proposed the name Celtium, with the symbol Ct, must be present. Lutecia from xenotime shows no trace of the new element. [396] _Compt. rend._ 1911, ~152~, 141. Spectroscopic evidence for the existence of a third ytterbium element had previously been brought forward by Auer von Welsbach[397] and also by Exner and Haschek.[398] [397] _Monats._ 1908, ~29~, 204. [398] Exner and Haschek, _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 771.