Numberless experiments have been made with organic acids in the hope that an easy method of separation might be found, but though some useful results have been obtained, precipitation has always to be effected in neutral solution, so that all such processes involve the tedious preliminary work of which an outline has been given above. Metzger[509] finds that a quantitative separation of thorium can be effected from a solution in 40 per cent. alcohol by use of fumaric acid; a second precipitation is needed to secure the complete removal of the cerium elements. Neish[510] uses meta-nitrobenzoic acid, which precipitates the thorium salt from a boiling solution; cerium earths, if present, are carried down in small quantities, and are removed by dissolving the precipitate in dilute nitric acid, adding a further quantity of the organic acid, and treating carefully with ammonia to almost complete neutralisation. The compound obtained by this second precipitation is the pure thorium salt. More recently, Smith and James[511] have shown that sebacic acid gives a quantitative precipitation of the thorium salt, from boiling neutral solution, as a voluminous granular precipitate, readily filtered and washed; sebacic acid is very sparingly soluble in cold water, but dissolves readily at 100°, and since, in virtue of this property, it can be readily recovered after use, the authors suggest it as a suitable reagent for the technical separation of thorium from monazite. In all cases where thorium is precipitated as an organic salt in quantitative analysis, the precipitate is dried and ignited, and the residue weighed as the pure dioxide. [509] _J. Amer. Chem. Soc._ 1902, ~24~, 275 and 901. [510] _Ibid._ 1904, ~26~, 780. [511] _Ibid._ 1912, ~34~, 281. An interesting method has been worked out by Giles.[512] If pure moist lead carbonate be stirred into a neutral solution of rare earth compounds, thoria is completely precipitated. Only the tetravalent elements are separated by this method, so that if ceric compounds are present, they must first be reduced by means of sulphuretted hydrogen or sulphur dioxide; zirconium, if present, must afterwards be separated from the thorium. One precipitation is said to ensure almost complete separation from the trivalent elements. The precipitate is collected, washed, and dissolved in hydrochloric acid; after filtering, if necessary, the solution is saturated with sulphuretted hydrogen, to ensure complete removal of the lead, and thorium hydroxide is then precipitated by ammonia. The drawback to this method lies probably in the fact that it is necessary to use absolutely pure lead carbonate, a substance which, as the author’s elaborate process of purification seems to show, could not be obtained very cheaply on a large scale. [512] _Chem. News_, 1905, ~92~, 1 and 30.