Another method which can be carried out in acid solution is based on the insolubility of the hypophosphite, ThP₂O₆,11H₂O, in dilute acids.[516] To the boiling acid solution, an aqueous solution of sodium hypophosphate, Na₂H₂P₂O₆,6H₂O, is added drop by drop. The precipitate, which contains any titanium and zirconium present in the original solution, is best treated with a mixture of sulphuric and fuming nitric acids; the phosphates produced by the oxidation are freed from nitric acid by evaporation, dissolved in water, with addition of sulphuric acid, and thorium precipitated as the oxalate, which is then ignited as usual. This method has been suggested for the technical separation of thorium from monazite (_vide_ p. 278). Since the precipitations by means of sodium hypophosphate and potassium iodate can be carried out with solutions obtained directly from the product of the action of sulphuric acid on the mineral, these two methods are probably more suitable for the rapid and accurate estimation of thorium for technical purposes than any of the others mentioned. [516] Wirth, _Zeitsch. angew. Chem._ 1912, ~25~, 1678; see also Koss, _Chem. Zeitg._ 1912, ~36~, 686, and Rosenheim, _ibid._ p. 821. CHAPTER XIX THE MANUFACTURE OF MANTLES FROM COTTON AND RAMIE The fabric chosen for the manufacture of the original Welsbach mantles was a specially selected cotton, woven from threads of a specified thickness. The oxide skeleton left after burning off the impregnated fabric, however, showed many serious defects. Gradual shrinkage occurred during use, so that the mantle was gradually withdrawn from the hottest zone of the flame; the contraction also resulted in crumpling, which caused the fragile fabric to fall to pieces. The light-giving power showed a gradual but continuous diminution, so that after a hundred hours, the decrease sometimes amounted to thirty per cent. of the original intensity. Lastly, owing to the fragility due to the torsion introduced by the twisting together of so many short fibres in the spinning of the fabric, the life of these mantles was very short, and their susceptibility to shock very great.