The amorphous metal is a dark powder, which when washed with water on the filter paper passes through as a dark blue colloidal solution; it burns readily when heated in the air. According to Wedekind and Lewis,[459] amorphous zirconium is really the colloidal form of the metal. The fused metal is very hard (7-8, Mohs’ scale--it scratches quartz but not topaz) and very brittle; it has the density 6·4, and is of a whitish colour, with good metallic lustre on freshly broken surfaces. The atomic heat is abnormally high, being approximately 7·3; the element is paramagnetic. The melting-point was given by Wedekind and Lewis[460] as 2330°-2380°, but later work of the former author[461] gives the much lower value of 1530°, which seems more probable in view of the fact that the element cannot be employed for electric lamp filaments (see p. 322). [459] _Ibid._ 1910, ~371~, 367. [460] Weiss and Neumann, _loc. cit._; also Wedekind, _loc. cit._ [461] _Annalen_, 1913, ~395~, 149. Metallic zirconium is highly resistant to acids; it is attacked only by hydrofluoric acid and by aqua regia. In the compact form it burns in the air only at very high temperatures, though when powdered it glows in the air at a red heat, forming probably a mixture of lower oxides. It is attacked by chlorine and by hydrogen chloride at a red heat, with formation of the chloride; fused potash also oxidises it, with evolution of hydrogen. When heated in a current of hydrogen at a red heat, it forms the _hydride_, ZrH₂,[462] as a velvet-black powder, which burns with an intense bluish flame in oxygen, forming the sesquioxide, Zr₂O₃. When heated in nitrogen or ammonia, amorphous zirconium yields _nitrides_, which are also obtained when any attempt is made to reduce zirconium compounds to the metal in air. The most definite is the compound Zr₂N₃,[462] which forms a bronze-coloured powder, resistant to all mineral acids except hydrofluoric acid. Chlorine and bromine transform this to the halide. [462] Wedekind and Lewis, _Annalen_, 1910, ~371~, 367.