silt above A will flow down and join the silt collected over B. This operation may be repeated until a perfect separation is effected. Finally B is opened and the heavy silt collected in a beaker, and the lighter silt then removed from the upper part of the apparatus. [Illustration: FIGURE 61. APPARATUS OF WÜLFING. ] =272. Method of Wülfing.=—A somewhat more convenient method of purifying the silt segregates and freeing them of mechanically occluded particles of differing specific gravities has been proposed by Wülfing.[178] An elliptical ring of heavy glass tubing carries glass stop-cocks A and B, Fig. 61, at the two extremities of the ellipse, each arm of which is provided with a lateral glass-stoppered neck. The perforation in the stop-cocks has the same diameter as the sides of the ellipse. The apparatus has an interior cubical content of about forty cubic centimeters. Thirty cubic centimeters of the separating fluid are introduced through one of the lateral apertures and brought to the same height in the two arms by opening the cock B. The silt is then introduced in equal quantities into each of the arms. The stoppers having been inserted, the whole is well-shaken. At the beginning of the separation, the apparatus being held in position 1, the lighter soil above and the heavier soil below are somewhat mixed by reason of flocculation and mechanical entanglement. At this point B is opened and the apparatus placed in the inclined position 2. The heavier particles S + l, on the right arm, are thus united with the same class of particles in the left arm making 2S + 2l. This operation is hastened by opening A and allowing the higher column of liquid in the right arm to pass into the left. The liquid in the left arm is allowed to rise to A. After all of S + l in the right arm has passed into the left B is closed, the apparatus then placed back in position 1 and inclined in the opposite direction until L + s in the top of the left arm has been transferred to the L + s in the top of the right, and the same quantity of liquid is found in each arm. The operation is then repeated and this continued until all S + s is found in the bottom of the left arm and all L + l in the top of the right arm.