Sulphuric Acid, is obtained either by simple distillation from copperas, (which was the original method,) or by the combustion of sulphur, in large leaden chambers, in combination with substances yielding a large supply of oxygen; which is the method generally practised at the present day. The latter process is conducted in various ways; the most usual method in this country is the following. Common brimstone, coarsely ground, is mixed with saltpetre, in the proportion of 8lbs. of the former to 1lb. of the latter; the mixture is spread upon iron plates set upon stands of lead, in a large chamber, lined with lead, and covered at the bottom with a thin sheet of water. The materials are ignited by means of a hot iron, and the door is closed. The sulphur in vapour, then combining with the oxygen of the nitre, forms sulphuric acid, and condenses in the water, which is afterwards drawn off and concentrated, first in leaden vessels, and then in glass retorts (which are sometimes lined internally with platinum); but some manufacturers dispense with the leaden boilers altogether.
In some of the more recently established manufactories in France, the following process has been employed with very advantageous results: - The sulphur is burned upon an iron plate, set over a furnace, beneath a leaden cylinder, opening into a leaden chamber, containing about 20,000 cubic feet: at the same time, a retort, placed in a sand bath, and containing 93/4 lbs. of nitric acid, and 11/4 lb. of molasses, is heated, the nitrous gas disengaged being conducted into the leaden cylinder about 2 feet above the burning sulphur, and this operation being continued until all the nitrous gas is disengaged.
From the residue in the retort oxalic acid is obtained. About two hours after the combustion has commenced, steam is admitted into the chamber by a pipe, which enters about the middle of the chamber. Soon after the introduction of the steam, a condensation is perceived in the chamber, and a small hole is opened to admit a supply of atmospheric air. When the condensation is completed, which is commonly in about three hours' time from the commencement of the operation, a door in the cylinder, and two valves placed under a tall chimney, are opened, in order to renew the air in the chamber, after which the operation may be repeated. The bottom of the chamber should always be covered with liquid, and it is inclined to the horizon, so that the liquid may be 9 inches deep at one end, and only 11/2 at the other, and only the overplus is drawn off daily. The acid may be concentrated in the chambers to about 1.450 spec. grav.; after which it is removed to leaden boilers, and brought to a spec. grav. of 1.600; the remaining requisite concentration is effected in glass and platinum retorts.
A quantity of acid comes over during the evaporation, which is condensed by a leaden worm fixed to the neck of the retort.
The annexed cut represents the apparatus employed in this process, a is a portion of the chamber lined with lead; 6 the leaden cylinder entering the chamber at one end of it, and rising about 10 inches above the floor c.
The cylinder at its lower part d, turns inwards and upwards, forming a gutter e, concentric with the cylinder; in this gutter a portion of weak acid is always kept up as high as the line g, to prevent the lead from getting too much heated. The whole is placed on a mass of brickwork h, in the middle o which there is an iron dish k, having a convex bottom, and a rim 3 inches high , it is mounted over a furnace I. m is a door in the leaden cylinder, and n, an air-hole in the door, fitted with a stopper; r, a glass mattress, containing the nitric acid and molasses; s, the steam boiler; t, the steam pipe. Sulphuric acid is very extensively used in the chemical arts, particularly in bleaching, and some of the processes of dyeing; in the manufacture of the nitric and muriatic acids, and in numerous other branches of manufacture. The combinations of this acid with various bases are called sulphates, many of which are of great utility in medicine and the arts. The acidulons sulphate of alumina, combined with potash or ammonia, forms the important article called Alum,