There is scarcely any article that is used for such a different variety of purposes, and one, too, that is so highly useful, of which there is so little known of its production, as oil of vitriol. Although it may not belong to the peculiar province of the manufacturer of liquors to manufacture this acid, yet a knowledge of its formation and general properties is necessary to a complete practical knowledge of the manufacture of wines, liquors, etc.

Sulphuric acid is obtained by burning sulphur, mixed with one eighth of its weight of nitre, over a stratum of water, contained in a chamber lined with sheet lead; if the sulphur was burned by itself, the product would be sulphurous acid, which contains only two thirds as much oxygen as sulphuric acid; the object of the nitre is to furnish, by its decomposition, the requisite additional of oxygen.

The leaden chambers vary in size, but are generally from thirty to thirty-two feet square, and from sixteen to twenty feet in height; the floor is slightly inclined to facilitate the drawing off of the acid, and covered to the depth of several inches with water. There are several modes of burning the mixture of sulphur and nitre, and otherwise conducting the process, but that pursued in France is as follows: near one of the sides of the chamber, and about a foot from its bottom, a cast iron tray is placed over a furnace, resting on the ground, its mouth opening externally, and its chimney having no communication with the chamber; on this tray the mixture is placed, being introduced by a square opening, which may be shut by means of a sliding door, and the lower side of which is level with the surface of the tray; the door being shirt, the fire is gradually raised in the furnace, whereby the sulphur is inflamed, and the products already spoken of are generated. When the combustion is over, the door is raised, and the sulphate of potassa removed; a fresh portion of the mixture is then placed on the tray, and the air of the chamber is renewed by opening a door and valve situated at its opposite side; next, the several openings are closed, and the fire is renewed. These operations are repeated, with fresh portions of the mixture, every three or four hours, until the water at the bottom of the chamber has reached the sp. gr. of about 1.5, it is then drawn off, and transferred to leaden boilers, where it is boiled down until it has attained sp. gr. 1.7. At this density it begins to act on lead, and therefore its further concentration must be conducted in large glass or platinum retorts, where it is evaporated as long as water distils over. This water is slightly acid, and is thrown back into the chamber. When the acid is filly concentrated, opaque, greyish-white vapors arise; the appearance of which indicates the completion of the process. The acid is allowed to cool, and is then transferred to large demijohns of green glass, called carboys, which, for greater security, are surrounded with straw or wicker work, and packed in square boxes, inclosing all the carboy, except the neck.

Another method of manufacturing this acid consists in spreading the mixture on iron or leaden plates, resting on stands of lead within the chamber, placed at some distance from each other, and a foot or two above the surface of the water; the sulphur is then lighted by means of a hot iron, and the doors are closed. If the sulphur and nitre be well mixed, the combustion will last for thirty or forty minutes, and in three hours from the time of lighting, the con densation of the gases having in that interval been completed, the doors are thrown open for from fifteen to twenty minutes, to admit fresh atmospheric air, and to allow time for the residuary nitrogen to escape. Preparatory to the next burning, the operations are repeated with fresh charges of the mix ture, every four hours, both night and day, until the water has attained the requisite acid-impregnation. When it is transferred to leaden boilers, and otherwise treated, as just explained, the quantity of the charge for each burning is determined by the size of the chamber, allowing one pound of the mixture for every three hundred cubic feet of atmospheric air which it may contain.

As in the manufacture of sulphuric acid, the nitre is the most expensive material. Many plans have been resorted to for the purpose of obtaining the necessary nitrous acid at a cheaper rate. One plan has been to treat molasses, or starch, with common nitric acid. In this case the manufacturer obtains oxalic acid as a collateral product, which serves to diminish his expenses.

In some manufactories of sulphuric acid nitrate of soda is substituted for nitre; the advantages of the, former salt are its greater cheapness, and the cir cumstance of its containing a larger proportional amount of nitric acid.

A new method is now practised by some manufacturers, for making sulphuric acid; it consists in filling the leaden chamber with sulphurous acid, by the ordinary combustion of sulphur, and afterwards admitting into it nitrous acid and steam; the nitrous acid is generated from a mixture of sulphuric acid with nitrate of potassa, or nitrate of soda, placed in an iron pan, over the burning sulphur, in the sulphur furnace, where the draught serves to conduct the nitrous acid fumes into the chamber; as under these circumstances sulphurous and nitrous acid, and the vapor of water, are intermingled in the chamber, it follows, that all the conditions necessary for generating crystalline compounds, already alluded to, are present. Of course, the rationale of this new process is the same as that already given.

What has been said above relates to the mode of preparing common sulphuric acid; but there is another kind known on the continent of Europe by the name of the "Fuming sulphuric acid of Nordhausen," so called from its properties and a place in Saxony, where it is largely manufactured. This acid is obtained by distilling sulphate of iron in large stone ware retorts, heated to redness, and connected with receivers of glass, or stone ware; the acid distils over, and sesquioxide of iron is left in the form of colcothar.

The process for making sulphuric acid by the combustion of sulphur with nitre, was first mentioned by Lemry, and afterwards put in practice by an English physician, of the name of Ward. As practised by him, the combustion was conducted in very large glass vessels. About the year 1746, the great improvement of leaden chambers was introduced by Roeback, an eminent physician of Birmingham, where the first apparatus of this kind was erected. In consequence of this improvement, the acid immediately fell to one fourth of its former price, and was employed for many purposes for which, previously, it could not be used, on account of its high cost.

Properties. - Sulphuric acid, or, as it is commonly called, "oil of vitriol," is a dense, colorless, inodorous liquid, of an oleaginous appearance, and possessing strong corrosive qualities; on the living fibre it acts as a powerful caustic. In the liquid form, it always contains water, which is essential to its existence in that form. When pure, and as highly concentrated as possible, as manufactured in the leaden chambers, its sp. gr. 1.845, a fluid ounce weighing a small fraction over fourteen drachms when of this specific gravity, it contains about 18 per cent, of water; whenever its density exceeds this, the presence of sulphate of lead, or some other impurity, is indicated. The commercial acid is seldom of full strength, and it generally is of the sp. gr. of only 1.8433, an contains 22 per cent, of water. This acid acts po erfully on organic bodies, whether vegetable or animal, depriving them of the elements of water, developing charcoal, and turning them black. This acid will absorb ninety-five per cent, of carbonic acid. When diluted with distilled water, it ought to remain limpid. When this acid is present in small quantities in solution, it is detected unerringly by chloride of barium, which causes a precipitate of sulphate of baryta. The most usual impurities in sulphuric acid, are the sulphates of potassa and lead; the former derived from the residue of the process, the latter from the leaden boilers in which the acid has been concentrated.

Occasionally nitre is added to render dark samples of acid colorless; this addition will give rise to the impurities of sulphate of potassa; these impurities often amount to three or four per cent. The commercial acid cannot be expected to be absolutely pure, but when properly manufactured, it ought not to contain more than one fourth of one per cent, of impurity. The fixed impurities are discoverable by evaporating a portion of the suspected acid, when they will remain. If sulphate of lead be present, the acid will become turbid on diluting with an equal bulk of water. This impurity is not detected sulphuretted hydrogen, unless the sulphuric acid be saturated with an alkali. If only a scanty muddiness arises, the acid is of good commercial quality.

Other impurities occur in the commercial sulphuric acid. Nitrous acid is always present in a greater or less amount, and may be detected by gently pouring a solution of green vitriol over the acid, when the solution at the line of contact will acquire a deep red color due to the sesquioxidation of the iron by the nitrous acid. The commercial acid is not to be rejected on account of the indications of this test, unless it shows the presence of nitrous acid in unusual quantities. The mode of removing this impurity by the aid of sugar, consists in heating eight fluid ounces of the acid, with twelve grains of refined sugar, at a temperature not quite sufficient to boil the acid, till the dark color at first produced, shall have nearly or altogether disappeared.

The dangerous impurity of arsenic is often present in sulphuric acid, and the test is so simple and economical, that no manufacturer should make use of this acid, without first testing for arsenic.