In enumerating the conceits of Physic, as relating to the Materia Medica, we must not pass over the idea, so prevalent at one period, that _all poisonous substances possess a powerful and mutual elective attraction for each other_; and that consequently, if a substance of this kind were suspended around the neck, it would, by intercepting and absorbing every noxious particle, preserve the body from the virulence of contagious matter. Angelus Sala, accordingly, gives us a formula for what he terms his _Magnes Arsenicalis_, which he asserts will not only defend the body from the influence of poison, but will, from its powers of attraction, draw out the venom from an infected person. In the celebrated plague of London, we are informed that amulets of arsenic were upon this principle suspended over the region of the heart, as a preservative against infection. There is yet to be mentioned another absurd conceit which long existed respecting the subject of Antidotes,—a belief that every natural poison carried within itself its own antidote; thus we learn from the writings of Dioscorides, Galen, and Pliny, that the _virus_ of the _Cantharis Vesicatoria_ existed in the body of the fly, and that the head, feet, and wings, contained its antidote; for the same potent reason were the hairs of the rabid dog esteemed the true specific for Hydrophobia.[52] DEVOTION TO AUTHORITY, AND ESTABLISHED ROUTINE. This has always been the means of opposing the progress of reason—the advancement of natural truths—and the prosecution of new discoveries; whilst, with effects no less baneful, has it perpetuated many of the stupendous errors which have been already enumerated, as well as others no less weighty, and which are reserved for future discussion. To give general currency to an hypothetical opinion, or medicinal reputation to an inert substance, requires only the talismanic aid of a few great names; when once established upon such a basis, ingenuity, argument, and even experiment, may open their ineffectual batteries. The laconic sentiment of the Roman Satirist is ever opposed to our remonstrance—“Marcus dixit?—ita est.” “Did Marcus say ’twas fact? then fact it is, No proof so valid as a word of his.”