The complicated history of the terbium group has been entirely cleared up by the work of Urbain and his co-workers during the early years of the present century, and processes have been devised by which the separation of the three members of the group from one another, and from the related elements of the erbium group on the one side, and samarium on the other, can be satisfactorily accomplished. The chemistry of this group, therefore, may be regarded as satisfactorily settled, though relatively little is known of the properties of the elements and their compounds. In their general chemical relations, elements of the terbium group occupy an intermediate position between the cerium group and the elements of the yttrium group in the narrower sense. In the solubility relations of the double salts, they are bounded on the one side by samarium and the less soluble cerium group, on the other by dysprosium and holmium and the more soluble yttrium group. They show only very slight differences in electropositive character, and methods based on differences in basic strength of the oxides, therefore, are of very little use for separating them from one another. Fractional precipitation with ammonia separates them in the order terbium, samarium, gadolinium, and europium--samaria being less strongly basic than the oxides of gadolinium and europium; this constitutes an exception to the general rule regarding the solubilities of the double nitrates and sulphates with increasing electropositive character.[310] The difficulties of separation are greatly increased by the very small proportions in which the elements are usually found in rare earth minerals. Gadolinium usually occurs in the largest quantities; in consequence of this, there is little doubt that most of the material described by the earlier workers as terbia consisted very largely of gadolinia. [310] See Lecoq de Boisbaudran, _Compt. rend._ 1890, ~111~, 394. The group is not characterised by well-marked absorption spectra; europium and terbium show weak absorption in the blue region. Terbium, of which the salts are colourless, forms a very strongly coloured peroxide, analogous to that of praseodymium; small quantities of this give to the mixed oxides obtained by ignition the characteristic yellow colour, whilst mixtures richer in the peroxide become correspondingly darker and darker. SEPARATION