In the state in which chemistry was, even as late as the time of John Bell, the chemical power of the heat produced by respiration at the lungs could not have been understood. SECTION II. Publication of the Theory, in 1846, in "A treatise on the Motive Powers which produce the Circulation of the Blood." Its Reception: Critique in the New York "Journal of Medicine," September, 1846. My Reply, in the same Journal, March, 1847. TO DR. MARCY.--In the years immediately succeeding 1840, (in which year, as you will recollect, I had the honor to receive your countenance and advice respecting my theory,) I was almost exclusively devoted to the revision and enlargement of my historical works; but early is 1846, having determined on making the tour of the United States, I resolved first to prepare my theory for the press. In the introduction, I remarked, "The house of clay in which the mind dwells must receive a portion of its care; and that which I have bestowed on mine has proceeded on a belief in the truth of the theory herein advocated, as undoubting as that in the laws of gravitation; and when any new fact, or any remark of an author, relating to my theory came under my observation, I noted it down and laid it by with its kindred. About to set out on a long journey, and aware that my field of vision had thus enlarged, I felt it my duty to put together the principal of my remarks, that I might so leave the subject, that, in case anything should prevent my return, it would be in a form equal to the present slate in which the theory exists in my own mind." The time I had spent in devotion to this theory, the many rebuffs I had met in seeking to promulgate it--sometimes, unhappily, affecting my social life--had made painful the duty of publishing it. My historical works had been received with favor; but I believed that, in publishing this, it would be charged against me that I chose a subject unsuited to my sex. I therefore said, in my preface, "This is not so much a subject which I chose, as one which chooses me; and if the Father of Lights has been pleased to reveal to me from the book of his physical truth a sentence before unread, is it for me to suppose that it is for my individual benefit? or is it for you, my reader, to turn away your ear from hearing this truth, and charge its great Author with having ill-chosen his instrument to communicate it?"