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compounds of sulphur and oxygen have also been described under the Sulphurous a-names of Hyposulphurous and Hyposulphuric Acids; but these can scarcely be said to exist, except in combination with bases, and what is known respecting them will be found under the articles Hyposulphite of Lime (630) and Hyposulphate of Baryta (639). If they be admitted as distinct compounds, sulphur may be considered as susceptible of four states of combination with oxygen, and these compounds will consist respectively of

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313. Sulphurous acid is a gaseous body, which may be obtained by several processes. It may be procured directly, by burning sulphur in oxygen gas; or indirectly, by boiling mercury in sulphuric acid. It must be collected and preserved over mercury; for water takes up rather more than 30 times its bulk of this gas, forming the liquid sulphurous acid, which, when recently prepared, has a sulphurous astringent taste, and destroys many vegetable colours; but, by keeping, it acquires a sour flavour, and reddens the generality of vegetable blues.

314. If sulphur be burned in oxygen, sulphurous acid is produced without any change in the volume of the gas, so that its composition is easily learned by the increase of weight; and as 100 cubic inches of oxygen (weighing 33.88 grains) dissolve 33.88 grains of sulphur, it is obvious that the sulphurous acid is composed of equal weights of sulphur and oxygen; and if we regard it as consisting of two proportionals of oxygen and one of sulphur, the latter element will be represented by the number 16; and the sulphurous acid, consisting of 1 proportional of sulphur = 16, and 2 of oxygen = 16, will be represented by 32, which is also its relative specific gravity to hydrogen, considering the latter as = 1*; 100 cubical inches of sulphurous acid gas weigh 67.75 grains. This gas has a suffocating nauseous odour, and an astringent taste; it extinguishes flame, and kills animals.

315. Sulphurous acid suffers no change at a red heat, but if mixed with hydrogen, and passed through a red hot tube, water is formed and sulphur deposited; under the same circumstances, it is also decomposed by charcoal, by potassium and sodium, and probably by several other metals. Prepared by the combustion of sulphur, it is much used for bleaching cotton goods (Quarterly Journal of Science, iv. 196,) and also for whitening silk and wool; in wine countries it is sometimes used to check vinous fermentation.

316 When sulphurous acid is mixed in equal volume with ammonia, a yellowish salt is produced, which is a sulphite of ammonia, and which consists of 32 sulphurous acid + 17 ammonia. When formed by saturating liquid ammonia with sulphurous acid, it crystallizes in four and six-sided prisms, soluble in their own weight of water and having an acrid taste. It is deliquescent, and becomes a sulphate by exposure to air.

* When sp. gr. of oxygen =1, that of sulphurous acid =2 this multiplied by 2 gives 4 for the atom of this acid, also 4 is just 32 times the atom (0.125) of hydrogen.

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317. Sulphuric Acid. - This body was formerly obtained by the distillation of green vitriol, and called oil of vitriol. It is now procured in this country by burning a mixture of about 8 parts of sulphur and 1 of nitre in close leaden chambers containing water, by which the fumes produced are absorbed, and by evaporation the acid is procured in a more concentrated state. This improved method of preparing sulphuric acid was invented by Dr. Roebuck, about the year 1746.PARKES'S Chemical Essays, Vol. ii. -Prefatory History of Chemistry. Vol. i. p. 36.

318. Sulphuric acid, as usually met with, is a limpid colourless fluid, sulphuric ahaving a specific gravity of 1.80; it boils at 620°, and freezes at 15°, cid. contracting at the same time considerably in its dimensions. But the temperature at which the diluted acid congeals is singularly modified by the quantity of water which it contains. At the specific gravity of 1.78 it freezes at about 40°; but if the density be either increased or diminished, a greater cold is required for its congelation (KEIR, Irish Phil. Trans. iv. 88.) Its boiling point diminishes with its dilution; acid of the specific gravity of 1.78, boils at 435°, and acid of the specific gravity of 1.650 boils at 350°. - DALTON'S Chem. Phil. ii. 404.

It is acrid and caustic, and when diluted with water, produces a very sour liquid. It rapidly absorbs water from the atmosphere, and upon sudden mixture with water, condensation ensues, and much heat is evolved. Even a boiling temperature, when it is concentrated, does not prevent its taking up moisture from the air; hence it cannot be concentrated so well in an open as in a close vessel, on which account retorts of glass or platinum are used for the last stage of its concentration by the manufacturers.

It chars animal and vegetable substances, and is apt to acquire a brown tinge from any small particles of straw, resin, or other matters that may accidentally have fallen into it.

319. In sulphuric acid 1 proportional of sulphur = 16, is combined with 3 of oxygen = 24, and, consequently, dry sulphuric acid is correctly represented by 16+ 24 = 40; but it only exists in this state (like the nitric and chloric acids) when united with bases, and in its ordinary state contains water, and may, therefore, be called hydrated sulphuric Acid. It has been found by experiment, that 100 parts of sulphuric acid, specific gravity 1.85, contain 18.5 of water; consequently, it may be looked upon as composed of 1 sulphur + 3 oxygen + 1

water:

Or of 16 sulphur

24 oxygen
9 water

49 number for liquid sulphuric acid.

100 Parts of liquid sulphuric acid, of the specific gravity of 1.3530, contain 37.5 parts of dry sulphuric acid; or 46 of liquid acid, of the specific gravity 1.85.

320. The strength of sulphuric acid is best judged of by its saturating power, and by its specific gravity. Mr. Dalton (New System of Chemical Philosophy, Vol. ii. p. 404,) has published a Table, exbibiting the specific gravity and boiling point of the acid of various strengths.

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Dr. Ure also has given several valuable tables relating to this subject, in his Experiments to determine the Law of Progression, followed in the Density of Sulphuric Acid at different Degrees of Dilution (Quarterly Journal of Science and the Arts, Vol. iv. p. 114.) An extremely useful table of this kind will also be found in Mr. Parkes's Essays above quoted (Vol. ii. p. 444.)

The following is Dr. Ure's Table :

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Formation of

321. The formation of sulphuric acid by the combustion of sulphur sulphuricacid and nitre is as follows:

The sulphur, by burning in contact with atmospheric, air forms sulphurous acid. The nitre gives rise to the production of nitric oxide, which, with the oxygen of the air, produces nitrous acid gas. When these gases (i. e.), sulphurous and nitrous acids) are perfectly dry, they do not act upon each other, but moisture being present in small quantity, they form a white solid, which is instantly decomposed when put into water; the nitrous acid reverts to the state of nitric oxide, having transferred one additional proportional of oxygen to the sulphurous acid, and, with water, producing the sulphuric acid; while the nitric oxide, by the action of the air, again affords nitrous acid, which plays the same part as before.

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hence every two portions of sulphurous acid require one of nitrous acid, which transfers two of oxygen, and passes back into the state of nitric oxide, sulphuric acid being, at the same time, produced.

The gases, therefore, before decomposition, may be thus represent

ed:

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Analysis of 322. The decomposition of sulphuric acid may be effected by passsulphuric acid. ing it through a red-hot platinum tube, when it is resolved into sulphu

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rous acid, oxygen, and water.

When heated with charcoal, sulphuric acid gives rise to the production of carbonic and sulphurous acids; with phosphorus it produces phosphoric and sulphurous acids; and, with sulphur, sulphurous acid is the only product. It is decomposed by several of the metals, which become oxidized, and evolve sulphurous acid, as shown in the production of this acid, by boiling sulphuric acid with mercury (313), tin, lead, &c.

323. Sulphuric acid is largely consumed in a variety of manufactures. It is used by the makers of nitric, muriatic, citric, and tartaric acids; by bleachers, dyers, tin-plate makers, brass-founders and gilders. For these purposes it is generally sufficiently pure as it comes from the wholesale manufacturer; but, as traces of lead, lime, and potassa, are usually found in it, it often requires to be purified by distillation for the use of the experimental chemist.

The distillation of this acid in glass retorts requires some precaution, in consequence of the violent jerks which the production of its vapour occasions, and which often break the vessel; this may be prevented by putting some strips of platinum into the acid; it then boils quietly. and it is only necessary to take care that the neck of the retort and receiver are not broken in consequence of the high temperature of the condensing acid. This very useful contrivance to the practical chemist was first shown me by Mr. James South.

If the acid of commerce contain dissolved sulphate of lead, it becomes turbid on dilution, so that its remaining clear when mixed with water, is some proof of its purity, as far at least as lead is concerned. 324. When sulphuric acid was procured by the distillation of green vitriol, it was frequently observed that a portion concreted into a white

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