What was long known as _didymium_ (διδυμος = a twin) was discovered by Mosander in 1841. It owes its name to its close chemical relationship to, and almost constant association with, _lanthanum_—both elements occurring in many minerals, more particularly in _cerite_, _allanite_, and _monazite_. In 1885 Auer von Welsbach announced that the didymium of Mosander was, in reality, a mixture of two elements which could be separated by the systematic fractional crystallisation of the double ammonium nitrates; to these elements he gave the names _praseodymium_ (πράσινος, leek-green) and _neodymium_ (νέος, new). Neodymium salts are rose-coloured, whereas those of praseodymium are green, and the elements are further characterised by differences in their absorption and spark-spectra. When mixed, the substances give the spectrum originally considered to be characteristic of didymium. _Samarium_ was discovered in 1879 by Lecoq de Boisbaudran in _samarskite_. Its salts are yellow, and afford in solution characteristic absorption bands. It is not improbable that many of the minerals from which the so-called rare earths are obtained contain elements hitherto unrecognised, and it is possible that certain of the substances now assumed to be elements may, like didymium, turn out to be mixtures. In fact, additional elements have from time to time been announced, as for example, the _decipium_ of Delafontaine (1878) and the _monium_ or _victorium_ of Crookes (1899), pronounced by Urbain to be identical with gadolinium: their individuality cannot as yet be said to be established. Didymium itself was stated by Krüss and Nilson (1888) to be even more complicated than the work of Auer von Welsbach would seem to indicate, and to contain no fewer than eight elementary substances. As yet, however, no confirmation of this surmise has been obtained. The chemistry of the rare earths has of late years been greatly extended owing to the employment of certain of the members of the group in the manufacture of the “mantles” used in gas-lighting, and which consist substantially of thoria, mixed with about one per cent. of ceria. Large quantities of _monazite_, _thorianite_, _thorite_, _cerite_, and other minerals, are now worked up for the sake of the thoria and ceria they contain, and considerable amounts of residual products, consisting largely of other members of the family, are now available for investigation. It is reasonably certain, therefore, that our knowledge of this section of inorganic chemistry will be largely augmented in the immediate future. Indeed, the application of thoria to the construction of gas-mantles may be said to have removed that substance from the category of the rare elements. No sooner was it discovered that it was capable of useful application than unexpected sources of supply were found.