(Na, SO„) and sulphuric acid (H, hydrogen. There are:however, several objections to this view of the constitution of sulphates.
Sulphuric acid in its concentrated state, and at common tempe ratures, acts only slightly, if at all, on the metals; but when they are heated together, the acid is generally decomposed, sulphurous acid and a sulphate of metallic oxide being produced: thus, when copper is heated in concentrated sulphuric Reid, tho acid yields one of its three equivalents of oxygen to the copper, which becomes protoxide ; while the sulphur combined with two equivalents of oxygen is given off in the state of sulphurous acid; and this, as already indicated, is one of the best methods of procuring sulphurens acid. on the other hand, sulphuric acid is diluted, it has no action, even when heated, on copper ; but on zinc, iron, and such other metals as readily decompose water, it acts with great readiness : the metal, being oxidised by the oxygen of the decomposed water, is dissolved by the acid, while the hydrogen of the water is given out in the gaseous state; by this operation, then, we procure a metallic sulphate and hydrogen gas.
Sulphuric acid and all sulphates are decomposed by the salts of baryta and of lead, minute quantities either of the acid or soluble salts being rendered evident by the formation of a dense white precipitate either of sulphate of baryta or lead ; 116.5 parts of the former or parts of the latter sulphate, when dry, indicate 40 parts of anhydrous sulphuric acid.
Commercial sulphuric acid generally contains sulphate of lead derived from the vessels used in its manufacture ; it is nearly all precipitated as a whits powder on diluting the acid with water. It frequently also contains traces of arsenic derived from the pyrites ; in this case, sulphuretted hydrogen gives a yellow precipitate when passed through the diluted acid, or the contamination may be detected by Marsh's test. Small quantities of nitric and sulphurous acids arc also sometimes present in common sulphuric acid. If required to be purified and concentrated, it is boiled and distilled. As lead would melt at the beat here employed, and as most other common substances would be corroded, platinum is often employed as a material for tho still, at a heavy expense. Glass is another material employed for sulphuric acid stills. When the acid has been condensed to a specific gravity nearly double that of it is ready for sale.
_Hyposulphurous 2cid (HO, 5,00. Dithionous acid. In 1817 Dr. Thomson inferred the existence of an acid of sulphur in the salts called sulphoretted sulphated : this acid and its compounds have been particu larly examined by Sir John Herschel.
Hyposulphurous acid has not been obtained in a separate state, but its composition has been determined to be in accordance with the above formula.
This acid is procured by dissolving zinc or iron in close vessels in an aqueous solution of sulphurous acid : solution is effected in this acid without the evolution of any gas, which hardly happens in any other case : the metals thus dissolved form crystalliaable salts, which, when decomposed by other acids, yield sulphurous acid and sulphur, pre viously existing in the proportions above stated, and constituting hyposulphurous acid : this acid is also formed when aulphites are digested in close vessels with sulphur, in which case the sulphur takes half the oxygen of the sulphurous acid ; when iron is dissolved in sulphurous acid, it takes half the oxygen of the sulphurous acid, which, by this loss becomes hyposulphurous acid, and, combining with the oxide of iron formed, they constitute hyposulphito of iron.
Most of the salts containing hyposulphurous acid, or the hyposul phites, are unimportant; the acid undergoes decomposition when they are strongly heated or acted upon by an acid.
H&posalplite of soda (NaO, 5,0, + 5 Aq.), in solution, is largely used in photography to " fix," or render perinanently unalterable the photo graphic picture. This it does by dissolving the aqueously insoluble silver salts, with which it forms double compounds having an intensely sweet taste. Hyposulphite of soda is made by dissolving sulphur in sulphite of soda, NaO, + S becoming NaO, llyposulphuric Acid Dithionic Acid. This acid was dis covered in 1519 by Gay-Lussac and Welter. It is prepared by sus pending finely-powdered binoxide of manganese in water, and passing sulphurous acid gas into the mixture : if this be not kept cold, sul phuric acid is formed : but otherwise the oxide of manganese is dis solved and hyposulphate of its protoxide formed : to the filtered solu tion sulphide of barium is to be added, by which sulphide of manganese is precipitated, and hyposulphate of baryta remains in solution : when the proper quantity of sulphuric acid is added to this, sulphate of baryta is precipitated, and the hyposulphurie acid is liberated, the filtered solution of which is to be evaporated in vacuo over sulphuric acid till it acquires a density of : if it be carried further than this, it is decomposed into sulphurous acid, which escapes, and sul phuric acid, which remains dissolved.