5. The helium ratio has also been used, but appears less trustworthy, owing to escape of helium, and uncertainty as to geological age of the minerals employed.
6. Some connection between radioactivity and the presence of the yttrium or cerium metals appears highly probable, but no satisfactory theories have been advanced on this point; it has been shown that actinium is very closely allied to lanthanum.
PART II
THE CHEMISTRY OF THE ELEMENTS
CHAPTER IX
GENERAL PROPERTIES OF THE CERIUM AND YTTRIUM GROUPS
The chemistry of the rare earth elements begins in the year 1794, with Gadolin’s discovery of the new oxide ‘Ytterbia,’ for which the name Yttria was subsequently proposed by Ekeberg, and generally adopted (see [Chapter I], and under [Gadolinite], p. 35). The discovery of Ceria followed in 1804 (see under [Cerite], p. 32). The classical work of Mosander, carried out between 1838 and 1842, showed the complex nature of the new oxides. From ceria he separated three new earths, Ceria proper, Lanthana, and Didymia. Yttria was shown to be a mixture of at least three oxides, for which the names Yttria, Erbia, and Terbia were proposed. These oxides were believed to have the general formula RO, by analogy with the alkaline earths, which they were found to resemble in many respects, notably in their strongly basic character.
The properties of the new oxides were examined during the next twenty years by many chemists, the chief workers being Marignac, Rammelsberg, and Hermann, but the next important advance was the investigation of the absorption spectra of solutions of the rare earth salts, first suggested by Gladstone in 1856, and developed more fully by Bunsen and Kirchhoff in 1860 and the following few years. The introduction of the methods of spectrum analysis furnished a very delicate and valuable method of examining and identifying the various oxides, and so greatly assisted the laborious processes of separation.
Sixteen elements (excluding thorium and zirconium) are at the present time recognised as belonging to the rare earth group. With one or two exceptions, these show the closest resemblance to one another, both in chemical behaviour and in the properties of their compounds, so that the difficulties of separating and purifying them are very great. They may be said to form a series, in which the properties vary continuously but gradually from member to member, so that no sharp differences are anywhere perceptible. The method of division into groups is, therefore, almost entirely one of convenience, and has arisen from the course which the separations have followed.
The elements are divided into two chief families or groups, that of the cerium metals and that of the yttrium metals respectively. The cerium elements are separated by a process depending on the relative insolubility of their alkali double sulphates; in this group are included cerium, lanthanum, praseodymium, neodymium, and samarium. The yttrium family is further divided into four sub-groups: the first consists of scandium and yttrium; the second or terbium group of europium, gadolinium, and terbium; the third or erbium group of dysprosium, holmium, erbium, and thulium; and the fourth or ytterbium group of ytterbium and lutecium—the element celtium, recently discovered by Urbain, will also fall into this sub-group, but the discovery awaits confirmation. Whilst scandium and yttrium fall into somewhat abnormal positions, corresponding to their low atomic weights, the terbium elements occupy an intermediate position between the cerium elements and the remaining yttrium elements, or yttrium group proper, and so are frequently classified as a third or intermediate group.