II.—Substances are added together which mutually decompose each other, whence their original virtues are changed, or destroyed.
This is a more serious, but not a less frequent source of error; it has been already shewn in this Analysis (IV. B) that the judicious and scientific application of chemical science has furnished new and endless resources to the physician, by exalting the efficacy and correcting the acrimony of established remedies, or by combining inert substances so as to create new and powerful medicines. With equal truth and confidence it may be asserted, that the abuse of these means not only destroys the virtues of the most valuable articles in the Materia Medica, but that the mildest remedy may be thus converted into an instrument of torture, and even of death. In a lecture delivered at Apothecaries’ Hall, Mr. Brande stated that he had seen a prescription in which the blue, or mercurial pill, was ordered in conjunction with nitric acid, and that the patient was brought to “death’s door” from the formation of nitrate of mercury in his stomach! I have myself lately seen a Recipe, professing to afford a preparation similar to the “Black Drop,” and which directed a mixture of a Tincture of Opium, made with rectified spirit, with Nitric Acid; in this case, it may be very safely inferred that the author was not only ignorant of the chemical habitudes of these bodies, but that he never performed the experiment in question, or he would have learnt from dire experience, that in consequence of the rapid evolution of nitric ether, the contents of the phial will explode with violence, to the imminent hazard of the operator’s eye-sight. During the course of my professional practice I have witnessed more than an ordinary share of consumptive cases, and I can confidently state that in the treatment of Hæmopthysis, the styptic properties of Acetate of lead are entirely invalidated by combination with Alum,[[279]] or by its exhibition being accompanied with that of the acidulated infusion of roses, or with small doses of sulphate of magnesia; and yet, I would ask, whether this practice is not usual and general? The practitioner however cannot be too often reminded that he is not to reject a remedy whose value has been ascertained by experience, merely because it appears to be unchemical: the popular and certainly useful pill, consisting of calomel, rhubarb and soap, may be adduced as an example of this kind. Of the Mistura Ferri Composita, I will only say that it is a most valuable combination; and whether it be the product of accident, or the result of philosophical induction, it equally deserves a distinguished place in our list of tonic remedies: but it cannot be denied that many of our esteemed arrangements, which are in apparent contradiction to all the laws of composition, owe their efficacy to the operation of affinities altogether blind and fortuitous.
It has been observed that the practice of combining certain vegetable tonics with lime-water, although very common, is in cases where we are desirous to obtain their astringent effects, of very doubtful propriety; for the fact is, that Tannin forms with the alkalies and alkaline earths, compounds that are not soluble in water,[[280]] and which are therefore probably inefficacious. It may perhaps be said that such an argument cannot avail, because if the astringent matter be even introduced into the stomach in its purest form, it will immediately form an insoluble compound on its contact with gelatine.[[281]] We know so little of the laws of gastric chemistry, that it is difficult to learn what changes take place in the animal laboratory; but it would seem probable that the powers of the stomach rather consist in decomposing the ingesta into simple forms, than in complicating them by favoring new combinations; besides which, if such a compound were formed, it would be subsequently decomposed in transitu; for the experiments of Sir H. Davy shew that vegetable astringent matter passes through the body unchanged. (page 82.)
It is impossible to furnish any general rule that may enable the practitioner to avoid mixing together substances which are incompatible with each other; a knowledge of their chemical habitudes must in every case direct him, and these are enumerated in the second part of this work, under the history of each medicinal substance. The physician however will find it useful to retain in his remembrance the simple and beautiful law which has been so ably developed by the eminent author of the “Statique Chimique,” that whenever two salts in a state of solution are brought together, which contain, within themselves, elements capable of producing a soluble and insoluble salt, a decomposition must necessarily arise;[[282]] he illustrates this law by the example of Nitrate of Silver and Muriate of Potass, whose elements are capable of forming within themselves a soluble salt, Nitrate of Potass, and an insoluble salt, Muriate of Silver. It deserves also to be remembered, that a table of chemical affinity will not upon all occasions prove to the medical practitioner an unerring pilot; in those cases for instance, where a super or sub Salt is readily formed, a substance less weakly attracted by another than a third, will sometimes precipitate this third from its combination with the second, thus in the production of Nitric acid, we decompose the Nitrate of Potass by virtue of the superior affinity of the Sulphuric acid for its base, the nitric acid is accordingly disengaged, and a Sulphate of Potass remains in the retort; now, paradoxical as it may appear, if nitric acid be poured upon the Sulphate of Potass, a quantity of nitre will be reproduced, in consequence of the saturation of a portion of the base, in such a proportion as to enable the remaining atoms to form a Bi-Sulphate of Potass. In the same manner the Tartrate of Potass (Soluble Tartar) is, contrary to the usual affinities, decomposed by all sub-acid vegetables, which neutralize a portion of the base, and convert the salt into the Bi-tartrate of Potass (Cream of Tartar). The same effect is even produced by Carbonic acid.[[283]]
There are besides certain cases wherein Triple Salts are produced, which afford apparent exceptions to the usual affinities of the bodies involved in the combination; we have a very good illustration of this truth in the decomposition of the Liquor Ammoniæ Acetatis by Magnesia; if the practitioner refers to a table of affinities, he will perceive that Acetic acid has a greater attraction for Ammonia than for Magnesia; but if upon this assurance he were to administer these bodies together, he, or his patient, would soon discover that ammonia is developed with considerable pungency; now in this case the Magnesia forms a triple Acetate with one part of the ammonia, and consequently sets the remainder at liberty.
A popular error exists with respect to the subject of chemical incompatibility, against which it may be here advisable to caution the inexperienced prescriber, viz. that no important change is produced, on the admixture of solutions, unless precipitation is occasioned. This however occurs only when the new compound produced is insoluble; thus Sulphuric acid may be added to Lime water, by which a Sulphate of Lime is formed, but as its proportion is not too large for the water to dissolve, no precipitate occurs; so again, a solution of Nitrate of Silver is not apparently disturbed by the addition of Ammonia, because the resulting Ammoniuret is a soluble compound. We should, nevertheless, commit a great error in supposing that, for such a reason, these bodies were not incompatible. On the other hand, the medicinal powers of a solution are not necessarily destroyed by the occurrence of a precipitate, although such a result should always be regarded with suspicion.
III.—The Methods directed for the preparation of the Ingredients are either inadequate to the accomplishment of the object, or they change and destroy the efficacy of the Substances.
The observations already offered upon Formula 40, will sufficiently explain the nature of the various errors comprehended under this head: so, again, if the virtues of a plant reside in essential oils, which are easily volatilized, or in extractive matter, which readily becomes oxidized, Decoction must necessarily destroy its efficacy; a striking example of this fact is presented us in the history of the Laurel and Bitter Almond: the poisonous influence of the essential oil and distilled water of these vegetable substances is well known, but their watery extracts are perfectly innocuous. A still more familiar example is to be found in the onion, or in garlic, which by simple coction is deprived of all its acrimonious qualities. On the other hand, an error equally injurious would be committed, by directing a simple infusion of a vegetable, whose medicinal properties depended upon resino-mucilaginous principles. Orfila found that an extract of Hemlock, prepared by boiling the dried powder in water and evaporating the decoction, was entirely destitute of activity. See Decocta, Infusa, Extracta.
An instance of the baneful effects which may arise from an erroneous method of preparation happened some time ago to fall under my immediate notice and care; it was in preparing an infusion of the root of the Veratrum with Opium, as directed by Mr. James Moore, when the dispenser ignorantly substituted a spirituous for a vinous menstruum.
A very common error may be here noticed, which is that of prescribing a substance in such a form, as not to be acted upon with any effect by the solvent; as an example it may be stated, that in preparing an infusion of Juniper Berries, unless pains be taken by strong contusion to break the seed, it will contain but little power as a medicine.