The medicinal substances recognized as irritants are subdivided into rubefacients, epispastics, suppurants and escharotics.

Rubefacients are agents employed to redden the surface by exciting the action of the capillaries, and occasioning an afflux of vascular and nervous power to the part with which they come in contact; hence pain is a usual consequence of their employment.

They are used for the same purposes as blisters, and are often a good substitute for them; for example, in low degrees of inflammation, as local anodynes, as general stimulants; their efficacy as such depending upon their action on the capillary circulation, and also on the pain they occasion. They are especially serviceable in the coma and asphyxia resulting from poisons and drowning, but in cases of cerebral oppression are inferior to blisters. Rubefacients are used until redness and pain occur, but their persistent application will cause vesication, and even gangrene.

In the class of rubefacients are included mustard, capsicum, oil of turpentine, ammonia, liniment, Burgundy pitch, Canada pitch, ginger, black pepper, garlic and spice plaster.

Epispastics, also called vesicants and blisters, are medicinal agents, capable of producing, when applied to the skin, inflammation followed by an effusion of serum beneath the cuticle. As a general rule, blisters should remain on the surface of the skin six or eight hours, in order to insure their full effect. When the skin is very delicate a shorter application will answer every purpose; and in the case of children it is seldom necessary for them to remain longer than three or fours hours. When applied to the scalp, twelve hours are generally required. After removing a blister, the usual dressing is some non-irritating ointment, such as simple cerate. In acute diseases, blistering ought never to precede such means as have a tendency to reduce inflammatory action, and the application should be as near the affected part as possible.

Covering blisters with fine gauze renders them much less irritating, and does not retard their operation.

If a blister is applied long enough to redden the skin, a simple poultice will complete the vesication; and in the case of children . this method should always be pursued. When it is necessary to keep the blister open, weak epispastic or savin ointment will prove sufficient. When the circulation is languid in the extremities, they seldom act efficiently, and may cause gangrene by exhausting what vitality remains. When strangury is produced, the blister must be removed after three or four hours, and the part bathed with olive oil, or a poultice applied, and diluent drinks used; an opium suppository or injection will prove serviceable. Epispastics are employed as local stimulants in the treatment of inflammations; to create a healthy inflammatory action, as in various cutaneous eruptions; to relieve pain; to destroy morbid associations by causing a powerful impression; to stimulate the absorbing or secreting vessels of parts in the neighborhood of the affected part; to stimulate generally; to relieve threatened gangrene and paralysis; to produce local depletion as evacuants, and to prepare a surface for the endermic application of medicines.

The class of epispastics include such agents as cantharides, cantharidal collodion, water of ammonia, etc.

Suppurants are medicinal agents, which, when rubbed on the skin, cause rubefaction, accompanied by a pustular eruption; their beneficial effects being due to the counter-irritation set up. The agents of this class are generally employed in subacute, chronic laryngeal and bronchial affections, diseases of the joints, etc., etc.

Included as suppurants are croton oil, antimonial ointments, etc., etc.

Escharotics, called also Cauterants or Caustics, are medicinal agents capable of destroying the structure and vitality of the parts with which they come in contact, producing an eschar or slough, which is followed by inflammation and suppuration of the neighboring tissues to such a degree that the slough separates from the living parts. The mode of action of an escharotic is as follows: After being applied to the skin, so as to chemically disorganize it, or destroy its vitality, a new action is set up in the vessels beneath the slough, so as to cause it to be thrown off. The excavation resulting is then kept open by inserting some irritant, which maintains a copious secretion of pus from the ulcerated surface. Escharotics are divided into Actual and Potential; the actual being fire itself, while the potential are substances which destroy the living solids, either by excessive stimulation, or by producing a chemical decomposition.

Iron heated to a white heat and the moxa (cones or cylinders of inflammable substance) represent the actual cautery; and caustic potash, nitrate of silver, burnt alum, chloride of zinc, chromic, sulphuric and nitric acids, and the nerve or arsenical paste employed in dental practice, represent the potential cautery. A sub-class is composed of what are known as Issues and Setons; the blister issue, where the skin is removed by a blister, and the discharge promoted by means of stimulating applications, as the cantharidal ointment, for example; also, the pea issue, where an incision made by the lancet is kept open by means of a pea, beet, or piece of orris root. The Seton is prepared as follows: A seton needle, to which is attached a skein of silk, is passed completely through the part chosen for the operation, after which it is removed, and the ends of the silk left hanging from the wound. It is dressed once, or several times a day, with some mild ointment; or, if this is not sufficient to keep up the discharge, a more stimulating ointment is used. Escharotics are employed to destroy morbid growths, warts, polypi, condylomata, fungous granulations, etc.; also to relieve violent inflammation by their substitutive action; to stimulate indolent ulcers, sinuses, etc.; to open abscesses of the liver, and other internal viscera, the method of "aspiration" being preferred; to remove cancer, lupus and other morbid growths; to decompose the virus of rabid and venomous animals, and of chancres and malignant pustules, and prevent their absorption.

Escharotics include such agents as caustic potassa, fused nitrate of silver, caustic soda, solution of nitrate of mercury, corrosive chloride of mercury, bichromate of potassium, the mineral acids, sulphate of copper, and the substances before referred to.