=267. Method of Bréon.=—Instead of a solution of a salt, Bréon[173] has proposed to use salts in a fused state for separating mineral particles. Lead and zinc chlorids may be used for this purpose in a melted state, having the specific gravities of 5.0 and 2.4, respectively. By mixing the molten salts in different proportions, any desired specific gravity between the extremes mentioned may be secured. The fusion is accomplished at 400° in a test-tube. The silt is added gradually with constant stirring until a sharp separation is secured between the sinking and floating particles. After cooling, the tube is broken, the two parts separated, and the silt recovered by dissolving the mixed salts in hot water containing a little nitric acid. Only the coarser silts can be separated by this method. Fused silver nitrate, melting point 198°, specific gravity 4.1, has also been used for separation. =268. The Separation.=—Forty cubic centimeters of the solution in the Thoulet process are placed in the separatory tube A, Fig. 58, together with from one to two grams of the silt and the stopper F inserted. The tube G is connected with a vacuum apparatus by means of which any air particles adhering to the mineral fragments are removed. The silt which sinks in the solution is removed after G has been disconnected by opening the cock C and sucking through B at I. The cock C is closed and the separated particles washed into a beaker at H after opening D. Water is next added to the materials left in A in quantities previously determined to secure a given specific gravity and thus a second, a third, etc., separation secured. An intimate mixture of the solutions in A can be effected by closing D, opening C, and blowing through B in such a way that no liquid is allowed to pass through C. [Illustration: FIG. 58. THOULET’S SEPARATING APPARATUS. ] The quantity of water to be added in each case to secure a given specific gravity is determined by the formula _v_₁ = (_v_(D − _d_))/(_d_ − 1), in which _v_ is the volume of the solution, D its specific gravity, and _d_ and _v_₁ the specific gravity desired and volume of the water to be added. _Example._—Let the specific gravity of the original solution be 3.2, its volume thirty cubic centimeters, and the desired specific gravity of the new solution 2.85. Then _v_₁ = (30(3.2 − 2.85))/(2.85 − 1) = 5.68. The desired specific gravity is therefore secured by adding 5.68 cubic centimeters of water, which is easily accomplished by means of the graduations on the tube.