The Euxenite-Polycrase series was studied by Hauser and Wirth in 1909,[71] in an endeavour to establish their theory that the proportions in which the various earths and acids occur in this group of minerals is subject to definite laws beyond the ordinary laws of combination. Thus of the erbia earths they state that the proportion of holmia and dysprosia increases relatively to erbia as titanium dioxide increases, _i.e._ as we pass from the euxenites to the polycrases; at the same time scandia and yttria increase relatively to the other yttria earths (the terbia group), whilst in the ceria group samaria and praseodymia decrease relatively to the others. Thus samaria is found in appreciable quantities only when the titanium content is low. The original paper must be consulted for full details. [71] _Ber._ 1909, ~42~, 4443. It was stated above that zirconium was found in euxenite in 1879. In 1901 Hofmann and Prandtl[72] declared that zirconia was an unfailing constituent of the mineral, and that it was always accompanied by a new oxide, which they named Euxenia (‘Euxenerde’). This was characterised by the solubility of its oxalate in acid solutions, the insolubility of the precipitated hydroxide in excess of alkali, and the gradual precipitation by hydrogen peroxide from a slightly acid solution of its salts. In their paper quoted above, Hauser and Wirth state that zirconia is never present in typical euxenites. In a second paper[73] they state that after exhaustive treatment of every known zirconia mineral, they can find no trace whatever of the ‘new earth,’ and conclude that Hofmann and Prandtl must have made some experimental error. During this examination, they observed radioactivity in some minerals which contained no traces of uranium or thorium. [72] _Ibid._ 1901, ~34~, 1064. [73] _Ber._ 1910, ~43~, 1807. ~Risörite.~[74]--A columbate of yttria earths, with titanium; ferric oxide, alumina, lime and lead monoxide are present in small quantities. It resembles fergusonite in composition, but differs in the almost complete absence of uranium, the high loss on ignition, and the amount of titanium present, which is here considerable (TiO₂ = 6·5 per cent.). Hauser regards it as an orthocolumbate, R´´´(Cb,Ta)O₄, with an isomorphous admixture of metatitanate, R´´´₂(TiO₃)₃. [74] Hauser, _Ber._ 1907, ~40~, 3118; _Zeitsch. anorg. Chem._ 1908, ~60~, 230. The rare earths are chiefly yttria, with some erbia earths and a little terbia; ceria, lanthana and didymia are also present. The mineral contains a considerable amount of helium, which is remarkable in view of the very small content of uranium and thorium (cf. Thalenite). It is radioactive, the active constituent being precipitated with the lead (and to a very small extent with the rare earths).