The astringent salts of Iron are less active in this kind of influence than the medicines already referred to. They have very little action on the glands. But in cases of hæmorrhage they are particularly appropriate, for in addition to their astringent action they tend to restore the deficient red colouring matter of the blood. In many such instances the Sulphate or the Sesquichloride of Iron may be advantageously prescribed along with Sulphuric acid.
Acetate of Lead is used internally as an Astringent, but being a more dangerous remedy than the medicines above mentioned, it requires greater care in its application. It should not, if possible, be used long at a time. When it has been for some time prescribed, the blue line at the edge of the gums, which is characteristic of a saturation of the system by the poison of lead, will indicate that its further administration is unadvisable. The Acetate of Lead is much esteemed as an Astringent in the case of Hæmoptysis. It is frequently combined with Opium, in spite of the chemical decomposition which follows the admixture. But it must certainly not be prescribed with Alum.
The soluble salts of Silver, Zinc, and Copper, and the Bichloride of Mercury, are used with advantage as external Astringents, but can seldom be prescribed internally so as to act in this way, because they are poisonous when given in any quantity. The Sulphates of Zinc and Copper have, however, been occasionally used in diarrhœa.
When thus applied as Astringents externally, their solutions must be very dilute, for all the mineral Astringents are caustic and corrosive, destroying texture, when in the solid state, or in strong solutions. These solutions may be applied to any part of the surface of the skin or mucous membrane, when relaxed, inflamed, or ulcerated. Their operation is simple and obvious. When one of them is applied to an inflamed eye, or to a red ulcerated sore, that is turgid with blood and discharges an unhealthy matter, it tends to promote the contraction of the dilated vessels, and thus dispels the congestion and diminishes the discharge.
Various other substances have wrongly obtained the title of Astringents, regard being had only to the result of their operation, and not to the mode in which it is produced. Thus Chalk is often of benefit in diarrhœa. It acts both by mechanically absorbing irritating fluids, and protecting the surface of the bowel, and by chemically neutralizing an acid matter by which the irritation is maintained. And when applied to the surface of inflamed ulcerated parts, it does good by absorbing the discharge, and protecting from the contact of air. Thus its action in both cases is very different from that of the true Astringents.
The action of some of the minerals above named, when applied externally in the solid state, as caustics, must not be confounded with their astringent power. Its efficacy depends upon the revulsive effect which follows a local destruction of tissue.
Ord. II. Vegetable Astringents.
(Tannic Acid. Gallic Acid. Kino; Catechu; Logwood; Oak-galls; Rhatany; Bistort; Pomegranate-rind; Rose-leaves; Uvæ Ursi; Tormentil. Creosote.)
Tannic Acid, Gallic Acid, and Creosote are the three chief Vegetable Astringents. To the first two the vegetable substances above enumerated appear to owe their efficacy. All contain Tannic Acid, or some modification of it; and Oak-galls contain also Gallic Acid.