Euxenite and Polycrase are members of an isomorphous series and vary considerably in composition. The composition of the series is that of mixed columbates and titanates of yttria earths (with, as usual, some ceria earths), with uranium and zirconium, and water. Before the isomorphous relation was recognised, Rammelsberg gave for Euxenite the formula R´´´(CbO₃)₃,R´´´₂(TiO₃)₃,1¹⁄₂H₂O. The ratio of the acidic oxides, Cb₂O₅ : TiO₂, is here 1 : 2. This is the greatest value of the ratio, which varies for the series between 1 : 2 and 1 : 5.[69] The end members, the pure metacolumbate and pure metatitanate respectively, are unknown; all the members occurring in nature are to be regarded as mixtures of these within the limits set by the ratios ¹⁄₂ and ¹⁄₅. Brögger[70] suggests that the name Euxenite be retained for all members for which the ratio is between ¹⁄₂ and ¹⁄₃, whilst for those minerals in which it is less than ¹⁄₄ the name Polycrase be kept; these views have been supported by Lange, who has analysed members of the series. [69] Lange (_Abstr. Chem. Soc. 1911_, ~100~, ii. 499) gives the limits ¹⁄₂ and ¹⁄₆. [70] _Abstr. Chem. Soc. 1907_, ~92~, ii. 885. The members of this isomorphous series, however, are themselves dimorphous, that is, can each crystallise in two different ways. The second form corresponding to the Euxenites is known as Priorite, whilst that corresponding to Polycrase is known as Blomstrandine; and these second forms are themselves members of a parallel isomorphous series of the same chemical composition, of course, as the first series. It is, perhaps, undesirable to cite this as a typical example of an isodimorphous series, since no end members of unmixed composition are known. A perfect example of such a series is furnished by the oxides of antimony and arsenic. Each of these compounds exists in two distinct crystalline varieties, antimony trioxide, Sb₂O₃, as Valentinite (orthorhombic) and Senarmontite (cubic), arsenic trioxide, As₂O₃, as Claudetite (orthorhombic) and Arsenolite (cubic); and these two modifications are isomorphous with one another, senarmontite with arsenolite, and valentinite with claudetite. In the case we are considering, the name Euxenite is applied to one crystalline modification (A) of a number of isomorphous compounds within certain limits of composition, the name Priorite to the second crystalline modification (B) of the same compounds; the name Polycrase is applied to compounds having the crystal form A, and a composition varying within a second set of limits in the same chemical series, whilst this second set of compounds in the crystalline form B is known as Blomstrandine.