As regards the higher oxide or the _thallic oxide_, Tl_{2}O_{3}, the thallium is trivalent in it--that is, it forms compounds of the type TlX_{3}. The hydroxide, TlO(OH), is formed by the action of hydrogen peroxide on thallous oxide, or by the action of ammonia on a solution of thallic chloride, TlCl_{3}. It is obtained as a brown precipitate, insoluble in water but easily soluble in acids, with which it gives thallic salts, TlX_{3}. Thallic chloride, which is obtained by cautiously heating the metal in a stream of chlorine, forms an easily fusible white mass, which is soluble in water and able to part with two-thirds of its chlorine when heated. An aqueous solution of this salt yields colourless crystals containing one equivalent of water. It is evident from the above that all the thallic salts can easily be reduced to thallous salts by reducing agents such as sulphurous anhydride, zinc, &c. Besides these salts, thallic sulphate, Tl_{2}(SO_{4})_{3},7H_{2}O, thallic nitrate Tl(NO_{3})_{3},4H_{2}O, &c., are known. These salts are decomposed by water, like the salts of many feeble basic metals--for example, aluminium. Gallium, indium, and thallium belong to the uneven series, and there should be elements of the even series in group III. corresponding with calcium, strontium, and barium in group II. These elements should in their oxides R_{2}O_{3} present basic characters of a more energetic kind than those shown by alumina, just as calcium, strontium, and barium give more energetic bases than magnesium, zinc, and cadmium. Such are _yttrium_ and _ytterbium_, which occur in a rare Swedish mineral called _gadolinite_, and are therefore termed the gadolinite metals. To these belong also the metal _lanthanum_, which accompanies the two other metals _cerium_ and _didymium_ in the mineral _cerite_, and it therefore belongs to the cerite metals. All these metals and certain others accompanying them, give basic oxides R_{2}O_{3}. At first their formula was supposed to be RO, but the application of the periodic system required their being counted as elements of groups III. and IV., which was also confirmed by the determination of the specific heats of these metals,[42] and better still by the fact that Nilson and Clève, in their researches on the gadolinite metals (1879), discovered that they contain a peculiar and very rare element, _scandium_, which by the magnitude of its atomic weight, Sc = 44, and in all its properties, exactly corresponds with the metal (previously foretold on the basis of the periodic system) _ekaboron_, whose properties were determined by taking the cerite and gadolinite metals as forming oxides R_{2}O_{3}.[43]