The first stage in the manufacture of sulphuric acid is the preparation of sulphurous acid by the burning of sulphur or of iron pyrites. Previous to the year 1838, Sicilian sulphur was almost exclusively used in the manufacture, but in that year the very ill-advised establishment of a monopoly of the sulphur trade by the Sicilian gov ernment, and its consequent increase in price, diverted the minds of manufacturers to the employment of iron pyrites. (sulphuret of iron), the'use of which, as a source of sul phurous acid, was already not unknown. The monopoly was quickly abolished, on the representations of the English government, but not until it was demonstrated that the world was independent of Sicily both for sulphuric acid and sulphur. Iron pyrites is now much more used than sulphur, and the only hindrance to its universal adoption is the presence of foreign matter in the pyrites, the most deleterious being arsenical compounds; and it has hitherto been found impracticable to free the sulphuric acid wholly from the arsenious acid which consequently accompanies it. This renders the acid prepared from pyrites inapplicable for many purposes.
When sulphur is the material used for producing the sulphurous acid, it is burned in an oven or "burner" (A) of brick-work, having a sole or bottom of iron, termed the "burner-plate." Under this a small fire is at first lighted, which is allowed to go out after the sulphur has ignited. A little above the sulphur, a small pot, called the niter pot, m„ is either placed on a stand or hung from the roof, filled with a quantity of either nitrate of soda or nitrate of potash, with sulphuric acid sufficient for its decomposition —8 or 10 lbs. of the niter, with 5 or 6 lbs. of sulphuric acid, being allowed for every cwt. of sulphur. The decomposition of the niter by the action of heated sulphuric acid furnishes nitric acid fumes, which go over into the chamber along with the sulphurous Acid. The sulphurous readily abstracts from the nitric acid the additional equivalent of oxygen required for its conversion into sulphuric acid, reducing the nitrous com pound from nitric acid, to nitrous oxide, the reaction being thus: = Nitrous oxide in its turn quickly converts itself into nitrous acid, by the abstraction of two additional equivalents of oxygen from the air that is constantly entering the chamber -through the burners. Again, in the presence of moist ure, which is supplied by a jet of steam from the boiler C, sulphurous acid readily deprives the nitrous acid of two equivalents of oxygen, and thus forms two more volumes of sulphuric acid, and again liberates nitrous oxide; which is ready once more to seize upon the oxygen of the air, and would continue so acting and reacting ad infini Cum, were it not carried forward and out by the chimney provided for the escape of the freed nitrogen.
The chamber is an immense box or room of lead, bound together with a strong frame work of timber, and generally raised on arches several feet above the ground. Cham bers vary in size from 60 to 140 ft. in length, and from 20 to 90 ft. in width and height. Curtains of lead proceeding alternately from the bottom to near the top, and vice versa, are frequently used; they serve to retard the progress of the gases, and thus insure the transformations desired. The floor of the chamber is covered with water, into which the sulphuric acid falls as it is formed; and when this solution attains a certain strength, it is tapped off for concentration. When the gases reach the chimney, on account of
the reactions of the nitrous compounds already explained, a large amount of nitrous acid would not only be wasted, but would also he deleterious to the neighborhood, were steps for its recovery not adopted. This recovery is usually effected by means of a tower filled with coke, E, down which a constant stream of strong sulphuric acid trickles, the acid absorbing the nitrous fumes in their way upward. Instead of a single chamber, curtained off or not as the case may be, sometimes three or five distinct chambers, con nected by pipes, are employed, those communicating directly with the burners being termed working chambers, and the others receiving chambers, the last either acting as or communicating with a condenser or chimney. In France and Germany, the appa ratus employed is generally of a more complicated nature, but in principle the operations are identical.
When iron pyrites is used as the source of sulphurous acid, a furnace somewhat on the principle of the ordinary lime-kiln is required. The pyrites is broken into pieces like nuts, washed, and spread in layers on plates heated to redness, and frequently stirred; or a quantity of coke is introduced with the first charge; and the heat evolved by the burning sulphur is thereafter sufficient fuel for the fresh charges. The exhausted ore is frequently sufficiently rich in copper for its extraction; indeed, when there is not more than 2-1 per cent of that present in pyrites, it is now recovered, and this has led to the establishment of copper smelting works in connection with great chemical works near Newcastle and Manchester. The use of pyrites necessitates a chamber of comparatively larger size fbr the production of a given amount than is required when sulphur is used. The increased labor, with the greater quantity of niter wasted, and other circumstances, tend to make the cost of acid from both sources nearly equal.
In consequence of strong sulphuric acid absorbing both sulphurous acid and nitrous acid, the acid requires to be drained off from the chamber while the solution is compara tively weak, at which strength, viz., of a specific gravity of about 1.4, it is used for some purposes in the arts, under the name of " chamber acid." This is concen trated by evaporating, in lead pans, D, till it reaches the specific gravity of 1.6, then boiling in a platinum retort, on which strong acid does not act, even at high heat, or in large flint-glass retorts. Platinum retorts are extremely expensive; one to hold 30 gallons costing nearly £900. Large glass retorts which were used before the introduction of the platinum retorts, are again coming into favor with manufac turers. The only objection to their use is the great expense arising from frequent breakages, and consequent loss of both acid and retort.
The manufacture of sulphuric acid is a very extensive industry; immense quantities of it being consumed in the manufacture of soda, in that of bleaching-powder, in calico printing and dyeing, and, in fact, in most chemical operations both in the manufactory and She laboratory. In South Lancashire alone upward of 700 tons of concentrated acid are produced weekly. A process of purification is required for the acid in several of its minor applications; but for general purposes, it is sufficiently pure as supplied by the works.