The apparatus of Knorr, described in volume first, page 338, may be conveniently used. The weight of the sample to be used should be regulated by the content of carbonate. When this is very high, from one to two grams will be found sufficient; when low, a larger quantity must be used. Hydrochloric is preferred as the solvent acid. Those forms of apparatus which are weighed as a whole and the carbon dioxid determined by reweighing after its expulsion, are not as reliable as the absorption apparatus mentioned. =26. Soluble and Insoluble Matter.=—Five grams of the fine phosphate are put into a beaker, twenty-five cubic centimeters of nitric acid, (specific gravity 1.20) and 12.5 cubic centimeters of hydrochloric acid (specific gravity 1.12) are added. The beaker, covered with a watch-glass, is placed upon the water-bath for thirty minutes[18]. The contents of the beaker are well stirred from time to time, and at the end of the period the beaker is removed from the bath, filled with cold water, well stirred, and allowed to settle. The solution is next filtered into a half liter flask, and the residue is thoroughly washed with cold water, partially dried, and then ignited, (finishing with the blast-lamp) and brought to constant weight. The figures thus obtained will, however, be incorrect, because the fluorin liberated during the solution of the phosphates dissolves a portion of the silica. Hence, the results are too low. Nevertheless, as the same action would occur in the manufacture of a superphosphate from the material, the determination may be considered, as a fair approximation to commercial practice. The ignited residue must be tested for phosphorus pentoxid. =27. Preparation of the Solution.=—The flask containing the filtrate is filled to the mark with cold water, and the solution is thoroughly mixed by twice pouring into a dry beaker and returning it to the flask. Cold water is used for washing the residue, since if hot water be used, the sesquichlorids are apt to become basic and insoluble, and hence to remain in the residue and on the filter paper. Besides, as the flask is to be filled to the mark, the contents must be cold before any volumetric measurements can be made.