Sulphur is then burned in a furnace so contrived that the current of air which supports the combustion conducts tho gaseous products, including the nitric acid fumes and excess of air, into large leaden chambers, the bottoms of which are covered to the depth of a few inches with water. The nitric acid of the nitre, composed of oxygen and nitrogen, is here decomposed, yields oxygen to a portion of the sulphurous acid formed by combustion, and converts it into sulphuric acid. The nitric acid, on losing oxygen, is converted into nitric oxide, which, by mixing with the air at the moment of its separation, com bines with its oxygen, and gives rise to red nitrous acid vapour. The gaseous substances present in the leaden chambers are therefore sulphurous and nitrous acid., atmospheric air, and watery vapour. Now, when dry sulphurous acid gas and dry nitrous acid gas are mixed together, no action occurs between them; but when a little moisture Is added. a white crystalline compound of sulphuric acid, hyponitrous acid, and water is formed ; and when this falls into the water of the chamber, or comes into contact with steam, which is for this purpose blown into the chamber, it is instantly decomposed, the sulphuric acid is dissolved, and nitrous acid and nitric oxide gases escape with offer TCSCCUCO. The nitrous acid thus act free, as well as that reproduced by the nitric oxide uniting with the oxygen of the atmosphere, is again intermixed with sulphurous acid and moisture, and thus gives rise to a second portion of the crystalline body, which undergoes the same change as the first ; and this operation is repeated until the water at the bottom of the lead-chambers is sufficiently acid to be removed for ulterior operations. Tho amount of nitric acid required in this ope ration is of course exceedingly- small, inasmuch as the nitric oxide contained in it is not consumed, but acts only as a carrier of oxygen from the air to the sulphurous acid. Some nitric oxide is, however, conveyed out of the chambers by the escaping nitrogen of the air, and is in most works lost. A few manufacturers cause the escaping gases to pass over strong sulphuric acid that is trickling through a coke Miim, by which the nitric oxide is dissolved and subsequently recovered from the liquid by exposure to a stream of sulphurous acid in a second similar coke column.
It thus appears that sulphur during combustion can combine only with sufficient oxygen to become sulphurous acid; but is curious is, that sulphurous acid becomes sulphuric acid by taking oxygen from nitrous acid, the nitric oxide of which appears nevertheless to have a stronger affinity for oxygen, since it can take that element rapidly from the air, which sulphurous acid cannot. The first attempt at explaining the mode in which nitric acid acts in this operation was made by MM. Clement and Desormes : it was subsequently further explained by Davy and other chemists.
Of late years also sulphuric acid has been made from iron pyrites, the sulphurous acid being formed by combustion, and converted into sulphuric by the agency of nitrous acid, in the manner already described.
When the sulphuric acid in the chambers has acquired a density of about it is drawn off and further concentrated in open leaden vessels by heat ; after this it is again removed either to glass or plating retorts, and heated till it has acquired a density of about This is then the sulphuric acid, or oil of vitriol, of commerce, composed of one equivalent of sulphuric acid and one equivalent of water (HO, SO,).
This acid is a limpid, inodorous, colourless fluid, of an oily con sistence. It boils at about 620°, and distils over unchanged; the boiling-point diminishes with dilution : thus, when of specific gravity it boils at 435°, and when 1'65 only at 350°, the concentrated acid freezes at —15°, but when it contains two equivalents of water instead of only one, and has a specific gravity of 118, it freezes at —40°.
Sulphuric acid is intensely caustic and acrid, and readily decomposes animal and vegetable fibre ; even when diluted to a very great extent it has an extremely sour taste, and turns vegetable blues strongly red : on the other hand, when concentrated, it turns turmeric-paper of a brownish colour, as the alkalies do; but the effect is not permanent, for it is removed by water. Its affinity for water is very great, attracting it so readily from the air that in moist weather three parts increase to four iu 24 hours, and by longer exposure the quantity is increased. When suddenly mixed with water, much heat is evolved, and, on cooling, condensation is found to have taken place, the two fluids occu pying less space than before mixture. When sulphuric acid is mixed in certain proportions with snow, heat is given out, or cold generated, according to the quantities employed : thus four parts of acid and one of snow evolve heat, but four of snow and one of acid occasion cold.
Sulphuric acid is employed for a vast number of purposes : thus, on account of its great chemical power, it is used for the purpose of sepa rating other acids from bases, as in preparing nitric, hydrochloric, acetic, phosphoric, and carbonic acids, &c. It is also used in preparing sulphates, a class of salts we shall presently again refer to. Perhaps the greatest consumption of sulphuric acid is in the decomposition of salt at alkali works where carbonate of soda is manufactured.
The strength of sulphuric acid is readily ascertained by taking its specific gravity and referring to such a table as the following, con structed by Ure Tho salts which sulphuric acid forms with various bases are termed sulphates, sesqui-sulphates, or hi-sulphates, &c., according to circum stances. They arc a very important class of saline bodies, and those of most use will be found described under their respective hues. With sulphuric anhydride, dry rumuouiacal gas forms largo transparent crystals, containing NH,, SO,, and termed sulphatammon ; by long boiling with water, it is converted into sulphate of ammonia (N11,0, SO,).
According to the binary theory of the constitution of salts, sulphates are not combinations of the basic oxide and sulphuric acid, but of a hypothetical body, salphion (SO.), with the metal, or radical, of the base: thus, sulphate of soda (NaO, SO,) is sulpltionide of medians.