(7) _Details of the Manipulation._—On the addition of the citrate solution there should be no permanent troubling of the liquid but there should be a total clearing up thereof. In order to facilitate this, after the addition of the citrate solution, the flasks should be gently shaken in order to distribute the solution throughout the mass. Solutions from bone-black superphosphates show sometimes, after the addition of the citrate solution, a more or less strong opalescence, but this opalescence does not influence the results. Should it happen that with superphosphates which are made from raw material containing large excesses of iron or clay, fifty cubic centimeters of the citrate solution are not sufficient to prevent the other bases from being precipitated, an additional quantity up to twenty-five cubic centimeters may be added. The addition of the magnesia mixture must follow as quickly as possible after the addition of the citrate solution to avoid a separation of crystalline calcium phosphate. On the addition of the citrate solution there is always a rise in temperature. Inasmuch as the precipitation of the phosphoric acid with magnesia must take place in the cold, the liquid must be cooled after the addition of the citrate,[54] and the cooling should take place as quickly as possible. The above method was adopted by the chemical section of the International Agricultural Congress held at Vienna, September, 1890.[55] [Illustration: FIGURE 4. SHAKING MACHINE FOR AMMONIUM MAGNESIUM PHOSPHATE.] In order to hasten the precipitation of the ammonium magnesium phosphate and to prevent the fixation of the precipitate on the walls of the erlenmeyer, the flask should be shaken for half an hour. For this purpose the flasks should be closed with smooth well-fitting rubber stoppers and placed in a shaking machine. The shaking machine of the form given in Fig. 4, recommended by the Halle station, is very conveniently used for this purpose. On a vertical axis are carried two stages for holding the flasks. The flasks are prevented from striking each other by means of the partitions shown. The apparatus is conveniently driven by a small water-motor, as indicated, which imparts to the stages a partial back and forth revolution. After shaking for half an hour, any precipitate adhering to the rubber stoppers is carefully washed off with ammonia water into the flask. The filtration can be made immediately after the shaking or after two or three days; the results are the same. [Illustration: FIGURE 5. RÖSSLER IGNITION FURNACE.]