All medicines act on the system either directly or indirectly. Of the first class, or those which act directly, we have examples in the Ergot of Rye, on the uterine muscular fibre; in Cantha-rides, on the neck of the bladder; in Belladonna, on the iris; and in caustics applied to ulcerations, &c. The second class comprises by far the larger portion of medicinal substances; one example may suffice. In Neuralgia depending upon acidity of the primAe-vise, carbonated alkalies are given to correct the acidity; the cause being removed, the effect ceases, and the alkali thus indirectly cures the neuralgic affection.

Liebig proposes to divide all medicinal substances into three great orders; the first (which includes metallic poisons) cons-of substances which enter into chemical combination with certain parts or constituents of the body, while the vital force is insufficient to destroy the compounds thus formed. The second division, consisting of the essential oils, camphor, antiseptics, &c, possesses the property of impeding or retarding those kinds of transformation to which certain very complex molecules are liable - transformations which, when they take place out of the body, are usually designated by the names of fermentation and putrefaction. The third division is composed of bodies, the elements of which take a direct share in the changes taking place in the animal body. When introduced into the system, they augment the energy of the vital activity of one or more organs; they excite morbid phenomena in the healthy body; all of them produce a marked effect in a comparatively small dose, and many are poisonous when administered in larger quantity. None of the substances in this class can be said to take a decided share in the nutritive process, or to be employed in the organism in the production of blood; partly, because their composition is different from that of the blood, and partly because the proportion in which they must be given to exert their influence is as nothing compared with the mass of the blood. (Liebig.*) A therapeutic arrangement of medicines, on the groundwork thus proposed by the greatest chemist of the age, would have many advantages over those now in use.

The means of discovering the medicinal properties of various substances, previous to their administration to the human subject, have in all ages attracted the serious attention of medical men. Many plans have been proposed of late years, but they are all, more or less, defective, and we are at last obliged to confess that the only sure way of ascertaining the true properties of all substances is by carefully observing their several effects upon the human economy, when, either from accident or design, they have been introduced into the system. A brief review of some of the means proposed may prove instructive.