~Analysis of a Monazite or Monazite Sand for Thorium.~--Since the commercial value of a monazite sand or concentrate, or of the pure mineral, depends, at present, entirely on the percentage of thoria, it is important to have a rapid and reliable method of estimating this constituent. The only reliable methods of quantitatively decomposing the mineral, however, all involve acid treatment, and excess of acid must always be present to prevent precipitation of phosphates. Until recently, no way was known for estimating thorium in an acid solution, and all the earlier methods therefore involved tedious processes for complete removal of phosphoric acid, so that the salts could be obtained in neutral solution. This was usually effected by precipitation of the whole rare earth content with oxalic acid, and thorough washing of the oxalates; these can then be dissolved directly in fuming nitric acid on the water-bath, or ignited to the oxides, which may then be dissolved in the same reagent. The solution of nitrates is evaporated to dryness, to effect removal of the excess of acid, the nitrates dissolved in water, and the thorium estimated in the neutral solution. Among the earliest methods employed for the estimation in neutral solution was the thiosulphate precipitation.[505] Thorium thiosulphate is not known; when sodium thiosulphate is added to a neutral solution of a thorium salt, a precipitate of thoria mixed with sulphur is obtained, by hydrolysis of the potential thiosulphate, and decomposition of the unstable thiosulphuric acid. The method, however, leaves much to be desired; other earths are partly precipitated, and the separation of thoria is not complete. For analytical purposes the precipitate obtained is redissolved in hydrochloric acid, and a second precipitation with thiosulphate effected. The filtrates from the two precipitations are collected, and the whole earth-content precipitated from these with ammonia; the hydroxides are dissolved in hydrochloric acid, and again treated with thiosulphate to throw down any thoria which has escaped the previous precipitations. The three precipitates of thoria are then collected, dried, and ignited for weighing as pure thorium dioxide, ThO₂. [505] Full accounts of this and the two following methods will be found in an important paper by Benz, _Zeitsch. angew. Chem._ 1902, ~15~, 297