—The determinations of the earlier workers, being carried out with impure material, gave results which differ very widely, and are quite unreliable. Cleve’s value of 1880, for material free from ytterbia, but not apparently free from earths of lower equivalent, was 166·25; Brauner,[378] using the same material in 1905, obtained the much higher value 167·14. The determinations of Hofmann and Burger[379] in 1908 gave the mean value 167·38; with purer material, Hofmann in 1910[380] obtained the mean value 167·68, on which is based the value accepted by the International Committee, 167·7.

[378] Abegg, III. i. 318.

[379] Loc. cit.

[380] Loc. cit.

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:

667weak492
654strong487strong
541very weak450
523very strong442weak
519shadowy

[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:

3230·733692·85
3264·913896·40
3312·563906·47
3372·923938·79
3499·28