It would seem that it has been generally supposed that medicines,--bodies of active properties, and more or less peculiar construction,--are able to enter and to pass through a fluid which is still more complicated and changeable in nature than themselves, yet that while so doing they produce no impression upon it, but proceed passively to the solid tissues, or quietly pass out of the body, without ever leaving behind them any trace of their action. Certain writers have made some amends by including in their arrangements a class termed Alteratives. But even then they have not unfrequently laid it down as a rule that they produce, in the first place, an impression on the nervous system, and that this has subsequently the effect of altering the manifestations of vital force. How varied and peculiar in nature must these nervous impressions be, to account for actions which are often so widely different! The medicines thus grouped as Alteratives correspond very nearly with my division of Catalytics. The permanency of their effects, and the fact that the disorders which they cure are blood-disorders, will sufficiently stamp the character of the operation of these remedies. In many works, as in those of Dr. Duncan, of Dr. A. T. Thomson, and more lately, of Drs. Ballard and Garrod, the existence of a class of Alteratives or blood-medicines is not even recognised or alluded to. But in the learned classification of Dr. Pereira blood-medicines hold an important place. All except Chalybeates are grouped together in a sub-class, called Spanæmics, because they tend to impoverish the blood. In addition to those which I have named Catalytics, there are here the acids and alkalies. It is true that all these medicines, after long use, impoverish the blood, which, indeed, suffices to prove my first point, viz,: that they act in the blood, and that their effect is permanent. But Dr. Pereira has founded his classification on the physiological action of medicines, and not on their therapeutical uses, which form the basis of mine. Thus he takes no notice of the questions of what their ultimate destination may be, or what their primary _modus operandi_, whether they have to remain in the blood, or to pass out of it, and whether they can act by supplying a material to it, or by neutralizing a material in it. The subsequent statements which Dr. Pereira has chosen as the basis of his subdivisions, founded often on therapeutical action, seem in certain cases to be somewhat problematical. Thus he calls Acids "adipsa," or thirst-quenching, which very imperfectly expresses their action. Alkalies are grouped with Salts, Iodine, and Sulphur, under the general title of "resolventia seu liquefacientia," _i.e._ resolvents, or liquefacients, which involves an assumption that some might be inclined to dispute. The title of Spanæmics, applied to the whole, though unobjectionable in a classification which is founded upon their