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.
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; but by increasing its proportion of Tartaric Acid, we convert it into a super-tartrate or “Cream of Tartar,” which is a substance characterized by a comparative degree of insolubility, and a correspondent change is produced in the medicinal activity of the salt; its purgative effects are considerably diminished, whilst its diuretic powers are rendered more considerable. We may even extend this experiment by adding to the Cream of Tartar, Boracic Acid, a substance capable of increasing to a certain extent its solubility, when we shall again find that its purgative properties are strengthened in an equal proportion.
It has been observed that a mixture of different saline cathartics is more efficient than an equivalent dose of any single one, a fact which is strikingly exemplified in the prompt and active operation of Cheltenham Salts, in comparatively small doses, as well as in that of sea water. I submit whether this may not in some degree depend upon increased solubility; for it is a law well known to the chemist, that when water has ceased to act upon a salt, in consequence of its having obtained the term of saturation, the solution may still take up another salt of a different kind. I apprehend that an advantageous application of this law might be frequently made in practice, and the energies of a remedy thereby considerably extended.[[267]]
Where the active principle of a cathartic is not sufficiently soluble, it is apt to vex and irritate the bowels, producing tormina instead of exciting a free and copious excretion; hence the reason why the operation of resinous purgatives is so commonly attended with griping, and why relief may be obtained by combining them with neutral salts. Thus also Senna, whose virtues reside in extractive matter, is apt by decoction, or long exposure to the air, to act with griping, in consequence of the extractive matter becoming by oxidation, resinous and comparatively insoluble: this effect is best counteracted by the addition of soluble Tartar, that will quicken its action, or by an alkaline salt that will increase its solubility.
It appears then to be established as a pharmaceutical maxim, that the intensity and even specific action of a purgative medicine may be modified or completely changed, by changing the degree of solubility possessed by the principles in which its activity resides.