~Detection.~--Salts of erbium give in solution absorption spectra which are well defined and highly characteristic, though not so intense as those of praseodymium and neodymium. Hofmann and Bugge[381] give the following absorption maxima for a 10 per cent. solution of their pure nitrate in a layer of 15 mm. thickness: 667 weak 654 strong 541 very weak 523 very strong 519 shadowy 492 487 strong 450 442 weak [381] _Ber._ 1908, ~41~, 3783. The arc spectrum has been mapped by Eder and Valenta[382] and Exner and Haschek. The following lines are used by Eberhard[383] for purposes of detection: 3230·73 3264·91 3312·56 3372·92 3499·28 3692·85 3896·40 3906·47 3938·79 [382] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 18. [383] _Publ. astrophys. Observ. Potsdam_, 1909, ~20~, No. 60. ~Thulium~, Tm = 168·5 The thulia isolated in 1879 was described by Cleve[384] as pale rose in colour; in the following year, having obtained it in larger quantity, he found that it was white, and dissolved in acids to form colourless solutions which showed absorption bands in the red and blue. The spectra of the thulium compounds prepared by Cleve were examined by Thalèn,[385] who concluded that a new element was certainly present, though it had not been freed from ytterbium and erbium. Incidental observations on the new oxide were made by various investigators, but no extensive researches were carried out upon it until 1911, when James[386] published an account of the separation and purification by the bromate method, stating that after some 15,000 operations, his products remained unaltered; he gives, however, no spectroscopic determinations, though part of his material, spectroscopically examined by Sir William Crookes, was described as ‘Very good thulium, with a trace of ytterbium.’ In the same year Auer von Welsbach[387] published an account of a spectroscopic investigation, as a result of which he concludes that thulium is a mixture of at least three elements, of which the second, Tm II, agrees fairly well in properties, so far as the two accounts allow of comparison, with the thulium of James. [384] _Loc. cit._ [385] _Compt. rend._ 1880, ~91~, 376. [386] J. _Amer. Chem. Soc._ 1911, ~33~, 1333. [387] _Zeitsch. anorg. Chem._ 1911, ~71~, 439.