The thorium hydroxide to be purified is dissolved in sulphuric acid, and in the first form in which the method was employed, the thorium sulphate obtained by evaporation of the solvent was heated until it became anhydrous. This was dissolved to saturation at 0°, and the solution raised to the boiling-point, the 4-hydrate being precipitated; this treatment was repeated several times. It was pointed out by Bunsen, from theoretical grounds, that this method could never yield a pure thorium salt, and Krüss and Nilson accordingly introduced a modification. The impure sulphate, after dehydration, as before, is dissolved at 0°, and allowed to come to ordinary room temperature, 20°; the hydrate which separates (the 8-hydrate) is collected and dried at high temperature and the crystallisation repeated. This method gives a fairly pure salt after three recrystallisations, but the process is very tedious, owing to the time required for drying and heating the hydrate. For this reason the method was further modified by Cleve and Witt. The crude sulphate is boiled with ammonia, and the hydroxide obtained dissolved in hydrochloric acid; addition of sulphuric acid to the concentrated solution in the cold transforms the chloride into the sulphate, which separates as the 8-hydrate at ordinary temperatures. Three repetitions give a satisfactory product, and in this form the method is now much used. The work of Koppel and Holtkamp referred to above has placed the process on a sound basis. These authors have examined the solubilities of the various hydrates in presence of hydrochloric, nitric, and sulphuric acids, and mixtures of these, at different temperatures. They find that hydrochloric acid is to be preferred to nitric acid, in the process of Cleve and Witt, as besides its lower price, its use involves less loss than that of the latter acid; excess of hydrochloric acid is not harmful within wide limits, whilst a slight excess of sulphuric acid over the quantity required to form the sulphate is desirable, to secure the greatest yield. Finally, the temperature at the addition of the sulphuric acid must not be allowed to rise above 25°, for in the presence of so much acid the transition temperature to the 4-hydrate, normally 42°, is considerably lowered; it is necessary to avoid separation of the 4-hydrate, which is a flocculent unworkable precipitate. Recently it has been proposed to carry out the purification by use of alkyl hydrogen sulphates,[504] as it is stated that the differences of solubilities of the alkyl sulphates of thorium and the cerium metals are greater than in the case of the sulphates themselves. It is also claimed that the presence of a small quantity of the alkyl sulphate in the thorium nitrate which forms the final product has a good effect on the quality of the mantles made from it. [504] Kreidl u. Heller, _D. R. P._ 233023, March, 1911; _F._ 414463, June, 1910.