The _formate_ and _acetate_ can be obtained in the form of neutral salts by the action of the acids on the hydroxide; by double decomposition, amorphous precipitates of basic salts are obtained. With _tartaric acid_ stable complex compounds are formed, as shown by the fact that alkalies will not precipitate the hydroxide from a solution in presence of that reagent, and by the elevation of the specific rotatory power. Many _complex salts_ are known, the simplest having the composition ThO(C₄H₄O₆R´)₂,8H₂O, where R´ = K,Na,NH₄; these are obtained by dissolving thorium hydroxide in concentrated solutions of alkali hydrogen tartrates. _Thorium acetylacetone_, Th(C₅H₇O₂)₄, is precipitated by addition of ammonia to an aqueous solution of the nitrate mixed with acetylacetone dissolved in ammonia; the solid is recrystallised from alcohol, and melts at 171°. ~Atomic Weight of Thorium.~--The value adopted by the International Committee (1914) is 232·4, but most of the determinations carried out within the last thirty years show considerable discrepancies. The earlier work of Berzelius (1829) and Chydenius (1861) led to very widely varying results, and for the same reason little reliance can be placed on the results of Delafontaine (1863) and Hermann (1864). In 1874 Cleve determined the constant by ignition of the sulphate, obtaining the mean values 234·03 and 233·97; the figure 234 based on these results was for many years accepted as the true atomic weight. A series of determinations carried out by Nilson in 1882 led to much lower results. He employed the sulphate ennea- and octohydrates, first dehydrating these, and then igniting to oxide, and showed that Cleve’s value must be too high on account chiefly of the hygroscopic nature of the ignited oxide, which increases in weight when kept; but his own values show considerable discrepancies. The ratio Th(SO₄)₂,9H₂O-ThO₂ : ThO₂ (enneahydrate converted to oxide) gave the figure (corrected to vacuo) 232·51, whilst the ratio ThO₂ : 2SO₃ (anhydrous sulphate to oxide) gave 232·16; the ratio Th(SO₄)₂ : 9H₂O (hydrate to anhydrous salt) gave, however, 233·75. The value obtained for the ratio ThO₂ : 2SO₃ for anhydrous sulphate prepared from the octohydrate was 232·49 (corrected to vacuo). Five years later, Krüss and Nilson prepared the anhydrous sulphate from the pure octohydrate, and ignited this to the oxide. The ratio ThO₂ : 2SO₃ gave as a mean of very concordant results the figure 232·49.