This element is one of the rarest of the whole group, and occurs only in extremely small quantities. Monazite sand is said to contain about 0·002 per cent. of the oxide, though on account of the remarkable intensity of some of the stronger lines in the arc spectrum, Eberhard[313] was able to detect europium with ease in a mixture of rare earth oxides from that mineral, after the separation of cerium. The _oxide_ has a pale rose colour; the salts are also faintly coloured, and in solution show weak absorption bands. [313] _Zeitsch. anorg. Chem._ 1905, ~45~, 378. _Europium sulphate_, Eu₂(SO₄)₃,8H₂O, separates in pink crystals, which are completely dehydrated at 375°; _europic chloride_, EuCl₃, in the anhydrous state forms fine yellow needles; _europium oxychloride_, EuOCl, prepared by heating europic chloride in dry air to 600°, is a white solid, insoluble in water, but soluble in strong acids; _europous chloride_, EuCl₂, prepared by reduction of the higher chloride in hydrogen, is a white amorphous solid, soluble in water to a neutral solution, which on boiling throws down the oxide, Eu₂O₃.[314] Several organic salts have been prepared by James and Robinson.[315] [314] Urbain and Bourion, _Compt. rend._ 1911, ~153~, 1155. [315] _J. Amer. Chem. Soc._ 1913, ~35~, 754. ~Atomic Weight.~--Using the material isolated from samaria, Demarçay[316] in 1900, by the synthetic sulphate method, found the atomic weight of europium to be about 151. Urbain and Lacombe[317] determined the value in 1904, with material free from gadolinium and samarium, using the three ratios which they employed in the case of the latter element (see p. 182); their values, corrected by Brauner, were 152·00, 151·93 and 151·94 respectively. Another series of determinations was carried out by Jantsch[318] in 1908, the same method being employed; he obtained the mean value 152·03, with an error of ±·02. The International Committee have adopted the value 152·0. [316] _Compt. rend._ 1900, ~130~, 1469. [317] _Ibid._ 1904, ~138~, 627. [318] _Ibid._ 1908, ~146~, 473. ~Detection.~--The absorption spectrum was determined by Demarçay,[319] but is not sufficiently intense or characteristic for ordinary purposes of detection. The spark spectrum has been investigated by the same author (_loc. cit._); it is very bright, and shows the three blue rays which characterised Lecoq de Boisbaudran’s Z_{ε}. The reversal spectrum shows the characteristic band of Z_{ξ}. The pure oxide, according to Urbain,[320] shows no luminescence under the influence of cathode rays, but when impure, or very largely diluted with lime or gypsum, it gives very bright and characteristic spectra. [319] _Ibid._ 1900, ~130~, 469. [320] _Ibid._, 1906, ~142~, 205, 1518.