The filtration of the ammonium magnesium phosphate is made through perforated crucibles. The asbestos felt is prepared in the following way: The coarse fibers of asbestos are chopped up with a sharp knife on a glass plate and boiled for two hours with strong hydrochloric acid; afterwards, by repeated washing with distilled water they are freed from acid and the too fine particles of asbestos which would tend to make the filter too impervious. After the last wash-water is poured off, the asbestos is suspended in water and used for making the felt on the filter. The preparation of the crucible and the filtration under pressure are accomplished in the usual way. The ignition of the precipitate is accomplished in a Rössler ignition oven, Fig. 5. When the muffle of the furnace shows a white heat or a white-red heat it is at the proper temperature for the estimation. At higher temperatures, the asbestos felt is easily injured. Generally, an ignition of five minutes is sufficient, but with double superphosphates, ten minutes are required. =65. The Swedish Citrate Method.=[56]—This method of determining phosphoric acid is founded on the fact that phosphoric acid in the presence of calcium salts, without it being necessary, previously, to convert it into phosphomolybdate, is precipitated directly by magnesia mixture from a solution, to which ammonium citrate has been added, provided first, that the solution contain a sufficient quantity of sulfuric acid, and second, that only as much citrate be added as is required to keep the calcium salts in alkaline solution.[57] _Reagents._ (1) _Citric Acid Solution._—Prepared by dissolving 500 grams of citric acid in water, and completing to a volume of one liter. (2) _Ten Per Cent Ammonia_ of 0.959 specific gravity. (3) _Magnesia Mixture_, of the usual composition. The various processes are conducted as follows: (_a_) _Water-Soluble Phosphoric Acid._—Add twenty cubic centimeters of citric acid solution to fifty cubic centimeters of the water-soluble solution obtained according to the Swedish molybdenum method, and then add thirty-three cubic centimeters of ammonia. When the mixture has cooled, add slowly twenty-five cubic centimeters of the magnesia mixture, and then forty-two cubic centimeters of the ammonia. Keep the solution stirred by means of a closely clipped feather which is pressed tightly against the sides of the beaker; by this process the phosphate is precipitated after half an hour in pure condition and completely, without, in the least, sticking to the wall of the beaker; filter, wash, and ignite, as usually directed.