SULPHURIC ACID.

265

Characteristics. —Free sulphuric acid or the soluble sulphates arerecognized by a solution of any of the barytic salts, which throws down aheavy white precipitate (sulphate of baryta) insoluble in acids or alkalies.If this precipitate (or any insoluble sulphate) be ignited with charcoal, itis decomposed and converted into a sulphuret, which, on the addition ofhydrochloric acid, evolves hydrosulphuric acid, known by its odour andits blackening paper moistened with acetate of lead. If sulphuric acidbe heated with organic matter, sulphurous acid is given out: this may beknown by its odour, and by its occasioning the formation of the blueiodide of starch when mixed with iodic acid and starch.

To determine whether sulphuric acid is free or combined with a base,Dr. Christison ( Treatise on Poisons, 3d ed. p. 142) recommends theliquid to be boiled with pure carbonate of lead. If any free sulphuricacid be present sulphate of lead will be formed, which, being insolublein dilute nitric acid, may be thereby distinguished from carbonate of lead.But this test is liable to objection: alum and other supersulphates re-acton carbonate of lead like free sulphuric acid ; and, furthermore, sulphateof ammonia is decomposed at a boiling temperature by carbonate of lead,

Impurities. —Pure sulphuric acid should be colourless : the presenceof organic matter, as cork, communicates a brownish or black colour.The acid of commerce usually contains traces of sulphate of lead:this may be detected either by adding water, when the white sul-phate is precipitated; or by the addition of hydrosulphuric acid tothe dilute sulphuric acid, when a very slight change of colour isobserved, owing to the formation of sulphuret of lead. “ What remainsafter the acid has been distilled to dryness does not exceed thepart of its weight.”—( Ph . Lond.) The residuum here referred to issulphate of lead.

Physiological Effects, {a.) On vegetables. —In the concentrated statesulphuric acid chars the parts of plants to which it is applied. In thedilute form it destroys vegetables in a few hours (Marcet, in Decandolle’sPhys. Veg'et. p. 1345).

(b.) On animals generally .—The action of sulphuric acid on animalsgenerally is precisely the same as that on man. Thrown into the veinsof a dog, Orfila found that it coagulated the blood, and caused immediatedeath.—( Toxicol. Gener.)

(i c .) On man. —Properly diluted and administered in small but repeateddoses, snlphnric acid acts as a tonic and refrigerant, like the other mineralacids (see pp. 80, 84, 96). Thus it promotes the appetite, diminishesfebrile heat, allays excessive thirst, checks profuse sweating, and, notunfrequently, reduces the frequency of the pulse. Sometimes it increasesthe secretion of urine, and likewise renders this fluid unusually acid.Under its use the milk frequently acquires a griping quality.

The continued use of it, however, generates a slow kind of gastro-enteritis : heat and pain are experienced in the throat, stomach, andintestines; the digestive functions become disordered; gripings areexperienced ; sometimes purging, and subsequently fever. Of all diemineral acids, save the phosphoric, this may be administered for thelongest period without occasioning the above symptoms.

Concentrated sulphuric acid acts as an energetic caustic poison,promptly disorganizing the different tissues with which it is brought incontact. Its chemical action depends principally on its affinity for