This has but recently become officinal. The name is applied by the Pharmacopoeia to the liquid resulting from the passage of sulphurous acid gas through distilled water, by which it is absorbed. This gas is procured from sulphuric acid by heating it with charcoal, which deprives it of one of its equivalents of oxygen, thereby converting it into sulphurous acid (S02). The gas has a pungent, disagreeable, suffocating odour, familiar to all who have ever been in the vicinity of a common sulphur-match when burning. Water absorbs it very largely. it is quite irre-spirable in its pure state, causing a spasmodic closure of the glottis. Diluted with atmospheric air, it is admitted into the air-passages, where it acts as a powerful irritant, endangering life by bronchial and pulmonary inflammation.

In former times, the fumes of burning sulphur were used as a disinfectant, but it is doubtful whether they possess any power of this kind; at least they are much inferior to chlorine, by which they have been entirely superseded.

They are occasionally applied to the nostrils, for the purpose of making a strong irritant impression, and thereby exciting the cerebral centres, in cases of threatened or existing syncope or asphyxia. Care, however, must be taken not to apply them too freely, lest inflammation might be induced, or spasm of the glottis brought on at the time of revival.

But the chief use of sulphurous acid gas is in the sulphur vapour both. The patient is enclosed in a perfectly air-tight box, made for the purpose; his head projecting, and the aperture around the neck thoroughly closed. Some sulphur is placed upon a piece of heated iron within the box, where it burns, filling the space around the patient with its gaseous product; or the gas is introduced by means of a tube, connecting with a small outer furnace, in which sulphur is undergoing combustion. The effect of the gas is to produce heat, itching, prickling, and pain. it probably operates more by a direct alterative or stimulant influence on the skin, than through absorption. it has been used for the cure of scabies, which it will usually effect; in other obstinate skin diseases, as chronic eczema, impetigo, prurigo, etc., and in various constitutional affections, as chronic rheumatism, scrofulous complaints, old palsies, and obstinate neuralgia.

The sulphurous acid of the Pharmacopoeia, or liquid sulphurous acid, is a nearly saturated solution of the acid gas in distilled water. it is a colourless liquid, with the smell of burning sulphur, and a peculiar somewhat astringent taste. The sp. gr. of the U. S. acid is 1.035, of the British 1.04. When exposed to the air, it slowly absorbs oxygen, and is partially converted into sulphuric acid, which gives it a sour taste.

As sulphurous acid is a powerful poison to plants, and the lower kinds of animals, its aqueous solution ought to be efficient in the treatment of the animalcular and cryptogamous skin affections, as scabies or the itch, porrigo or favus, trichosis or ringworm of the scalp, mentagra or sycosis, and pityriasis versicolor, which have all been shown to be at least essentially associated with, if not dependent on, microscopic animals and fungi. On the same principle it is applicable to the thrush of children, which is believed to depend on a parasitic fungus. As fermentations are now generally believed to be essentially connected with microscopic plants or animalcules, it may be used to check this process when going on abnormally in the stomach; and on the same principle may be used in the various zymotic diseases, which may be supposed to depend on an absorbed organized poison. The dose for internal use is a fluidrachm, which should be largely diluted when taken. For local application it should be mixed with twice or thrice its bulk of water or glycerin.