—The determinations of this constant made by the earlier workers were all carried out with impure material and gave results which were considerably too low. The International Committee have adopted the value 157·3, which is based on the work of Urbain.[331] In employing the analytical sulphate method, that author observed that the anhydrous sulphate did not remain constant in weight when allowed to remain in a desiccator, and that it could not be accurately weighed. He therefore determined the ratio Gd₂(SO₄)₃,8H₂O : Gd₂O₃, by converting the octohydrate directly to oxide, and obtained the mean value 157·24.
[331] Compt. rend. 1905, 140, 583.
Detection.
—Pure gadolinium compounds show no absorption in the visible spectrum, but there are four strong bands[332] in the ultraviolet, viz. 311·6-310·5; 306·0-305·7; 305·6-305·5; and 305·4-305·0. The arc spectrum[333] is very rich in lines, of which the most intense are the following:
| 3082·15 | 3719·63 | 4050·05 | 4184·48 |
| 3100·66 | 3743·68 | 4063·62 | 4251·90 |
| 3422·62 | 3768·60 | 4070·51 | 4262·24 |
| 3545·94 | 3796·62 | 4073·99 | 4325·83 |
| 3549·52 | 3814·18 | 4085·73 | 4327·29 |
| 3585·12 | 3852·65 | 4098·80 | 4342·35 |
| 3646·36 | 3916·70 | 4130·59 | 6114·26 |
| 3671·39 | 4037·49 | ||
[332] Urbain, ibid. 1905, 140, 1233.
[333] Exner and Haschek; Eder and Valenta, Sitzungsber. kaiserl. Akad. Wiss. Wien, 1910, 119, IIa, 21.
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.