till it boils moderately. Conduct the vapour through a tolerably wide glass tube into an empty flask, not shown

Fig. 55.

in the cut, which is placed in a vessel filled with cold water. In summer time the water may easily be made colder by adding a few tea-spoonfuls of powdered saltpetre. If the vapour be suffered to escape into the air, it appears in thick white fumes, having a pungent acid smell; but if conducted into the flask, it is con

densed into a glistening white, solid mass. This is sulphuric anhydride. The distillation is stopped as soon as the boiling ceases, and the glass tube becomes too hot for the hand to bear. What remains in the flask no longer fumes; it has become common sulphuric acid. To cause this to boil, you must apply a ten-times stronger heat than before, for it does not begin to boil till above 572° F. (300° C.), while the anhydride boils even at a little above 86° F. (30° C.) This is the reason why the boiling ceases when the latter has escaped.

Experiment 10.-Take out some of the anhydride by means of a glass rod, and introduce it into a dry test-tube; it will fume violently, and after a time become fluid; that is, it attracts water from the air, and is thereby converted into Nordhausen sulphuric acid. On longer standing, it absorbs still more water, and ceases to fume; it thus becomes common sulphuric acid. By evaporation this water cannot again be removed.

Experiment 11.-If anhydrous sulphuric acid be thrown into water, it is dissolved with a hissing noise and the violent evolution of heat.

Experiment 12.--It is likewise dissolved by common sulphuric acid, converting this into the fuming acid. Fuming sulphuric acid may therefore be regarded as a compound of the anhydride with ordinary sulphuric acid, H2SOSO.

Fuming, or Nordhausen sulphuric acid is obtained by the distillation of green vitriol, or ferrous sulphate.

Experiment 13.-Put a crystal of green vitriol into a hard

glass tube, and heat it; aqueous vapour escapes, and the green crystal becomes white (anhydrous). On further heating the white colour passes into reddishbrown, and sulphurous and sulphuric anhydrides are evolved, while the iron remains as ferric oxide:

Fig. 56.

In preparing the fuming acid on a large scale, earthen retorts are used, and the anhydride is conducted into common sulphuric acid, which dissolves the sulphuric anhydride, and is thereby converted into the fuming acid, while the sulphurous anhydride escapes. It is a thick liquid, like oil, and is called Nordhausen sulphuric acid, because this city supplied Germany with it for centuries. It has the specific gravity of 1.9.

If this acid be exposed to the air, the anhydride in it evaporates, and unites with the watery vapour contained in the air; accordingly, common sulphuric acid is formed, which, being less volatile, condenses in the cold air, forming white vapours, just as steam does. Consequently the fumes of this acid consist of the vapour of common sulphuric acid.

As long as the process of manufacturing sulphuric acid from green vitriol was the only one known, it was a very expensive article. A hundredweight is now obtained for the same sum that was formerly paid for two pounds.

Common sulphuric acid, HSO4.-Sulphur by burning is converted into sulphurous anhydride, SŌ2, and this combined with water gives sulphurous acid, H,SO. To convert this into sulphuric acid, H2SO,, an additional atom of oxygen is necessary, and this additional oxygen is generally supplied at the expense of nitric acid.

Experiment 14.-Fasten some pieces of sulphur to an iron wire, inflame, and hold them in a capacious bottle containing a little water, until the blue sulphur flame is extinguished; the bottle becomes filled with a white smoke, which is recog

nised by its odour to be sulphurous acid. If you now inFig. 57. troduce a shaving moistened with nitric acid into the vessel, reddish-yellow fumes will immediately form around the wood, gradually filling the whole bottle. These fumes are nitric peroxide, and their evolution indicates that the sulphurous acid has withdrawn oxygen from the nitric acid, and has been oxidized and converted into sulphuric acid. After some time the bottle becomes clear again, because the vapour of the

sulphuric acid formed sinks to the bottom, and dissolves in the water, and we can now again burn sulphur in the bottle. If we repeat this operation several times, we can soon prepare a few ounces of diluted sulphuric acid.

Experiment 15.-Add some drops of a solution of barium chloride to a portion of the acid liquid just obtained; a copious white precipitate is formed, which disappears neither by boiling, nor by the addition of water or nitric acid. This precipitate is barium sulphate, a salt quite insoluble in water and acids. Add one drop of the diluted acid to a wine-glassful of water, and add to this a little more of the solution of barium chloride; even at this great dilution a perceptible cloudiness will be produced. A solution of barium chloride, or nitrate, is the most certain test for detecting sulphuric acid and sulphates.

The manufacture of common sulphuric acid on a large scale is conducted on the same principle as in the last experiment but one. The sulphur is burnt on a sort of hearth, and the sulphurous anhydride thus formed is carried by the draught of air into an immense leaden chamber, the atmosphere of which is kept moist by jets of steam. The sulphurous anhydride thus becomes sulphurous acid. A little nitric acid (or saltpetre and sulphuric acid, which yield nitric acid) is placed in a basin, supported by a tripod above the burning sulphur, and the heat drives it in vapour into the chamber. In the leaden chamber this nitric acid immediately oxidizes a portion of sulphurous acid into sulphuric acid, and is itself reduced to nitric oxide, NO:

2HNO, +3H2SO, = H2O +2NO + 3H ̧SO

Now we shall hereafter learn that nitric oxide is a colour

=

less gas which, on contact with air, takes up oxygen and forms the brown gas, nitric peroxide, NO2, NO+0 NO2. This change immediately takes place in the leaden chamber, and as fast as the NO, is formed it is again reduced to NO by another portion of sulphurous acid:

NO, +H2SO, NO + H2SO.

=

Again the NO takes oxygen from the air of the chamber, and becomes NO2, and again a new portion of sulphurous acid is oxidized into sulphuric acid, so that the process, after the first stage, is continuous. The nitric oxide acts as a carrier, taking oxygen from the air, and giving it out again to the sulphurous acid, which is unable to take oxygen direct from the air (except very slowly), so that theoretically there is no limit to the quantity of sulphurous acid which may be oxidized by a given weight of nitric acid. As fast as the sulphuric acid forms, it condenses and settles down on the floor of the leaden chamber. From time to time it is drawn off and concentrated by evaporation, first in leaden, and afterwards in glass or platinum vessels. The acid thus obtained has almost exactly the composition of true sulphuric acid, H2SO,.

The above account of the sulphuric acid manufacture must only be regarded as an approximation to the truth, for the process is in fact extremely complex. If the quantity of steam be deficient, white crystals of complex composition are apt to form. On the addition of water, however, they are decomposed, and sulphuric acid formed from them.

Experiment 16.-Let some sulphuric acid remain in an open flask exposed to the air; it will increase every day in weight, for it very eagerly attracts water from the air. After standing for some months in a damp place it will become two or three times heavier than before. Some substances, especially gases, are dried by means of sulphuric acid.

Experiment 17.-A piece of wood introduced into sulphuric acid becomes black, and is charred just as when it is exposed to the flame of a candle. The sulphuric acid seizes upon its hydrogen and oxygen, which combine to form water, and the carbon is left behind. Wood may be charred in this way, in order to protect it from decay in moist situations. In the refining of lamp-oil, the mucilage of the oil is charred

by sulphuric acid. Sulphuric acid chars and destroys many vegetable and animal substances. If figures are inscribed on paper with very dilute sulphuric acid, and the paper warmed, the figures will come out black. As the acid becomes concentrated, it acts upon and chars the paper.

Experiment 18.-Pour a drop of oil of vitriol upon paper; decomposition takes place slowly, but it will take place instantaneously if a drop of water is added, because water and sulphuric acid unite together with the evolution of strong heat. For this reason, when sulphuric acid comes in contact with the skin, it should first be wiped off with dry paper or cloth, and then be immediately washed with a great quantity of water. If 50 measures of sulphuric acid are mixed with 50 measures of water, we do not obtain 100 measures, but only 97 measures, of liquid; consequently, a contraction or condensation has occurred, which condensation is itself attended with the liberation of heat.

Experiment 19.-Pulverise a small quantity of indigo, and form a thin paste of it with fuming sulphuric acid. After a few days add to it some water, and you obtain a deep blue liquid-solution of indigo. With this solution wool may be dyed of a fine blue colour (Saxon-blue). Common sulphuric acid dissolves indigo only imperfectly. Indigo, although a vegetable substance, is not carbonised by sulphuric acid, thus forming an exception to the general rule.

Experiment 20.-To half an ounce of copper scales, such as fall off at the coppersmiths', add two ounces of water, and then gradually two-thirds of an ounce of sulphuric acid, and put it in a warm place; you obtain a blue solution, from which afterwards blue oblique, rhomboidal crystals will be deposited. The edges of these crystals are usually obtuse, giving to the narrow sides a roof-like appearance. Copper scales consist of copper oxide; and they combine with the acid, forming copper sulphate (blue vitriol), a soluble salt. The following metallic sulphates are commonly called vitriols ferrous sulphate (green vitriol); zinc sulphate (white vitriol); and copper sulphate (blue vitriol). Copper or iron vessels are cleaned more rapidly and made brighter by water to which some sulphuric acid has been added, than by simple water alone, because the oxide, which tarnished the vessels, is dissolved by the acid.