Thorium, like zirconium, forms only one series of salts, in which the metal is tetravalent. The formula ThO was originally put forward by Berzelius for the oxide, from its resemblance to the ceria and yttria oxides, and its general occurrence with these. The true formula was deduced, when the valency of zirconium had been decided by the vapour density experiments of Troost and Deville, in 1857, from the isomorphism of zircon and thorite, and the close relationship between the compounds of the two elements, especially among the double fluorides, and was confirmed by a determination of the specific heat of the metal by Nilson in 1883. In its chemical relations, the element resembles zirconium, though, as is to be expected from the high atomic weight, it shows a much more marked electropositive character, approaching in this respect the elements of the yttrium group. The oxide has no longer acid properties, and the neutral salts, though they hydrolyse readily and are therefore acid to indicators in solution, may be recrystallised unchanged from aqueous solution. The tendency to form double salts is still present, though diminished; the oxalate is soluble in a large excess of alkali oxalate, but not in oxalic acid, and the double fluorides are less numerous and varied than those of zirconium and titanium. On the other hand, it forms a well-crystallised and characteristic series of double nitrates, R´₂Th(NO₃)₆, isomorphous with the analogous ceric salts. In the behaviour of its sulphate it differs markedly from zirconium, and closely approaches the rare earth elements. The hydroxide has the characteristic tendency to form colloidal solutions and gels. Thorium is peculiar, among the elements which have been considered, through its property of giving characteristic radiations, and disintegrating with formation of a whole family of new elements; or, as it is commonly expressed, through its radioactive properties.[474] The element has a half-life period of the order of 4 × 10¹⁰ years; in the course of decay, it gives rise to mesothorium 1, which is rayless, but decays to mesothorium 2, with its product radiothorium, both of which give powerful radiations. Mesothorium 1 of course occurs in all thorium-containing minerals, and may be separated from monazite by addition of a barium compound during the sulphuric acid decomposition; in consequence of the powerful radiating properties of its products, it is itself of considerable importance, and proposals for extracting it from monazite in the preparation of the thorium nitrate of commerce have been put forward (see p. 276). [474] The nature of the present work allows only the briefest reference to be made to the exceedingly interesting phenomena which centre about this subject; for a more complete account, the student should consult Soddy, _The Chemistry of the Radio-Elements_, Part I, 1911.