[Illustration: Fig. 29.--Photo-micrograph of crystals of oxalic acid, × 50. (R. J. M. Buchanan.)] _Chemical Analysis._--From organic mixtures the acid may be separated by dialysis, and the tests applied, or it may be obtained in crystals by precipitating it from the boiled and filtered organic mixture with acetate of lead. The precipitate washed is then decomposed by sulphuretted hydrogen and filtered, the filtrate concentrated to drive off excess of sulphuretted hydrogen, and then set aside to crystallise, which, if the acid be present, it does in slender prisms. From the contents of the stomach the acid may be separated by partial drying over a water bath, extraction with hot alcohol acidulated with a little hydrochloric acid, filtering the alcoholic solution, evaporating to dryness, and dissolving the residue in water. Should, owing to the treatment adopted, oxalate of lime in white chalky masses be found in the stomach, these should be washed and then boiled with pure carbonate of potash. A partial decomposition takes place, insoluble carbonate of lime and soluble oxalate of potash are present in the liquid, which, when filtered and neutralised with nitric acid, may be tested with the following reagents: 1. _Nitrate of silver_ gives a white precipitate, _soluble_ in _cold_ nitric acid; the precipitate dried and heated on platinum foil is dissipated in a white vapour with slight detonation. 2. _Calcium chloride or sulphate_ produces a white precipitate with oxalic acid; the test is more delicate if the acid be first neutralised with ammonia. The precipitate is immediately dissolved by hydrochloric or nitric acid, but not dissolved by oxalic, tartaric, acetic, or other vegetable acid. Lime-water should not be used as a test, as it gives precipitates with other acids; the sulphate largely diluted is not open to this objection. 3. _Lead acetate_ gives a white precipitate soluble in nitric acid. On clothes, parchment, &c., the spot or spots must be well boiled, and the above tests applied to the solution. The stains may vary from a brownish-red to orange-red colour. _Fatal Dose._--Three drachms have caused death in one hour; sixty grains caused the death of a boy sixteen years of age, but recoveries have been known to take place after an ounce and an ounce and a half had been swallowed. _Fatal Period._--Death has resulted in _ten minutes_ from a dose of _one ounce_. The shortest period has been three minutes. The time varies with individuals, even when the same quantity is taken. In the case of two girls who each swallowed _an ounce_ of oxalic acid, one died in _ten minutes_, and the other in _thirty minutes_. Death usually takes place within from half an hour to an hour, although it has been delayed for five days.