There is sufficient ground for arranging astringent medicines in two sections, one including the vegetable, and the other those of mineral origin.

1. The vegetable astringents are distinguished by a striking similarity of properties, which has been ascertained to depend on the presence of a peculiar proximate principle, or set of principles, denominated tannin or tannic acid. Though this, as found in different products, has been ascertained to differ somewhat in chemical character, yet, both in this respect, and in its sensible and therapeutical properties, it is so nearly identical that, in relation to its medical uses, it may be considered as one substance. It is undoubtedly the main astringent principle. This rank was at one time claimed for gallic acid, which is associated with tannic acid in certain astringents; and there are some who seem disposed to revive this claim; but it is quite sufficient simply to taste the two principles, to be convinced that gallic acid is incomparably inferior in styptic power, and that little of the effects produced by astringent vegetables can be justly ascribed to it. The fact would seem to be, that it is more readily absorbed than tannic acid, probably because it does not like this form insoluble compounds with albumen and gelatin; and its constitutional effect is probably greater in proportion to its local than is the case with the more astringent substance; but, when it is understood that the vegetable astringents which have been most used internally, and in favour of which experience has spoken most decidedly, contain no gallic acid, as kino, catechu, and extract of rhatany, its claim to be considered the prominent astringent principle must be admitted to be extremely feeble. Tannic acid seems to be purely astringent, and destitute of any other physiological property. The vegetables, therefore, which contain little or none of any other active principle than this, may be looked on as proper representatives of the class. But these are very few. Most of the vegetable astringents contain also a bitter principle, which somewhat modifies the influence of their tannic acid, and might entitle them to rank with the tonics, which they considerably resemble in their effects. But, so far as their mere astringency is concerned, they are essentially different from that class of medicines; resembling them only In this single point, that, in cases of debility connected with deficient vital cohesion of the tissues, they increase strength by restoring to the tissues the compactness necessary for the proper exercise of their functions. It is obvious that the bitter astringents are less applicable than tannic acid itself, or the pure astringents, to those cases in which it is desirable to stimulate, whether locally or generally, as little as possible.

2. The, mineral astringents have in general nothing in common but their astringency. Each has peculiar properties of its own, which render it applicable to peculiar purposes. Thus, the preparations of lead are sedative, alum has an alterative influence, sulphuric acid is refrigerant and tonic, and the preparations of iron have remarkable tonic powers, and a peculiar power of modifying the blood. Between the sulphates of zinc and of copper, however, there is a remarkable coincidence of properties, though the latter is vastly more powerful than the former.