_c._ OXALIC ACID.--The third portion of the residue is heated with sulphuric acid, and the evolved gas carefully collected. It should then be confirmed by an elementary analysis that the gas consists of equal volumes of carbonic oxide and carbonic acid. This test is not conclusive; it is also necessary to ascertain if the precipitate produced by the addition of a baryta solution (_vide_: under portion _B._) produces the same reaction, inasmuch as other organic bodies could give rise to carbonic oxide and carbonic acid, and the danger of both admitting the presence of oxalic acid, when it is absent, and omitting its detection, in case it is present, would be incurred. _d._ ACETIC AND FORMIC ACIDS.--The fourth portion of the residue is distilled with dilute sulphuric acid. After determining that a small portion, previously neutralized with a base, acquires a red color, upon addition of a solution of a persalt of iron, the distillate is divided into two parts. One portion is treated with bichloride of mercury: if _formic acid_ be present, metallic mercury is formed, with evolution of carbonic acid which produces turbidity in lime-water. The remaining portion of the fluid is digested, in the cold, with an excess of litharge: in presence of _acetic acid_, a soluble basic salt of lead, possessing an alkaline reaction, is produced. PORTION B.--The second portion of the solution is supersaturated with nitric acid, and this neutralized by addition of a slight excess of ammonia. The ammonia is then expelled by boiling the fluid, and a solution of nitrate of baryta added. If a _precipitate_ forms, it is collected and subsequently examined for sulphuric, phosphoric, oxalic and boric acids as directed below. The _filtrate_ is preserved and tested for hydrochloric, hydrobromic and hydriodic acids. _a._ OXALIC ACID.--A portion of the precipitate produced by the addition of nitrate of baryta is submitted to the test mentioned under the treatment of portion _A_. _b._ SULPHURIC ACID.--If an insoluble residue remains upon treating the remainder of the precipitate with dilute hydrochloric acid, it consists of sulphate of baryta and indicates the presence of _sulphuric acid_. _c._ PHOSPHORIC ACID.--An excess of solution of alum and ammonia is added to the portion of the precipitate dissolved in hydrochloric acid. If phosphoric acid be present, insoluble phosphate of alumina is precipitated. This is brought upon a filter: the _filtrate_ being preserved and subsequently examined for boric acid. Upon boiling the precipitate with solution of silicate of potassa, silicate of alumina is thrown down, and phosphate of potassa remains in solution. Chloride of ammonia is now added to the liquid--in order to eliminate the excess of silica from the silicate--and the solution filtered. The _filtrate_ is then tested for phosphates, by means of molybdate of ammonia (_vide_: _detection of phosphoric acid_, p. 48).