The amorphous variety is fairly stable in air, but burns vigorously when heated in air, oxygen, or halogens. Heated in nitrogen or ammonia, it reacts vigorously, forming the nitride TiN; if carbon is present, a peculiar substance of uncertain composition, known as _titanium cyanonitride_, is formed. This substance is also obtained when air is passed over a heated mixture of the dioxide with coke, and is found in blast-furnaces in which ores containing small quantities of titanium are worked; it forms brilliant red cubes, which are extremely hard and resistant to acids. This substance, as well as the nitride itself, yields ammonia when heated in steam, and has been proposed as a medium for ‘fixing’ atmospheric nitrogen (see p. 337). The amorphous element also absorbs hydrogen, when heated in the gas, but no definite hydride is known. It combines when heated with almost all the known non-metals, and forms alloys with all the common metals. Moissan[440] claims to have prepared a compound as hard as diamond by heating titanium with boron in the electric furnace. The element attacks steam at a red heat. [440] _Loc. cit._ The element is fairly resistant to acids in the cold, but is readily attacked, with evolution of hydrogen, on warming. Hot dilute hydrochloric acid gives the trichloride; but dilute sulphuric acid is variously reported to give the di- and tri-salt. Hot nitric acid oxidises it readily, forming the so-called metatitanic acid. Hydrofluoric acid attacks it very readily, forming the tetrafluoride. COMPOUNDS OF DIVALENT TITANIUM. The compounds of divalent titanium show resemblances to those of divalent iron, chromium and vanadium, but on account of the great difficulty of preparing them and protecting them from oxidation, little is known of their properties and behaviour; even the colour of the salts in solution is not known with certainty. In its divalent state; the element does not appear to act as a strongly positive metal; the salts in solution are said to show an acid reaction, whilst the precipitates thrown down with alkali oxalates and acetates are soluble in excess of the precipitant, forming deeply coloured solutions. With sodium phosphate the soluble salts give a bluish-black precipitate, with potassium ferrocyanide and ferricyanide, dark brown and reddish-brown precipitates respectively. They are distinguished from salts of the higher oxides of titanium by the brown colouration produced by potassium thiocyanate in presence of hydrochloric acid.[441] [441] v. d. Pfordten, _Annalen_, 1886, ~234~, 257; 1887, ~237~, 201; see also _Ber._ 1889, ~22~, 1485.