~Preparation of Thorium Nitrate from Mantle-ash.~--Since the ordinary incandescent mantle, in use, consists only of the pure thoria and ceria, with small quantities of alumina, lime, and magnesia, which have been employed to strengthen the ‘head,’ the working-up of mantle-ash gives an easy means of obtaining the nitrates, and high prices are accordingly paid for the ash in quantity. At one period of great competition between rival manufacturers, canvassers went from house to house in many large towns buying up mantle residues, to be used for the extraction of the thorium for ‘lighting-fluid.’ For this purpose, the oxides are treated with hot concentrated sulphuric acid, the cooled residue dissolved in water, and the thorium and cerium precipitated free from compounds of aluminium, magnesium, and calcium by oxalic acid. If pure thorium nitrate, free from cerium, is required, the oxalates are added to the last precipitate from the double carbonate purification in the treatment of monazite (_vide supra_), and the ordinary processes of refinement continued; more often, however, the mixed nitrate for impregnation of the mantle-fabric is required, and this is obtained by ignition of the oxalates and solution of the oxides so obtained in nitric acid, more cerium nitrate being added if necessary. ~Extraction of Cerium Nitrate.~--Since monazite is primarily a phosphate of the cerium metals, the percentage of thoria being usually quite low (_vide_ Monazite, Chapter VI), very large quantities of compounds of the cerium group of elements are annually produced in the process of extraction of thorium. There is at present a very limited demand for these compounds (_vide_ Chapter XXI), no important uses having yet been found for them. In the ordinary process of extraction of the thorium, these elements remain as the sparingly soluble double carbonates, whilst the thorium double carbonate is removed in solution. From the mixed salts which contain 50-60 per cent. of the cerium compound, the cerium nitrate required for the manufacture of mantles is prepared, but the amount so used is a small fraction of the whole, and large quantities of compounds of cerium and the allied elements are available as soon as profitable uses can be found.