SULPHUROUS ACID.

Sulphurous acid exists at ordinary temperature and pressure in the

gaseous form; it is one, however, of the most easily liquified

gases. It is produced always when sulphur burns, either in air or

in pure oxygen; sulphur not being capable of passing directly to a

higher degree of oxydation. In the burning of sulphur, the volume of

sulphurous acid gas formed is exactly equal to the amount of oxygen

consumed.

Sulphurous acid gas may also be simply prepared by heating three

parts of flowers of sulphur with four of peroxide of manganese. The

reaction is very simple: one part of the sulphur uniting with the

metal, and another with the oxygen, form sulphuret of manganese and

sulphurous acid.

Another and quicker way to obtain this gas in small quantities is, to

decompose a solution of hyposulphate of soda, by adding muriatic acid

to it, so as to liberate the hyposulphurous acid, which immediately

decomposes into sulphur and sulphurous acid.

Sulphurous acid is absorbed by water. It is colorless and

transparent, possessing an odor peculiarly irritating (the smell of

burning sulphur), and cannot be breathed. It is not combustible,

nor does it support combustion. Water dissolves about thirty-seven

times its volume of sulphurous acid; the solution possesses the

properties of the gas in a very high degree, and bleaches vegetable

colors with great power; when kept for some time it gradually absorbs

oxygen, and the sulphurous becomes changed into sulphuric acid. The

sulphuric acid is one of the feeblest acids, and is expelled from its

combinations by almost all but the carbonic acid.

As has been demonstrated, all these gases are absorbed by water,

and a saturated solution possesses the properties of the gases

themselves.