The extraction of pure thorium compounds from monazite is a process of very great technical difficulty. The percentage of thoria is small, whereas that of the ceria oxides is high. The mineral is almost always decomposed by heating with concentrated sulphuric acid, and when the resulting pasty mass is taken up with water, a large amount of free sulphuric acid must be present in order to hold the rare earth phosphates in solution. For the separation of thoria from ceria and yttria compounds in acid solution no processes were known until quite recently. When it is remembered that the thorium nitrate used for the manufacture of mantles must be of a degree of purity which very few commercial products ever approach, some idea of the difficulties of the extraction may be obtained. ~Decomposition of the Monazite.~--Two processes have been used for the working up of monazite. The first of these consists in fusing the mineral with soda, and extracting the sodium phosphate with water; the earths may then be taken into solution with acid, and the separation effected as outlined below. This method is very rarely used. A process has been proposed, in which the monazite is fused with carbon in an electric furnace; the cooled mass is treated with mineral acids, which take the earths into solution free from phosphoric acid. No technical application has so far been made of this proposal. The method commonly used is that in which the sand is decomposed by means of sulphuric acid. The charge usually employed, about two to three hundred kilograms, requires from four to six hours’ heating, about twice the weight of concentrated acid being needed. The operation is carried out in cast-iron vessels, and an efficient draught must be maintained to remove the acid fumes; the factories are usually isolated. The treatment with sulphuric acid converts the phosphates chiefly into sulphates; when the reaction is finished, the liquor fumes strongly and begins to thicken, heating being stopped when a thick broth is obtained. The cooled mass is extracted with water, care being taken to maintain a degree of acidity sufficient to prevent any precipitation of the phosphates.