III. THE CORRIGENS; that which corrects its operation. (“_Tuto._”) IV. THE CONSTITUENS; that which imparts an agreeable form. (“_Jucunde._”) 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