In this method, the medicament is placed in contact, only, with the epidermis, and friction, to hasten absorption, is not employed. Although the epidermis opposes a strong obstacle to absorption, it does not entirely prevent diffusion into the blood, as numerous facts show. The skin may be considered a colloidal septum. The rate and degree of absorption of any medicine will depend, in large part, on its power of diffusion. Various circumstances influence this—for example, the chemical position of the agent to be diffused. On one side of the colloidal septum—the skin—lie the bloodvessels, containing an alkaline fluid. An acid fluid on one side of the dialysing membrane, and an alkaline fluid on the other, are conditions most favorable to diffusion. Experiments are wanting on this point, but it is a reasonable presumption that solutions of medicinal substances, acid in reaction, will find their way most readily into the blood.

Besides the epidermis, the sebaceous matter of the skin offers more or less positive obstruction to cutaneous absorption. Medicinal substances in solution in water, therefore, very slowly permeate the skin to enter the vessels. Waller, who has made very careful experiments, has ascertained that alkaloids dissolved in chloroform are readily transferred through the skin into the blood, and produce characteristic phenomena, while "alcoholic and aqueous solutions are either not at all, or very slowly, absorbed."

His observations were made with chloroformic solutions of aconitine, atropine, strychnine, and morphine. Waller further ascertained that alcohol mixed with chloroform did not retard absorption, but alcohol alone caused an outward osmotic flow. It follows from these facts that, if, in the application of a medicinal agent to the skin by the enepidermatic method, the object be to promote absorption, the remedy should be dissolved in chloroform, or in a mixture of alcohol and chloroform, and not in alcohol alone, or in water.