It has been generally supposed, that substances whose application does not produce any sensible action upon the healthy system, cannot possess medicinal energy; and, on the contrary, that those bodies which occasion an apparent effect in health, must necessarily prove active in the cure or palliation of disease. Under certain limitations we may perhaps venture to assent to this general proposition; but it cannot be too forcibly or too frequently impressed upon the mind of the medical practitioner, that _Medicines are frequently but relative agents_, producing their effects in reference only to the state of the living frame; we must therefore concur with Sir Gilbert Blane in stating, that the virtues of Medicines cannot be fairly essayed, nor beneficially ascertained, by trying their effects on sound subjects, because that particular morbid condition does not exist which they may be exclusively calculated to remove;[127] thus in certain states of debility, _Tonics_ may excite the system when languid, by their sympathetic influence upon the _primæ viæ_, while in a robust condition of the body, the effects of the same agents may be wholly inappreciable. The MODUS OPERANDI of remedies, or the general principle upon which they effect salutary changes in the morbid states of the body, is involved in considerable obscurity, and has given rise to much ingenious speculation and scientific controversy. It would seem that the immediate impression of a remedy may depend upon mechanical, chemical, or vital agencies; and that the sanative impulse thus occasioned may either be Absolute, or Relative;—Primary, or Secondary;—Local, or General;—Direct, or Sympathetic;—Permanent, or Transient;—thus certain purgatives will occasion intestinal excretions in every condition of the body, and may therefore be justly considered as _absolute_ agents; while diuretics, since they generally require for their success, a certain state of the living system, may with equal truth be denominated _relative_ in their operation. That the obvious effect of a remedy may either depend upon its _Primary_, or upon its _Secondary_ and incidental operation, will at once be apparent by inspecting the diagram which exhibits the classification of diuretic medicines; the same scheme will also shew that remedies may be _local_ or _general_ in their effects, and may excite an action in distant organs, either by entering the circulating mass, and being thus brought into contact with their textures;—by occasioning an impulse conveyed through the nervous system,—or by exciting a local impression upon the stomach and _primæ viæ_, and thus arousing their energies through the mysterious medium of sympathetic[128] communication.