(2) _Citrate-Soluble Phosphoric Acid._—One gram of the basic slag or phosphate is placed in a 100 cubic centimeter flask and covered with Wagner’s acid citrate solution making the total volume up to 100 cubic centimeters. With frequent shaking the flask is kept at 40° for an hour, or it may be allowed to stand for twelve hours at room temperature with frequent shaking. In fifty cubic centimeters of the filtrate from this flask the phosphoric acid is determined by the magnesia mixture as described. Since, in the present case, the precipitate of ammonium magnesium phosphate contains some silicic acid it cannot be directly ignited but must be treated in the following manner: The precipitate and the filter are thrown into a porcelain dish, the filter-paper torn up into shreds with a glass rod, the precipitate dissolved in nitric acid, neutralized with ammonia, acidified with acetic, and treated with uranium solution. The phosphoric acid may also be estimated by the gravimetric method by dissolving the precipitate again in hydrochloric or nitric acid, evaporating to dryness, and drying for one hour at from 110° to 120°, dissolving again in hydrochloric acid, filtering, and washing the precipitate well. The filtrate, which is now free from silica, can be treated with Joulie’s solution, precipitated with magnesia mixture, the precipitate washed, ignited, and weighed as described. The molybdenum method is preferred in the estimation of citrate-soluble phosphoric acid, especially in slags. For this purpose fifty cubic centimeters of the filtrate from the solution of one gram of slag in 100 cubic centimeters of Wagner’s citrate liquid are treated with 100 cubic centimeters of molybdenum solution and thirty cubic centimeters of ammonium nitrate solution, warmed for twenty minutes at 80°, filtered after cooling, and the yellow precipitate washed with cold water. The water will gradually dissolve all the silicic acid from the yellow precipitate and carry it into the filtrate. The yellow precipitate is then dissolved in two and one-half per cent liquid ammonia and precipitated with magnesia mixture and the precipitate washed, ignited, and weighed in the way described.