The insertion of additions and improvements in the title of new editions of books, has been too generally, though sometimes justly, understood as little else than a contrivance of the bookseller, to animate a languishing sale; but this is far from being the case in respect to the works of our author, whose maturer sentiments on many of the subjects, he had before treated of, cannot be well comprehended, unless by a careful perusal of his later corrections, seeing the alterations he has thought fit thereby to make in his earlier productions, are not less necessary to be attended to by the prudent practitioner, than they are really interesting to the unhappy patient: the truth of which cannot be more manifestly evinced, than by his last publication of his essays on poisons; wherein he entirely subverts his former hypothesis, and builds his reasonings upon a new foundation; he also tacitly admits his former experiments to have been too precipitately made, and the conclusions deduced from them, to have been too hastily drawn. To illustrate what has been advanced upon this head, it will not be improper to observe, that when Dr. Mead first wrote these essays, he was of opinion, “That the effect of poisons, especially those of venemous animals, might be accounted for, by their affecting the blood only: but the consideration of the suddenness of their mischief, too quick to be brought about in the course of the circulation, (for the bite of a rattle snake killed a dog in less than a quarter of an hour)[6] together with the nature of the symptoms entirely nervous, induced him to change his sentiments,[7]” and to conclude, that the poison must be conveyed by a medium of much greater quickness, which could be no other than the animal spirits. [6] Philosophical transactions Nº 399. [7] Introduction to the last edition of the essays on poisons, page 12. From hence our author is led to prefix to the last edition of this performance, an inquiry into the existence and nature of this imperceptible fluid, with which we have been but very imperfectly acquainted. He has also added several new experiments, tending to confirm this theory, and explain the properties of the viperine venom, particularly by venturing to _taste_ it; at the same time he has likewise contradicted some of those he had formerly made, whereby he had been induced to believe, this poison partook of a degree of acidity: for instance, he formerly asserted that he had seen this sanies, “as an acid, turn the blue tincture of _heliotropium_, to a red colour;[8]” whereas his more modern trials convinced him, it produced no alteration at all. [8] Second edition of those essays, page 10.