_Characters._--Concentrated sulphuric acid or oil of vitriol (specific gravity, 1.800 to 1.845), as it is found in commerce, is a heavy, oily, colourless, or slightly brownish-coloured liquid, not fuming when exposed to the atmosphere; but, when added to water, causing a rapid increase of temperature, which may crack the glass vessel in which the mixture is made. Sulphuric acid chars and destroys the texture of organic bodies placed in it. _Dilute sulphuric acid_ is a colourless, strongly acid liquid, reddening litmus, and charring paper dipped into it when subsequently dried, care being taken not to scorch the paper. _Symptoms, &c._--The symptoms and _post-mortem_ signs have been already described, p. 246. _Chemical Analysis and Tests._--The acid will have to be examined under the following heads: (1) Simple, concentrated acid. (2) Dilute acid. (3) Mixed with organic liquids, food, vomit, &c. (4) On the clothes of the person injured. I. Concentrated Acid 1. _Chars Organic Matter._--A piece of wood or paper placed in the strong acid rapidly becomes blackened. 2. _Heat when added to Water._--Equal quantities of acid and water added together produce intense heat. 3. _Evolution of Sulphurous Acid._--When boiled with chips of wood, copper cuttings, or mercury, fumes of sulphurous acid are evolved, detected by their sulphur-like odour, and by their power of first bluing and then bleaching starched paper dipped in iodic acid. II. Dilute Acid 1. _Chars Paper._--This only occurs when the paper is dried by the aid of heat, subsequently to moistening it in the dilute acid. 2. _Precipitation of Sulphate of Barium._--A solution of the nitrate or of the chloride of barium is precipitated by sulphuric acid in the form of a white insoluble powder, unaffected by nitric or hydrochloric acid, even when heat is applied. This test is so delicate, that a liquid containing ¹/₂₅₀₀₀th part by weight of the acid is precipitated by either of the test solutions. 3. _Action of Heat._--The dilute acid is entirely volatilised by heat. III. Mixed with Organic Liquids, &c. In tea, coffee, or beer the mode of applying the tests are the same, the mixture being previously filtered, or the acid separated from the organic mixture by dialysis, or the following cautions are necessary: _Objection A._--Alum, or any acid sulphate, would give all the reactions with the nitrate and chloride of barium. _Answer A._--Evaporate a portion of the doubtful liquid; if pure acid, there will be no residue, sometimes only the _slightest trace_ of sulphate of lead. _Objection B._--Erroneous estimation of free sulphuric acid, in consequence of the presence of some saline or neutral sulphates. _Answer B._--Evaporate as before. The free sulphuric acid separated by warming the liquid is then added to a known weight of powdered carbonate of baryta until effervescence ceases. The resulting precipitate when weighed, represents the free sulphuric acid present.