The objects to be attained, and the resources which are furnished, by MEDICINAL COMBINATION, together with the different modes of its operation, and the laws by which it is governed, may with much practical advantage be arranged in the following order. I. TO PROMOTE THE ACTION OF THE BASIS, OR PRINCIPAL MEDICINE. A.—_By combining together several different Forms, or Preparations, of the same substance_. The utility of such a combination is obvious, whenever we desire the full and general effects of _all_ the principles of a medicinal body in solution; thus, where the _Bark_ is required in the cure of an intermittent fever, and the stomach will not allow the exhibition of the powder, it will be eligible to conjoin in one formula, the tincture, decoction, and extract, as exemplified by _Formulæ 42, 126, 127_. The necessity of such a combination may be expressed by the following canon. _Whenever the chemical nature of the medicinal substance will not admit of the full solution of all its active principles in any_ ONE _Solvent_, _and its exhibition in substance is at the same time impracticable_. For farther illustrations see _Form. 2, 25, 33, 38, 70, 109_. Practitoners, probably without having reasoned upon the theory, have very generally adopted the practice, of combining the different solutions of the same substance; for in the prescriptions of practical physicians we commonly find, that the decoction or infusion of a vegetable remedy is quickened by a certain portion of a corresponding tincture. B.—_By combining the Basis with Substances which are of the_ SAME NATURE, _that is, which are_ INDIVIDUALLY _capable of producing the same effect, but with less energy than when in combination with each other._ Dr. FORDYCE first established the existence of the singular and important law, that _a combination of similar[241] remedies will produce a more certain, speedy, and considerable effect than an equivalent dose of any single one_; a fact which does not appear to have been known to any ancient physician. The earliest mention of it that I can find is by VALISNIERI, the favourite pupil of Malpighi, who filled the medical chair at Padua in 1711, nearly ninety years before Fordyce published his valuable memoir on the combination of medicines, but he does not attempt any generalization[242] of the subject; he merely states, as the result of careful experiments, that twelve drachms of _Cassia Pulp_ are about equivalent in purgative strength to four ounces of _Manna_; and yet, says he, if we give eight drachms of _Cassia Pulp_, in combination with four drachms of _Manna_, we obtain double the effect! How, adds the professor, can this possibly happen? Surely the very contrary _ought_ to obtain, since four drachms of _Cassia_ are much more than equivalent to an equal weight of _Manna_; the strength of the former being to that of the latter as 8 to 3.