A further illustration of the value of the principle of periodicity, as developed by Mendeléeff, in indicating the existence of new elements, is seen in the discovery of _germanium_. In 1885 Weisbach discovered a new Freiberg silver mineral, to which he gave the name _argyrodite_. This on analysis by Winkler was found to contain a new element to the extent of about seven per cent. with properties identical with those predicted by Mendeléeff for a missing element in the fourth group of the periodic series, consisting of silicon, tin, and lead, and which he had provisionally termed _eka-silicon_. _Argyrodite_, in fact, is a double sulphide of silver and germanium, 2Ag2S.GeS2. Germanium is a greyish-white, lustrous metal of sp.gr. 5.5., melting at about 900°, and resembling silicon and tin in its general chemical relations. _Dysprosium_, _europium_, _gadolinium_, _lutecium_, _neodymium_, _praseodymium_, _samarium_, _thulium_, and _ytterbium_ (_neoytterbium_) belong, like scandium, to the group of the so-called rare earth metals. These substances have been detected in a great variety of minerals, many of which are extremely rare. The elements most frequently occur in nature associated with yttrium, cerium, thorium, and zirconium. _Dysprosium_ was first detected, in 1886, by Lecoq de Boisbaudran in the so-called erbium earth of Mosander, in which Cleve had previously (1880) announced the existence of two other elements, _holmium_ and _thulium_. There is some reason to believe that the holmium of Cleve is identical with dysprosium. _Ytterbium_ was discovered by Marignac, in 1878, in the mineral _gadolinite_. In 1906 Auer von Welsbach announced that Marignac’s ytterbia was a mixture, which was confirmed in the following year by Urbain, who separated it into two elements, which he named _neoytterbium_ and _lutecium_. _Europium_ was discovered by Demarçay in 1901. All these earths are met with in small quantities associated with yttria in _gadolinite_, _euxenite_, _samarskite_, _xenotime_, _cerite_, _orthite_, and other similar minerals. Their compounds, or such of them as have been described, resemble the corresponding compounds of yttria. They are recognised by differences in their spectroscopic behaviour. _Gadolinium_ was detected, independently, in 1886, by Marignac and Lecoq de Boisbaudran in the terbium earth of Mosander.