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.
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