But there remains for our investigation a still more important precaution respecting this law of medicinal combination;—that, in combining substances in the manner, and for the object just related, the practitioner should be well satisfied that their medicinal virtues are in reality _practically_ SIMILAR, or he will fall into an error of the most fatal tendency; it has been already shewn, and I hope I shall not be considered tedious by again directing the reader’s attention to the fact, that medicines are not necessarily similar because they have been arranged in the same artificial division of remedies; in order to establish a perfect similarity _their operations must be found by experience to continue similar under every condition of the human body; and that, moreover, they must owe such similarity to modes of operation which are compatible with each other, and consonant with the general mode of cure_; we have only to refer to the history of Diuretics (_page 92_) for a full illustration of this important truth; thus _Squill_, _Calomel_, and _Digitalis_, are each powerful Diuretics, but nevertheless they cannot be considered _similar_ remedies, since _Digitalis_ will entirely fail in its effects in the very cases that _Calomel and Squill_ succeed; and _Squill_ will prove inert when _Digitalis_ is capable of producing the most powerful influence; this arises from their modes of operation being dissimilar, and consequently requiring for their success such different states of the living system. _Squill_, it will be seen, acts _primarily_ on the urinary organs, by stimulating the secreting vessels of the kidneys; _Mercury_, on the contrary, acts primarily on the absorbents, and _secondarily_ on the kidneys; whereas _Digitalis_ produces its effects by diminishing arterial action, and increasing that of absorption. Dr. Blackall, in his “Observations upon the Cure of Dropsies,” has offered some remarks so valuable in themselves, and so illustrative of this important subject, that I shall take leave to quote the passage. “Many physicians,” he observes, “are fond of combining _Squill_, _Calomel_, and _Digitalis_, as a diuretic in dropsy; a practice unsafe, and not very decidedly possessing the merit even of being consistent. _Digitalis_ greatly depresses the action of the heart and arteries, and controls the circulation, and it seems most unreasonable to believe that its curative powers can be independent of such an effect; on the other hand, _Mercury_, if it does not pass off quickly, is always exciting fever, and raising and hardening the pulse; speaking from experience, where the urine is coaguable, and _Digitalis_ agrees, both the others are, often at least, positively injurious. On the contrary, where the urine is foul, and not coaguable, and _Squills_ with _Calomel_ render service, I have on that very account, made less trial of Digitalis, and cannot therefore speak of it from much experience.” See _Form. 103_, and the _note_ thereon.