III. TO OBTAIN A NEW REMEDY NOT AFFORDED BY ANY SINGLE SUBSTANCE. By combining medicines which possess different properties, and which by their union produce effects not exerted by either separately and not attributable to chemical action. The officinal pulvis ipecacuanha compositus will serve as an illustration. This preparation is remarkable for its diaphoretic properties, whilst neither of its ingredients, opium nor ipecacuanha, when taken separately, exert any powerful action on the skin. The development of this property by the union of these substances is inexplicable on the ground of chemical reaction. We can, however, produce new remedies by combining substances which are capable of reacting chemically on each other. My namesake's mixture, the mistura ferri composita of the Pharmacopoeia, is a good example; in this preparation sulphate of iron. and carbonate of potash are employed, and these reacting on each other form carbonate of iron and sulphate of potash. IV. TO AFFORD A SUITABLE FORM. It is not the least of the duties of the prescriber to order his remedy in a convenient and agreeable as well as in an efficacious form. When writing a prescription always have regard to the taste, appearance, consistence, or equable mixture, and preservation of the medicine. While it is thus our duty, as far as possible, to consult the tastes, and even the caprices of our patients, we must never consent to sacrifice the It efficacy of a medicine to its taste or appearance. would seem to be necessary thus to admonish you, for Dr. Paris has known medical men who, if they considered it needful to recommend a shower-bath, would suggest that the disagreeable sensations occasioned thereby might be obviated by an umbrella. In this lesson I have purposely avoided giving more than one or two illustrations of the laws of combination, inasmuch as in future lessons we shall have frequent opportunities of studying further exemplifications of them. LESSON VI. INCOMPATIBILITY. Substances are said to be incompatible when their combination gives rise to chemical changes, a new compound being formed which is either inert or possessed of distinct properties. Chemical incompatibility, however, does not always signify therapeutical inertness. Substances which are chemically incompatible are sometimes intentionally combined in order to obtain a new compound; as, for instance, in the officinal mistura ferri composita a decomposition occurs between the sulphate of iron and carbonate of potash. The subject of incompatibility is, as Parrish says, "too much of a stumbling-block to the student." The older text-books of materia medica contained a long list of so-called incompatibles, which the unfortunate student was expected to commit to his already over burdened memory. It is now known that many of these "incompatibles" are not incompatible at all, and that many of them might be excluded from the list, from the extreme unlikelihood of anyone ever prescribing them together. Incompatibility may be threefold, viz. :- i. Chemical. 2. Pharmaceutical. 3. Physiological and therapeutical. We have given above the signification of chemical incompatibility. By pharmaceutical incompatibility we mean combination of such substances as are physically incapable of mixing; thus, if nitrous ether be added to tincture of guaiacum a gelatinous mass will result, or if resinous tinctures be added to aqueous solutions the resins will separate. The following are some of the more striking examples of pharmaceutical incompatibles, and they should be carefully remembered by the prescriber: Compound infusion of cinchona with compound infusion of gentian. Infusions generally with metallic salts. Tinctures made with strong alcohol, with those made with weak alcohol, and with infusions and aqueous liquids. Essential oils with aqueous liquids exceeding one drop to f. 3j. Fixed oils and copaiba with aqueous liquids, except with excipients. In our last lesson we alluded to the subject of physiological or therapeutical incompatibility. By this term we imply combination of such substances as possess opposite therapeutical and physiological properties, and which are medicinally inconsistent; for instance, belladonna would be physiologically incompatible with calabar bean. I will now ask your attention to the following simple rules, a knowledge of which will be sufficient to guard you from the commission of any flagrant errors in the matter of chemical incompatibility. LAWS OF CHEMICAL INCOMPATIBILITY. 1. Two salts in solution may form, by the interchange of their acids and bases, two insoluble salts which are precipitated. 2. When two salts in solution form, by the interchange of their acids and basis, a soluble and an insoluble salt, the latter will generally be precipitated, or may form with the soluble salt a double salt. I have before me two clear solutions, one of chloride of barium, the other of sulphate of soda. As I mix them you observe the formation of a copious precipitate; this is insoluble sulphate of barium. If we now filter off the clear fluid, we shall find it to be a solution of chloride of sodium. 3. When two salts in solution do not give rise to an insoluble salt no precipitate will result, though there may be decomposition. 4. An acid will decompose a salt (a) If the acid added be more fixed or more soluble than that of the salt. (b) If the acid added can form an insoluble or a less soluble compound with the base of the salt. (c) If the acid added possess a greater affinity for the base of the salt. (d) If the acid of the salt be gaseous. I have in one vessel dilute sulphuric acid, and in another vessel I have some liquor ammoniæ acetatis. You observe that, as I mix them, acetic acid is given off, and the fluid on examination will prove to contain sulphate of ammonia. 5. Oxides of the alkalies decompose salts of the metals proper and of the alkaloids, and precipitate their bases, or the base may be soluble in excess of the alkali. Here is a solution of sulphate of zinc; as I add to it a little liquor potassæ, you see the formation of a precipitate of oxide of zinc; as I add more of the liquor potassæ, the precipitate becomes dissolved. 6. Metallic oxides combine with acids to form salts. 7. Vegetable substances containing tannic or gallic acids precipitate albumen, vegetable alkaloids, and most of the metallic oxides, and form with salts of iron inky solutions. Substances containing tannic acid also precipitate gelatine. 8. Glucosides are incompatible with free acids or emulsions. As a general rule the following substances should be prescribed alone, and are best given in simple solution : Acid. hydrocyanic. dil. Liq. calcis Liq. potassæ Liq. potassæ arseniatis Potassii bromid. |