In many of the States the statutes, or custom, prescribe that only the water and citrate-soluble acid shall be reckoned as available, the insoluble residue being allowed no place in the estimates of value. In many instances such a custom may lead to considerable error, as in the case of finely ground bones and some forms of soft and easily decomposable tricalcium phosphates. There are also, on the markets, phosphates composed largely of iron and aluminum salts, and these appear to have an available value often in excess of the quantities thereof soluble in ammonium citrate. As a rule the apatites, when reduced to a fine powder and applied to the soil, are the least available of the natural phosphates. Next in order come the land rock and pebble phosphates which, in most soils, have only a limited availability. The soft fine-ground phosphates, especially in soils rich in humus, have an agricultural value, almost, if not quite equal to a similar amount of acid in the acid phosphates. Fine-ground bones also tend to give up their phosphoric acid with a considerable degree of readiness in most soils. Natural iron and aluminum phosphates, have also, as a rule, a high degree of availability. In each case the analyst must consider all the factors of the case before rendering a decision. Not only the relative solubility of the different components of the offered fertilizer in different menstrua must be taken into consideration, but also the character of the soil to which it is to be applied, the time of application, and the crop to be grown. By a diligent study of these conditions the analyst may, in the end, reach an accurate judgment of the merits of the sample. =135. Direct Weighing of the Molybdenum Precipitate.=—It has already been stated that many attempts have, been made to determine the phosphoric acid by direct weighing as well as by titration, as in the Pemberton method. The point of prime importance in such a direct determination is to secure an ammonium phosphomolybdate mixture of constant composition. Unless this can be done no direct method, either volumetric or gravimetric, can give reliable results. Hanamann[122] proposes to secure this constant composition by varying somewhat the composition of the molybdate mixture and precipitating the phosphoric acid under definite conditions. The molybdate solution employed is prepared as follows: Molybdic acid 100 grams. Ten per cent ammonia 1.0 liter. Nitric acid (1.246 sp. gr.) 1.5 liters. The precipitation of the phosphoric acid is conducted in the cold with constant stirring. It is complete in half an hour. The ammonium phosphomolybdate is washed with a solution of ammonium nitrate and then with dilute nitric acid, dried, and ignited at less than a red heat. It should then have a bluish-black color throughout. Such a body contains 4.018 per cent of phosphoric anhydrid.