(4) _Pyritic Phosphates._—Certain phosphates contain pyrites which hydrochloric acid does not dissolve, and there is left consequently, a residue more or less colored. In this case it is necessary to add some nitric acid and to prolong the boiling until the pyrite has disappeared, since it might retain a small quantity of phosphoric acid in the state of iron phosphate. (5) _Sulfuric Acid._—Some chemists decompose the phosphates by means of dilute sulfuric acid. This method, which is certainly able to give good results for certain products and for certain processes, presents numerous inconveniences which tend to render its use objectionable for volumetric purposes. The calcium sulfate which is formed, requires prolonged washings which lead to chances of fatal error. If an aluminum phosphate be under examination, containing only very little or no lime, sulfuric acid is to be preferred to hydrochloric and nitric acids, since it attacks amblygonite, which, as has been before stated, resists the action of the other two acids. But these are cases which are met with very rarely, and which can always be treated by the general method by previously fusing the material with a mixture of sodium and potassium carbonate. In the great majority of cases the decomposition by hydrochloric acid is very easily accomplished by simply boiling in a glass vessel, and without effecting the separation of the silica. This operation is only necessary after the substance has been fused with alkaline carbonates, or, in case of substances which contain decomposable silicates giving gelatinous silica with hydrochloric acid. There are two methods [see (6) and (7)] of securing a solution of the sample taken which varies from one to five, and even ten grams, according to the apparent homogeneity of the material to be analyzed. (6) _Solution by Filtration and Washing._—The ordinary method can be employed consisting in decomposing the substance by an acid, filtering, and washing the residue upon the filter, and combining all the wash-waters to make a determinate volume. Afterwards an aliquot fraction of the whole is taken for the precipitation. This method is long, and presents some chances of error, when the insoluble residue is voluminous and contains silica which obstructs the pores of the paper and renders the filtration difficult.