(4) _Double Superphosphates._—In the case of double superphosphates, which sometimes contain large quantities of pyrophosphate, the solution is made in the usual way so that in 100 cubic centimeters there will be contained two grams of the substance. Usually ten grains are taken and the volume made up to half a liter. Twenty-five cubic centimeters of the filtrate are diluted with seventy-five cubic centimeters of water and the pyro converted to orthophosphoric acid by heating with ten cubic centimeters of strong nitric acid on a sand-bath. The heating should be continued until the volume be reduced to twenty-five cubic centimeters. The strongly acid liquid is made alkaline with ammonia, and afterwards slightly acid with nitric, and the rest of the process is carried on in the usual way. (5) _Phosphoric Acid in the Residue of Superphosphate Manufacture._—In the mixture of superphosphates and gypsum, the residue of the manufacture of double superphosphates, the phosphoric acid is estimated in the following manner: Five grams of the substance are placed in a dish, rubbed up with absolute alcohol, and washed into a 250 cubic centimeter flask. The flask is filled with absolute alcohol to the mark, closed with a stopper, and with frequent shaking, allowed to stand for two hours; it is thereupon filtered as quickly as possible; fifty cubic centimeters of the filtrate corresponding to one gram of the substance, are taken for the estimation. This fifty cubic centimeters is evaporated on a sand-bath to a sirupy consistence, diluted with water, and treated, as in the case of the soluble phosphates above mentioned. In all cases as described above, after the solutions are obtained they are treated with the ammonium citrate solution and the phosphoric acid estimated as in the first instance given. (6) _Solutions Employed._— (a) The citrate solution is made as follows: 1,500 grams of citric acid are dissolved in water, treated with five liters of twenty-four per cent ammonia, and made up to fifteen liters. (b) The magnesia mixture is made as follows: 500 grams of magnesium chlorid, 1,050 grams of ammonium chlorid, three and five-tenths liters of twenty-four per cent ammonia, and six and five-tenths liters of distilled water are used. In the case of the superphosphates fifty cubic centimeters of the citrate solution are employed and with the basic slags 100 cubic centimeters; and in both cases twenty-five cubic centimeters of the magnesia mixture.