This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
These act by inflaming or destroying the part affected. They are all external remedies. Medicines may sometimes cause inflammation, or even gangrene, when taken internally; but they are not given for the purpose. These external remedies are usually arranged in three classes; the rubefacients, epispastics, and escharotics; the first simply reddening, the second vesicating, and the third producing the death of the part. There might also be made a class of pustulating agents; but I have not thought it expedient to multiply subdivisions, and therefore include these among the rubefacients. The distinction between the classes is not absolute. The same medicine may belong to all the three; as in the case of solution of ammonia, which, when feeble, is simply rubefacient, when more concentrated, blisters, and when strongest, acts powerfully as a caustic; and there are few of the more energetic belonging to any one of the classes, which may not be so applied as to produce the characteristic effects of the others. But of the medicines belonging to this division, some are better calculated for one of the modes of action, and others for another; and they are both used and classified according to this preferable applicability. I shall treat first of the epispastics; because their consideration involves, besides what is peculiar to themselves, the principles which govern the others also, so that, in the general observations in reference to the rubefacients and escharotics, it will be requisite to call attention only to a few points of difference; and time will thus be spared.