simplex veri erroris
confessio; praecipueque in eo ministerio, quod utilitatis causa
posteris traditur._"

[3] Advertisement prefixed to the last edition of the essay
on poisons, p. 4.

[4] Epidem. lib. iv. Sec. 14.

[5] Medicin. lib. viii. c. 4.

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