The decomposition of _Calomel_ by lime water, forming the well known “_black wash_” and that of _corrosive sublimate_ in the same fluid, constituting the “_aqua phagadenica_,” furnish remedies which derive all their peculiar efficacy from the developement of the mercury in different states of oxidation. The reader will find another, and a very striking illustration of the same principle in the history of “_Alterative Drops_,” under the article “_Hydrargyri Oxy-murias_.” A substance separated by chemical precipitation is often a valuable remedy, being in a much more subtle and impalpable form than any body can be rendered by mechanical triture and levigation;[266] for example, the _Carbonate of Lead_, (Cerussa), when diffused in water, is according to the experience of our best surgeons, far less active as a topical application than the same substance when produced at once by precipitation from the _Sub-acetate_ of that metal. In some cases, also, the substance obtained by precipitation is in a different state of oxidation from that which is prepared by a different process, see _Mist. Ferri Comp:_ It is a question well worthy of consideration whether a more active preparation of the _Antimonial powder_ might not be formed by obtaining the oxide by the precipitation of _Tartarized Antimony_. Many interesting and important illustrations have been lately afforded by an extended knowledge of vegetable chemistry, recent analyses having developed principles of extreme activity from several of our most esteemed plants; thus have Sertuerner and Robiquet succeeded in separating a narcotic element from _Opium_ (_Morphia_); Majendie, and _Pelletier_, an emetic principle from _Ipecacuan_, (_Emeta_); and the last mentioned chemist, together with Caventou, a tonic one from _Peruvian Bark_, (_Cinchonia_), the properties and applications of which will be fully explained hereafter, under the history of the different substances which contain them. It is only here necessary to caution the practitioner against those fallacies into which the captivating theories of the chemist may seduce him; and, if the views which I have offered upon the subject of combination be correct, it will follow as a corollary, that the concentration _of an active element must in many cases abridge its powers as a remedy_; for although the matter thus removed may _individually_ be quite inert, yet, in combination, it may subdivide the particles of the essential constituent, or modify its solubility, and give impulse and steadiness to its operation; thus the vegetable alkali _Quina_, although it indisputably constitutes the active matter of bark, will be found inefficacious when separated from it, unless it be rendered soluble by the addition of sulphuric, or some other acid. C. _By combining substances, between which no other chemical change is induced, than a diminution, or an increase, in the_ SOLUBILITIES _of the principles, which are the repositories of their medicinal virtues_.