=251. The Official Agricultural Method.=—This method is based on the processes at first proposed by Lindo[201] and Gladding,[202] and is given below as adapted to mixed fertilizers and mineral potash salts.[203] (1) _In Superphosphates._—Boil ten grams with 300 cubic centimeters of water thirty minutes. To the hot solution add ammonia in slight excess, and then a sufficient quantity of ammonium oxalate to precipitate all the lime present; cool and make up to half a liter, mix thoroughly, and filter through a dry filter; evaporate fifty cubic centimeters, corresponding to one gram, nearly to dryness, add one cubic centimeter of dilute sulfuric acid (1 to 1), evaporate to dryness and ignite to whiteness. As all the potash is in form of sulfate, no loss need be apprehended by volatilization of potash, and a full red heat must be maintained until the residue is perfectly white. This residue is dissolved in hot water, plus a few drops of hydrochloric acid, and a slight excess of platinum solution is added. This solution is then evaporated to a thick paste in a small dish, and eighty per cent alcohol added. In evaporating, special precaution should be taken to prevent absorption of ammonia. The precipitate is washed thoroughly with alcohol by decantation and on the filter, as usual. The washing should be continued even after the filtrate is colorless. Ten cubic centimeters of the ammonium chlorid solution, prepared as hereinafter directed, are run through the filter, or the washing may be performed in the dish. The ten cubic centimeters will contain the bulk of the impurities, and are thrown away. Fresh portions of ten cubic centimeters of the ammonium chlorid are run through the filter several times (5 or 6). The filter is then washed thoroughly with pure alcohol, dried, and weighed as usual. Care should be taken that the precipitate is perfectly soluble in water. The platinum solution used contains one gram of metallic platinum in every ten cubic centimeters. To prepare the washing solution of ammonium chlorid, place in a bottle 500 cubic centimeters of water and 100 grams of ammonium chlorid and shake till dissolved. Now pulverize five or ten grains of potassium platinochlorid, put in the bottle and shake at intervals for six or eight hours; let settle over night, then filter off the liquid into a second bottle. The first bottle is then ready for preparation of a fresh supply when needed. (2) _Potassium Chlorids._—In the analysis of these salts an aliquot portion of the solution, containing a half gram, is evaporated with forty cubic centimeters of the platinum solution and a few drops of hydrochloric acid, and washed as before.