The crude mixture of cerous sulphate with basic sulphates of other elements of the cerium group, which has been patented for use as a catalyst in the contact process for the manufacture of sulphuric acid,[574] is prepared from the earth compounds obtained as by-products in the treatment of monazite. These are converted into the sulphates, and, after evaporation of the excess of sulphuric acid, heated for several hours at a low red heat (300°-600°C.). The porous mass is then broken, and is ready for use. It is stated that a nearly quantitative yield of sulphur trioxide is obtained, and that the mixture acts more efficiently than pure cerous sulphate. The reaction is said to depend on the continuous formation and decomposition of the ceric salt, thus: Ce₂(SO₄)₃ + SO₂ + O₂ = 2Ce(SO₄)₂ 2Ce(SO₄)₂ = Ce₂(SO₄)₃ + SO₃ + O [574] Hölbling, _D. R. P._ 142144 and _F._ 326321 of May, 1903. This process does not appear to have come into general use. A general patent had been taken out in 1901, protecting the use of oxides of the rare earth elements for ‘high-temperature catalysis’ in the manufacture of sulphuric acid,[575] but the oxides do not seem to be very efficient.[576] [575] Meister, Lucius and Brüning, _E._ 1385, 1901. [576] _Vide_ Plüddemann, Dissertation, _Beitrag zur Aufklärung des Schwefelsäurekontaktprozesses_, Berlin, 1907. It has also been proposed to utilise the oxidising power of ceric salts in acid solution[577] for the preparation of aldehydes, quinones, etc., from aromatic hydrocarbons, for which purpose they are claimed to be more efficient than chromates. By the use of a crude cerium dioxide (60-70 per cent. CeO₂) obtained by the ignition of the by-products of the thorium industry, good yields were obtained of benzaldehyde, naphthaquinone and anthraquinone from toluene, naphthalene and anthracene respectively. [577] Meister, Lucius and Brüning, _D. R. P._ 158609, March, 1905. Garelli[578] has examined the action of cerium salts in tanning; he states that with neutral solutions, effects very similar to those produced by aluminium salts are obtained, but Eitner, who has also examined the question,[579] is of opinion that the cost of isolating and purifying the salts from the monazite residues renders their employment for this purpose impossible. [578] _J. Soc. Chem. Ind._ 1912, ~31~, 830. [579] _Ibid._ 1911, ~30~, 1128. The fluoride, silicofluoride, and dioxide have also been proposed for the preparation of enamels,[580] but do not give satisfactory results. [580] Rickmann and Rappe, _D. R. P._ 99165, September, 1898; also _D. R. P._ 203773, October, 1908.