These elements however are not all necessarily present in every scientific formula, for many medicines do not require any addition to promote their operation, and the mild and tractable nature of others renders the addition of any corrective unnecessary; whilst many again are in themselves sufficiently manageable, and do not therefore require the intermede of any vehicle or constituent. It also frequently occurs that one element is capable of fulfilling two or more of the objects required; the Adjuvans for instance, may at the same time act as the Corrigens, or Constituens; thus the addition of Soap to Aloes, or Extract of Jalap, mitigates their acrimony, and at the same time quickens their operation (80.) So again Neutral Salts both quicken and correct the griping which attends the operation of resinous purgatives. The disposition of the key letters placed opposite to the elements of the following Formulæ, will furnish the practitioner with a farther elucidation of these principles, viz. 70, 71, 76, 77, 101, 102, 105, 135, &c. This coincidence, if possible, should be always attained, for it simplifies the formula, and by decreasing the bulk of the remedy, renders it less nauseous and more elegant.[[276]]
This division also affords the best general rule for placing the ingredients of a formula in proper order, for the order should correspond with that of the arrangement; and those elements intended to act in unity should be marshalled together. The chemical and mechanical nature however of a medicinal substance will occasionally offer exceptions to any general rule; thus the volatile ingredients should be those last added, and the constituent or vehicle should be placed next the particular element to which it is intended to impart convenience or efficacy of form, or a capability of mixing with the other ingredients, as may be seen in Formulæ 69, 71, 127, 136, &c. This consideration induced the Committee, appointed to revise the late Pharmacopœia, to alter the order of the ingredients in the “Mistura Ferri composita,” and to place the “Spiritus Myristicæ” next in succession to the “Potassæ Sub-carbonas” and Myrrh. If any substance require decoction or infusion, a question then arises, determinable only by a knowledge of its chemical composition, whether the remaining ingredients should be added previous to, during, or subsequent to, that operation; Formula 40, which is recommended by Pringle as a remedy for Typhus fever, may serve to exemplify this principle. The preparation of the ingredients is resolved into three distinct stages, and it is easy to discover that by any other arrangement their several virtues could not be fully obtained, and secured from change. The Cinchona, for instance, yields its full powers only by decoction, a process which would necessarily impair those of Serpentaria, connected as they are with an essential oil; whilst the addition of the acid at any other stage of the process than that directed, would produce decompositions in the vegetable substances; and it is evident that were the Spirit of Cinnamon added previously, it would be entirely lost by vaporization. So in making the Compound Decoction of Sarsaparilla, the Sassafras should be added after the other ingredients have undergone boiling. The Decoctions of Lichen Islandicus and Sarsaparilla constitute a popular remedy on the Continent, in certain forms of Phthisis; now it is evident that as the former plant loses its virtues by long coction, and the latter requires a protracted ebullition for the extraction of its virtues, they ought not to be included under the same general directions; each decoction should be separately performed, and the results subsequently mixed.
Compound Medicines have been divided into two Classes, viz.
I. Officinal Preparations,
which are those ordered in the Pharmacopœias, and kept ready prepared in the shops. No uniform class of medicines however can answer the indications of every case, and hence the necessity of
II. Magistral or Extemporaneous Formulæ.
These are constructed by the practitioner at the moment, and may be either arrangements altogether new, or officinal preparations with additions, or modifications. Too much importance cannot be assigned to the Art which thus enables the physician to adopt and graduate a powerful remedy to each particular case by a prompt and accurate prescription; without this knowledge, the practitioner of the nineteenth century, with all the collateral aid of modern science, will be as helpless in the chamber of sickness as the physicians of ancient Egypt, who were obliged by the laws to follow with servile exactness the unvarying mandates of their medical code. Extemporaneous are also preferable to Officinal Formulæ, whenever the powers of the compound are less liable to deterioration from being long kept; for examples, see Mistura Ferri composita; Infusum Sennæ; Liquor Hydrargyri Oxymuriatis, &c.
The Chemical and Pharmaceutical Errors, which may be committed in the composition of Extemporaneous Formulæ, are referable to the following Sources.
1.—Substances are added together which are incapable of mixing, or, of forming Compounds of uniform and suitable consistence.
This may be termed an error in the Mechanism of the Prescription, and has been generally regarded as being more inconvenient than dangerous, more fatal to the credit of the Prescriber than to the case of the Patient: the observations however which are offered in this work, especially under the article Pilulæ, must satisfy the practitioner that this error is more mischievous in its effects than has been usually supposed; it is so palpable and self-evident in its nature, that it will be unnecessary to illustrate it by more than one or two examples. Calomel, for instance, has been ordered in an aqueous vehicle, and certain resinous tinctures have been directed in draughts, without the necessary intervention of mucilage; so again, an intermixture of substances has been formerly ordered in powder that possess the perverse property of becoming liquid by triture (see Pulveres), and bodies have been prescribed in the form of pills, whose consistence[[277]] renders it impossible that they should preserve the globular form; or else they have been so hard and insoluble, that they might be fired through a deal board.[[278]] In the London Pharmacopœia of 1809, an error of this kind unfortunately passed without correction with regard to the Formula for preparing the Syrup of Senna.