pid 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 deg., and
the temperature of the blood in the lungs is 101 deg.. 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.

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 determi