(1) Reduction in acid solution by means of tin or zinc gives an intense violet colour, due to the formation of trivalent titanium salts. Various colours are given by vanadium, columbium, and tungsten, so that the test is not decisive if these are present. (2) Hydrogen peroxide in acid solution gives a reddish-yellow colour, which is very delicate, and is used in quantitative estimation; vanadium compounds interfere. (3) In sulphuric acid solution, characteristic colours are obtained with many phenolic compounds; thymol gives a blood-red colour which is exceedingly intense. (4) A very characteristic and intense colour is given in acid solution on the addition of 1:8-dihydroxynaphthalene-2:4-disulphonic acid (chromotropic acid). The methods for the estimation of titanium are given in Chapter XXII. CHAPTER XVI THE GROUP IVA ELEMENTS (_continued_)--ZIRCONIUM AND THORIUM ~Zirconium~, Zr = 90·6 The oxide zirconia was isolated as a new earth from zircon from Ceylon by Klaproth in 1789; six years later the new earth was obtained also from hyacinth, the gem-variety of the same mineral. The new oxide was examined in 1818 by Berzelius, who pointed out its resemblance to alumina, and gave it the formula Zr₂O₃; during the next two decades he thoroughly investigated its properties, preparing the element itself, and determining its equivalent. In 1857 a determination of the vapour density of the chloride, by Deville and Troost, showed that the element is really tetravalent, and that the formula ZrO₂ must be assigned to the oxide; this formula was shown to accord with the isomorphism of rutile (TiO₂) and zircon (ZrO₂,SiO₂) by Rose in 1859, whilst in the following year Marignac observed the isomorphism between the fluozirconates of zinc and nickel and the fluosilicates, fluotitanates, and fluostannates of these metals. The homogeneity of the oxide has been questioned; Svanberg in 1845 considered it to be a mixture of at least three earths, whilst Sorby and Forbes in 1869 claimed to have discovered in it a new oxide, ‘Jargonia.’ These claims, however, have been shown to have been founded on inaccurate experimental work, and the individuality of the element is at the present time considered to be well established. Zirconium is fairly widely distributed in nature, but generally in very small quantities, and can be rightly classed as one of the rarer elements. It occurs in some silicates, and in small quantities in almost all the rare earth minerals. The most important source of the element and its compounds was until quite recently the mineral Zircon, with its gem-varieties Hyacinth and Jargon, and the large number of secondary altered zircon minerals. Since its discovery in 1892, however, the naturally occurring oxide, Baddeleyite,[455] has become increasingly important for the extraction of zirconium compounds, especially for the preparation of the pure oxide for fire-resistant materials.