By one experiment, Mr Berthollet found that 69 parts of sulphur in combustion, united with 31 parts of oxygen, to form 100 parts of sulphuric acid; and, by another experiment, made in a different manner, he calculates that 100 parts of sulphuric acid consists of 72 parts sulphur, combined with 28 parts of oxygen, all by weight.
This acid, in common with every other, can only dissolve metals when they have been previously oxydated; but most of the metals are capable of decomposing a part of the acid, so as to carry off a sufficient quantity of oxygen, to render themselves soluble in the part of the acid which remains undecomposed. This happens with silver, mercury, iron, and zinc, in boiling concentrated sulphuric acid; they become first oxydated by decomposing part of the acid, and then dissolve in the other part; but they do not sufficiently disoxygenate the decomposed part of the acid to reconvert it into sulphur; it is only reduced to the state of sulphurous acid, which, being volatilised by the heat, flies off in form of sulphurous acid gas.
Silver, mercury, and all the other metals except iron and zinc, are insoluble in diluted sulphuric acid, because they have not sufficient affinity with oxygen to draw it off from its combination either with the sulphur, the sulphurous acid, or the hydrogen; but iron and zinc, being assisted by the action of the acid, decompose the water, and become oxydated at its expence, without the help of heat.
Table of the Combinations of the Sulphurous Acid with the Salifiable Bases, in the order of affinity.
| Names of the Bases. | Names of the Neutral Salts. | ||
| Barytes | Sulphite of | barytes. | |
| Potash | potash. | ||
| Soda | soda. | ||
| Lime | lime. | ||
| Magnesia | magnesia. | ||
| Ammoniac | ammoniac. | ||
| Argill | argill. | ||
| Oxyd of | zinc | zinc. | |
| iron | iron. | ||
| manganese | manganese. | ||
| cobalt | cobalt. | ||
| nickel | nickel. | ||
| lead | lead. | ||
| tin | tin. | ||
| copper | copper. | ||
| bismuth | bismuth. | ||
| antimony | antimony. | ||
| arsenic | arsenic. | ||
| mercury | mercury. | ||
| silver | silver. | ||
| gold | gold. | ||
| platina | platina. | ||
Note.—The only one of these salts known to the old chemists was the sulphite of potash, under the name of Stahl's sulphureous salt. So that, before our new nomenclature, these compounds must have been named Stahl's sulphureous salt, having base of fixed vegetable alkali, and so of the rest.
In this Table we have followed Bergman's order of affinity of the sulphuric acid, which is the same in regard to the earths and alkalies, but it is not certain if the order be the same for the metallic oxyds.—A.
Sect. XV.—Observations upon Sulphurous Acid, and its Combinations.
The sulphurous acid is formed by the union of oxygen with sulphur by a lesser degree of oxygenation than the sulphuric acid. It is procurable either by burning sulphur slowly, or by distilling sulphuric acid from silver, antimony, lead, mercury, or charcoal; by which operation a part of the oxygen quits the acid, and unites to these oxydable bases, and the acid passes over in the sulphurous state of oxygenation. This acid, in the common pressure and temperature of the air, can only exist in form of gas; but it appears, from the experiments of Mr Clouet, that, in a very low temperature, it condenses, and becomes fluid. Water absorbs a great deal more of this gas than of carbonic acid gas, but much less than it does of muriatic acid gas.
That the metals cannot be dissolved in acids without being previously oxydated, or by procuring oxygen, for that purpose, from the acids during solution, is a general and well established fact, which I have perhaps repeated too often. Hence, as sulphurous acid is already deprived of great part of the oxygen necessary for forming the sulphuric acid, it is more disposed to recover oxygen, than to furnish it to the greatest part of the metals; and, for this reason, it cannot dissolve them, unless previously oxydated by other means. From the same principle it is that the metallic oxyds dissolve without effervescence, and with great facility, in sulphurous acid. This acid, like the muriatic, has even the property of dissolving metallic oxyds surcharged with oxygen, and consequently insoluble in sulphuric acid, and in this way forms true sulphats. Hence we might be led to conclude that there are no metallic sulphites, were it not that the phenomena which accompany the solution of iron, mercury, and some other metals, convince us that these metallic substances are susceptible of two degrees of oxydation, during their solution in acids. Hence the neutral salt in which the metal is least oxydated must be named sulphite, and that in which it is fully oxydated must be called sulphat. It is yet unknown whether this distinction is applicable to any of the metallic sulphats, except those of iron and mercury.