The element gives the white _sesquioxide_, Tb₂O₃, and colourless salts.[345] The _peroxide_, of which the composition corresponds approximately to the formula Tb₄O₇, is obtained as a brownish-black powder by ignition of suitable salts. Its presence, even in small quantities, gives so deep a colouration to the other earths that some kind of salt formation seems probable. It is insoluble in cold acids; it dissolves in hot nitric acid with evolution of oxygen, forming a solution from which the _nitrate_, Tb(NO₃)₃,6H₂O, melting at 89·3°, separates on cooling. In hot hydrochloric acid, the peroxide dissolves with evolution of chlorine, forming solutions from which the _chloride_, TbCl₃,6H₂O, can be isolated with difficulty; this salt is extremely deliquescent, and easily forms supersaturated solutions. The _sulphate_, Tb₂(SO₄)₃,8H₂O, can be precipitated from a sulphuric acid solution of the oxide by addition of considerable quantities of alcohol; it is isomorphous with the other sulphate octohydrates, and is completely dehydrated at 360°. [345] The terbium compounds here described have been prepared by Urbain (_loc. cit._) from carefully purified material; other compounds have been described by Potratz (_Chem. News_, 1905, ~92~, 3), but her material contained a large proportion of gadolinium. ~Atomic Weight.~--The value adopted by the International Committee is 159·2, which was obtained by Urbain in 1905 (_loc. cit._) from the ratio Tb₂(SO₄)₃,8H₂O : Tb₂(SO₄)₃. This is the only determination on which reliance can be placed, as the material of the earlier workers was seldom even approximately pure. ~Detection.~--Solutions of terbium salts show only one band in the visible spectrum, at 487·7 in the blue. This band was observed by Lecoq de Boisbaudran in a specimen of terbia containing dysprosia, and assumed by him to belong to a new element, Z_{δ} (_loc. cit._) In the ultraviolet nine absorption bands have been observed (Urbain, _loc. cit._) The spark spectrum shows the lines observed by Demarçay in 1900, and attributed by him to the new element Γ. Lecoq de Boisbaudran’s element Z_{β} showed a green fluorescence with the reversed spark, a phenomenon which Urbain has found to be exhibited by pure terbium compounds. The arc spectrum of Urbain’s pure terbia was examined by Eberhard[346]--see also Exner and Haschek, and Eder and Valenta.[347] The element may be detected in minerals and earth mixtures by the following lines: 3523·82 3676·52 3703·05 3704·01 4005·62 4278·71 The chief lines in the arc spectrum (Exner and Haschek) are the following: 3324·53 3509·34 3531·86 3561·90 3568·69 3600·60 3628·53 3650·60 3659·02 3704·10 3711·91 3848·90 3874·33 3899·34 3925·60 3939·75 3977·01 3982·07 4005·70 4012·99 4278·70 4752·69