This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
The friction here alluded to is employed not to excite the surface, but, by deranging the epidermic scab's, to force an entrance for medicinal substances to the absorbent tissues beneath. It is made by the hand protected by a leather glove, or by means of a piece of flannel, or of coarse linen. Substances applied in this way are most frequently in the unctuous form, as in the case of the mercurial and iodine ointments; but oleaginous, aqueous, and spirituous medicines and solutions are also not unfrequently used. The medicine employed, if insoluble, should be brought to the finest possible state of division. The parts usually selected for the application are those in which the cuticle is most delicate, as the inside of the upper and lower limbs, especially the inner surface of the thighs; but reference should be had to the special object in view; and, when a particular organ or part is to be acted on, or a tumour to be dispersed, it is usually deemed best to apply the medicine as nearly as may be over the seat of disease. In affections of the absorbent glands, the portion of surface from which lymphatics run through the diseased glands should be preferred.
This mode of using medicines is uncertain in its results, and, ill consequence of the irritation produced in the skin, is often inconvenient. But it may be resorted to in aid of internal medication, or when from circumstances this cannot be employed; and it is often very efficient in the cure of local affections, as of neuralgic pains, for example, and tumefactions of various kinds.