The amorphous impure metal is a dark grey powder, of specific gravity 11·3; the hammered and strongly heated leaf has the density 12·16. It burns readily in air with great brilliance, and when finely powdered ignites if crushed or rubbed. When heated in the electric furnace, it melts, according to von Bolton,[476] at about 1450°; von Wartenburg[477] found the melting-point to be about 1700°; the fused beads resemble platinum in physical properties. It is somewhat resistant to acids, dissolving easily only in aqua regia, and more slowly in fuming hydrochloric acid. It combines directly when heated in sulphur or halogens, and in nitrogen and hydrogen. [476] v. Bolton, _Zeitsch. Elektrochem._ 1908, ~14~, 768. [477] _Ibid._ 1909, ~15~, 866. The _hydride_, ThH₄, is best obtained by heating the metal in hydrogen, an energetic reaction taking place at a red heat. Winkler observed that a mixture of the dioxide with magnesium absorbs hydrogen readily when heated. The hydride is a stable greyish-black powder, not attacked by water, but dissolving readily in hydrochloric acid, with evolution of hydrogen. The _nitride_, Th₃N₄, is prepared by heating the metal in the gas, or the carbide in a stream of ammonia. It is a brown powder, decomposed by water with evolution of ammonia and formation of the dioxide. The _azide_ has been used for purposes of detection and estimation, since in boiling solution it is hydrolysed with separation of the hydroxide; zirconium and ceric salts also show this reaction, but the rare earth salts give no precipitate. The _hydroxide_, Th(OH)₄,_x_H₂O, is precipitated from solutions of thorium salts by alkalies or ammonia, as a gelatinous white precipitate, insoluble in excess. It dissolves readily in mineral acids or in alkali carbonates. Hydrogen peroxide and ammonia throw down an hydrated _peroxide_, Th₂O₇; from neutral solutions hydrogen peroxide alone throws down _peroxy-salts_, which contain acid groups. This peroxide may also be obtained by the action of sodium hypochlorite or hydrogen peroxide on the hydroxide, as in the case of the zirconium compound. It readily gives up oxygen, passing into the more stable peroxide, ThO₃. Since in neutral or faintly acid solutions zirconium and the rare earths give no precipitate with hydrogen peroxide, the reaction is extremely useful in the detection and estimation of thorium.