The years 1836-'7-'8 witnessed, on my part, several extraordinary and fruitless efforts to get before the public the theory, of whose truth and importance I was then fully convinced. In 1839, Dr. C. Smith, then of Troy, an able medical lecturer, became a convert to the theory; and showed me, in post mortem dissections, the organs of respiration and circulation. At the close of that year, having carefully corrected and made out copies of my manuscript theory, which I had before written, I sent two to Paris--one to the two brothers, Drs. Edwards, members of the French Institute, and one to my friend, Madame Belloc. I also sent one to Edinburgh, to Dr. Abercrombie. Dr. Milne Edwards soon after wrote a book, in which he made it a point to show that animals could live several minutes without breathing; and Dr. Frederic Edwards wrote me a short letter of objections to my theory, and adherence to that of Harvey. This letter was copied and answered in my work published in 1846. About this time, Dr. Aikin, of Baltimore, wrote to me on the subject; and showed, by calculations, that the mere gradual expansion of the water of the blood was not sufficient, of itself, to produce a current as rapid as that of the blood was proved to be, even on the lowest estimate of its velocity. This did not shake my faith in the great fact that circulation was created by respiration. It must be so; for in life, such respiration as produces heat is the invariable antecedent of circulation, and nothing else is. There was something, then, which remained to be discovered. Again, I placed before me the conditions of the great problem, and set myself intently to its study; and I soon found what I thus sought, and then discerned for the first time that the blood moves, as does the railroad car, by steam. John Bell, my favorite author, had shown that the lungs work _in vacuo_. A great proportion of the blood is water, which, in a vacuum, springs into vapor at 67°, and the temperature of the blood in the lungs is 101°. Its expansion, then, was not merely the gradual increase of bulk by transmitted heat, but also that of instantaneous expansion, by the vaporization of so much of it as is needed; and what expanded water could not do, steam certainly could. At once, a throng of proofs came to my mind. The most apparent of these was the vapor _expired_ breathing. I recollected how, in former times, the stage horses, driven rapidly into my native village of a winter morning, had clouds of vapor wreathing upward from their nostrils, while the icicles of condensation were hanging below. The nurse, who stands over the dying, holds a mirror before the mouth and nose, and considers that life is only extinct when vapor ceases to be formed. Then came to mind the solution of that great mystery of physiology, why the arteries are empty at death, which so long hindered the discovery finally made by Harvey.