[186] Proc. Roy. Soc., 1903, 72, 295.
The Arc Spectra.
—The final criterion of purity in the examination of a rare earth element is in almost all cases the arc spectrum. Since for some of the elements, especially in the yttrium group, the entire spectrum has not yet been accurately mapped out, spectra are generally observed frequently throughout the course of a fractionation; by this means, the separation can be followed by the disappearance of some lines, and the appearing or strengthening of others, and such examinations have led occasionally to the discovery of new elements (see, for example, under [Separation of ytterbium earths], p. 205). Such determinations, however, require much time and extensive and complicated apparatus.
Carbon electrodes are generally employed, and it is immaterial in this case which is the anode, and which the cathode. The lower carbon is hollowed out, and the space filled with the oxide or sulphate of the element or mixture to be examined; or the electrode may be impregnated with a concentrated solution of a salt. The light is examined by means of a diffraction grating, and the spectrum photographed on a plate which bears a comparison spectrum for measurement. The lines are most numerous in the violet and ultraviolet regions, and the most characteristic spectra are given by the colourless earths. The method is naturally more delicate for some elements than for others; the great persistency of the scandium line 3613·984, for example, was found very valuable by Crookes and by Eberhard in the examination of various rocks and minerals for that element, whilst other intense and persistent lines have served for the detection of various rare earth elements in the sun and many stars.
The Cathode Luminescence Spectra.
—The phenomenon of cathode luminescence, which was observed and very fully investigated by Sir William Crookes, and which led that author to his theory of Meta-elements, is one of the greatest scientific interest. Crookes observed that certain of the rare earths, when subjected to the action of cathode rays in a vacuum tube, exhibit a brilliant phosphorescence, which, when examined by the spectroscope, show characteristic spectra, which differ greatly for fractions of apparently identical chemical composition, and are otherwise distinguishable by physical properties. The researches of Lecoq de Boisbaudran, and the more recent work of Baur and Marc,[187] have shown that this luminescence is observed when a small quantity of a coloured earth is present with a very large quantity of a colourless earth, the maximum phosphorescence being produced by about 1 per cent. of the coloured earth, or ‘phosphorogen.’ The question has recently been very fully examined by Urbain.[188] He shows that the sensitiveness of the phenomenon is so great that it cannot be employed for the ordinary purposes of chemical analysis, one part in a million of the phosphorogen being sufficient to cause a clearly perceptible luminescence in a pure colourless oxide.
[187] Ber. 1901, 34, 878.
[188] Ann. Chim. Phys. 1909, [viii.], 18, 222; see also Introduction à l’étude de la Spectrochimie, pp. 145 et seq.
The Magnetic Susceptibility.
—The fact that the rare earths differ very considerably from one another in their magnetic properties has been known for several years,[189] and has recently been employed by Urbain and Jantsch[190] as a means of identification, and a test of purity, and for following processes of fractionation. The magnetic susceptibility reaches a minimum at samarium, and rises very sharply on either side of that element, so that the presence of the closely related elements, neodymium on the one side, and europium and gadolinium on the other, which differ only very slightly from samarium in atomic weight and solubility, can easily be detected by this means. The property is highly additive, and can be used, therefore, to estimate the relative proportions of two oxides in a mixture; the determinations are said to be easily and quickly carried out.