~Applications of the Cerium Compounds.~[568]--One of the earliest investigations in which cerium compounds were examined with a view to technical employment, was that of Kruis,[569] who made experiments on the comparative value of the salts of different metals as catalysts in the manufacture of aniline black. He showed that a solution of aniline with an oxidising agent (potassium chlorate or chromate) develops no colouration unless a salt of a heavy metal is present. In the case of the fabric impregnated with the solution, the only metals of which compounds were found suitable for producing a colour were copper, which was then generally used for the purpose, and cerium, iron, and manganese. Of these, cerium, used in the form of the double sulphate, was found to be by far the most suitable, and moreover to have the advantage that only small quantities are required; the price was at that time too high to allow of its use, but it has since been employed.[570] Cerium compounds have also been proposed as mordants for alizarin,[571] but they do not appear ever to have come into general use. [568] An account of the various suggestions for the technical employment of the rare earth elements, by Dr. Max Speter, will be found in Dammer, _Die Chemische Technologie der Neuzeit_, Stuttgart, 1910, vol. i. pp. 500-504. [569] _Dingl. Polyt. J._ 1874, ~212~, 347. [570] _Vide_ Buhrig, _Dingl. Polyt. J._ 1879, ~231~, 77; and _Abstr. Chem. Soc._ 1879, ~36~, 683. [571] _Vide_ Witt, _Chem. Ind._ 1896, ~19~, 156. In photography, ceric sulphate has been employed for some time for the purpose of ‘reducing,’ _i.e._ removing silver from over-developed negatives.[572] It is said to act very evenly and rapidly, the small quantity of free sulphuric acid required to hold the salt in solution having no bad effect. More recently, cerium salts have been proposed for use in colour photography.[573] An emulsion is obtained by adding salts of iron, uranium, or cerium to a colloidal solution of albumen in ammonia, borax solution, or sodium carbonate solution; this is spread on the paper or negative, and is said to be readily sensitive to light. [572] _Vide_ Lumière, _Bull. Soc. franc. Photog._ (2) 1900, ~16~, 103. Also _E._ 470, 1900. [573] Fateau, _E._ 20740, 1907.