The salts formed by the combination of sulphuric acid with a base— are generally composed, asin the case of green vitriol (Fe0,S0, 7Ao), of 1 equivalent of acid and 1 of metallic oxide, with or without water of crystallization. With the alka lies this acid also forms acid salts, as bisulphate of potash, and in a few cases—copper, for example—it forms basic salts. The insoluble sulphates, such as that of baryta, may be obtained by precipitating a soluble salt of the base by a soluble sulphate; thus, nitrate of baryta and sulphate of soda yield an insoluble sulphate of baryta and nitrate of soda, which remains in solution. The soluble sulphates may be prepared by dissolving the oxide or carbonate in dilute sulphuric acid, in those cases in which the metal itself is not readily attacked by the acid. Sulphuric acid and the soluble sulphates are easily detected by their yielding, with a solution of a baryta salt, a white precipitate of sul phate of baryta insoluble in acids.
This acid is employed in the arts and manufactures for a large number of purposes. Its use as a desiccating agent for laboratory purposes has been already noticed, and its application to the development of oxygen gas has been described in the article on that element. But its greatest consumption, doubtless, is in the preparation of the which is used in the manufacture of carbonate of soda (q.v.).
In medicine a dilute sulphuric acid, formed by gradually mixing three fluid ounces of the strong purified acid with thirty-five fluid ounces of water, or aromatic sulphuric acid (known also as elixir of vitriol), prepared by mixing three ounces of sulphuric acid with a quart of rectified spirit, adding cinnamon and ginger, digesting for a week, and filtering, are almost always employed. In doses of from ten to thirty minims, properly diluted, these preparations exert a strong astringent power, and are serviceable in all forms of passive hemorrhages, and in checking inordinate discharges when they arise from debility. In ordinary diarrhoea, and even in the premonitory diarrhoea of cholera, dilute sulphuric acid is of great use. In painter's colic it is given in order to convert any lead that is absorbed into an insoluble sulphate, which is inert. Sulphuric acid lemonade is also used as a prophylactic against the disease. As this acid exerts a dele terious action on the teeth, it should be directed to suck it through a quill. In some cases it is prescribed not so much for its specific as for its solvent power; with this object it is usually prescribed with quinia. The strong acid is used in surgery as a caustic. In cases of poisoning with this acid the most prominent features are, burning pain extending from the mouth to the stomach, intense pain in the bowels, vomiting, great prostration, coldness of the surface, and fetor of the breath. The mucous mem
brane of the parts injured by the acid is at first converted into a white slough, which soon becomes black, and the patient usually dies from exhaustion within twenty-four hours. The best antidotes are the alkaline bicarbonates, or carbonate of magnesia. If the primary symptoms be conquered the patient often dies subsequently from stricture of the oesophagus.
Sulphuric Acid are two distinct processes by which sulphuric acid is at present prepared on a large scale—viz., by the distillation of green sulphate of iron—the original process of Valentine; and by the oxidation of sulphurous acid through the agency of nitrous acid and hyponitric acid. The first process is chiefly employed at Nordhausen, in Prussia, and is thus described by Fownes: "The sulphate of iron, derived from the oxidation of iron pyrites, is deprived by heat of the greater part of its water of crystallization, and subjected to a high heat in earthen retorts, to which receivers are added as soon as the acid begins to distill over. A part gets decomposed by the very high temperature; the remainder is driven off in vapor, which is condensed by the cold vessel containing a very small quantity of water or common sulphuric acid. The prod uct is a brown oily7liquid of about 1.9 specific gravity, fuming in the air, and very corrosive. It is chiefly used for the purpose of dissolving indigo." The second method is that universally followed in Great Britain, the germs of which were likewise discovered by Valentine. He observed that when the fumes of burning sulphur were collected under a bell jar, slightly moistened with water, a small quantity of liquid was deposited. This liquid, which was simply sulphuric acid, on being con centrated from its solution by boiling, was long sold as oil of sulphur per campanum, at prices as high as 2s. 6d. per ounce.
About the year 1740, the French chemists Lefevre and Lernery suggested that, by the use of niter along with the sulphur, the operation might be conducted in close ves sels, and a much greater quantity of acid might be produced. This idea was acted on in England by a Dr. Ward, who established works at Twickenham and Richmond, con ducting his manufacture by burning the mixed sulphur and niter in large stoppered glass receivers, into each of which a small quantity of water was first introduced. The substitution, by Dr. Roebuck of Birmingham, of lead chambers in place of glass vessels, may be regarded as essentially the establishment of the process of manufacture followed at the present day Dr. Roebuck established his first works at Prestonpans in 1749.