SULPHUROUS ACID or SULPHUROUS ANHYDRIDE occurs under the ordinary relations of temperature and pressure as a colorless gas, possessing the suffocating odor of burning sulphur. In its concentrated form it is quite irrespirable, and in a diluted state it excites cough, and produces the symptoms of an ordinary catarrh. It is not only incapable of burning, but it rapidly extinguishes the flame of burning bodies. It is very freely soluble in cold water, which at 32° takes up nearly 69 times its volume of the gas, while at 75° it only takes up 32 volumes; the solution known as aqueous sul phurous acid having at first the same smell and taste as the gas, but soon absorbing oxygen from the air, and becoming converted into sulphuric acid. By the action of cold, sulphurous acid may be condensed to a colorless transparent limpid liquid, which freezes at —105°, forming a transparent crystalline solid: The specific gravity of the gas is 2.247 (atmospheric air being the unit), and that of the liquid is 1.49 (water being the unit), the solid being considerably heavier. Although dry sulphurous acid gas and dry oxygen, when mixed, exert no action on one another, there are many conditions under which sulphurous acid rapidly absorbs oxygen, and is converted into sulphuric acid. It has been mentioned that this takes place if the gas be dissolved in water; a similar action takes place under the influence of hydrated nitric acid, iodic acid, and certain metallic oxides. For example, oxide of lead, when immersed in the gas, burns, and is converted into white sulphate of zinc + SO, = Hence, sul phurous acid is a powerful reducing or deoxidizing agent. This gas is a common and
abundant product of volcanic action, and is occasionally met with in solution in the springs in volcanic regions. It may be prepared artificially by simply burning sulphur in the air or in oxygen gas, or by heating in a flask 4 parts of flowers of sulphur mixed with 5 parts of powdered black manganese, sulphurous acid and sulphide of manganese being the products, as shown by the equation 2S + SO, + 31nS. In conse quence of its solubilty in water, this gas should be collected over mercury. In addition to the uses of sulphurous acid as a bleaching agent, it is valuable both as a disinfect ant agent and as a powerful antiseptic; its latter property has been applied to the pre servation of meat, which, after exposure to this acid, will keep fresh for years, if it be inclosed in metallic canisters filled with nitrogen, to which a little binoxide of nitrogen has been added, to remove any trace of oxygen. But by far its most important use is, as a first stage in the manufacture of sulphuric acid. In combination with bases, this acid forms the sulphites—a class of salts which, excepting the sulphite of soda, are of little practical importance, except for their power, when moist, of extracting oxygen, and thus acting as reducing agents. For example, the salts of the sesquioxide of iron are reduced by them to salts of the protoxide.