The degree of solubility possessed by a medicinal substance may perhaps be regarded by some practitioners as a circumstance of but little or no importance; it will however appear in many cases that _it not only influences the activity of a remedy, but, like its dose, goes far to determine its specific operation_; indeed, where a medicine is not, in itself, very soluble, the increase of its solubility by any chemical expedient, is tantamount to an increase of its dose. It is probably owing to the diversity which exists in the solubility of the active elements of certain purgatives, that so great a diversity occurs in their operation; it is, for instance, easy to conceive that a medicine may act more immediately and specially on the stomach, small, or large intestines, according to the relative facility with which its principles of activity enter into solution; that those which are dissolved before they pass the pylorus are quick and violent in their effects, and liable to affect the stomach, as is exemplified by the action of _Gamboge_, &c. whilst some resinous purgatives, on the other hand, as they contain principles less soluble, seldom act until they have passed out of the stomach, and often not until they have reached the colon. _Colocynth_ has a wider range of operation, since its principles of activity reside both in soluble and in insoluble elements. _Aloes_ again, being still further insoluble, pass through the whole alimentary canal before they are sufficiently dissolved, and act therefore more particularly upon the rectum, by which they are liable to produce piles, tenesmus, and the various effects which so usually attend their operation. The characteristic effects of _Rhubarb_, _Senna_, _Saline Cathartics_, and indeed of all individual substances which compose the class of the purgative medicines, will also admit of a satisfactory explanation from the application of these views. It ought moreover to enable the practitioner, by changing the solubilities of these substances, to change their medicinal effects. Experience shews that this is the fact, and that it may be effected either _by the intervention of substances that act_ CHEMICALLY; or, _by the addition of Ingredients whose operation is entirely_ MECHANICAL; thus by combining _Aloes_ with _Soap_ or an _Alkaline Salt_, we quicken their operation, and remove their tendency to irritate the rectum; the _Compound Decoction of Aloes_ affords a combination of this kind. _Gamboge_, whose too ready solubility it is an object to obviate, should be intimately incorporated with some insoluble purgative, as for instance _Aloes_; a formula of this nature was introduced by Dr. George Fordyce, and it has been since simplified and admitted into our Pharmacopœia, under the title of “_Pilulæ Cambogiæ Compositæ_.” _Tartrate of Potash_, which, on account of its comparative solubility, has gained the name of _Soluble Tartar_, acts with corresponding briskness upon the small intestines;