SULPHURIC ACID. The most im portant compound of sulphur with oxy gen. It occurs native in volcanic dis tricts in equatorial Asia, and in Italy and Sicily,. i and occasionally in springs, where there organic matter brought into con tact with a ferruginous sulphate : this occurs in some of the springs of western New York. When pure or anhydrous (without water) it consists of an equiva lent of sulphur united with three equi valents of oxygen, bat it is never known except as a hydrated acid. The manu factories where it is made require large room, and entail great expense for the purchase of platinum stills. The manu facture is but small in this country the greater portion being derived from Eng land, of which there are 50,000 tons an nually made.
Hydrated sulphuric acid has been known since the fifteenth century. There are two distinct processes by which it is at the present time prepared, by the distillation of green sulphate of iron, and by the oxidation of sulphurous acid by nitrous acid.
The first process is still carried on at Nordhausen in Saxony ; 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 red heat in earthen retorts, to which receivers are fitted as soon as the acid begins to distil over. A part gets decomposed by the very high temperature • the remainder is driven off in vapor, which is condensed by the cold vessel. The product is a brown oily li quid, of about 1.9 specific gravity, fuming i in the air, and very corrosive. It is chiefly made for the purpose of dissolv ing indigo.
The second method, which is perhaps, with the single exception mentioned, al ways followed as the more economical, depends upon the fact that when sul phurous acid, nitrous acid, and water are present in certain proportions, the sul• phurous acid becomes oxidized at the ex pense of the nitrous acid, which by the loss of one-half of its oxygen sinks to the condition of dentoxide of nitrogen. The operation is thus conducted :—A large and very long chamber is built of sheet lead supported by timber framing ; on the outside at one extremity a small fur nace or oven is constructed, having a wide tube leading into the chamber. In this sulphur is kept burning, the flame of which heats a crucible containing a mixture of nitre and oil of vitriol. A
shallow stratum of water occupies the floor of the chamber, and sometimes a jet of steam is also introduced. Lastly, an exit is provided at the remote end of the chamber for the spent and useless gases. The effect of these arrangerrients is to cause a constant supply of sulphurous acid, atmospheric air, nitric acid vapor, and water in the state of steam, to he thrown into the chamber, there to mix and re-act upon each other. The nitric acid immediately gives up a part of its oxygen to the sulphurous acid, becoming nitrous ; it does not remain in this state, however, but suffers further de-oxidation until it becomes reduced to deutoxide of nitrogen. That substance in contact with free oxygen, absorbs a portion of the lat ter, and once more becomes nitrous acid, which is again destined to undergo a de oxidation by a fresh quantity of sulphur ous acid. A very small portion of nitrous acid, mixed with atmospheric air and sul phurous acid, may thus in time convert an indefinite amount of the latter into sulphuric acid, by acting as a kind of car rier between the oxygen of the air and the sulphurous acid. The presence of water is essential to this re-action.
We may thus represent the change :— Such is the simplest view that cau be taken of the production of sulphuric acid in the leaden chamber, hut it is too much to affirm that it is strictly true; it may be more complex. When a little water is put at the bottom of a large glass globe so as to maintain a certain degree of hu midity in the air within, and sulphurous and nitrous acids are introduced by se parate tubes, symptoms of chemical so tion become immediately evident, and after a little time, a white crystalline mat ter is observed to condense on the sides of the vessel. This substance appears to be a compound of sulphuric acid, hypon itrous acid, and a little water. IN hen thrown into water, it is resolved into sulphuric acid, dentoxide of nitrogen, and nitric acid. This curious body is certainly very often produced in large quantity in the leaden chambers, but that its production is indispensable to the success of the process, and constant when the operation goes on well and the nitrous acid is not in excess, may per haps admit of doubt.