entific men of Europe, was proposed by Dr. Crawford. He agreed with
Dr. Black that heat not only was generated in the lungs, but that the
arterial blood had a greater capacity for heat than the venous, and
that this increase of capacity takes place in the lungs. At the moment
heat is generated, a portion of it, under the name of latent heat, is
absorbed and conveyed to the different parts of the body Wherever
arterial blood is converted into venous, this latent heat is given
out. But, unfortunately for this theory, Dr. Davy proved the capacity
of both, for heat, to be nearly the same.

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555. What inquiries are naturally suggested? 556. What was the theory
of the ancients? What did Mayow assert at a later period? 557. What
was the theory of Dr. Black? The objection? 558. What was the theory
of Dr Crawford?

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559. No one can doubt that respiration and animal heat are closely
connected. Those animals whose respiratory apparatus is the most
extended, have the highest temperature. An example is seen in birds,
whose organs of respiration extend over a large part of the body, and
their temperature is 12 deg. above man; while the respiratory apparatus of
cold-blooded animals, as some kinds of fish, is imperfect, and only a
small quantity of blood is subjected, at any time, to the effects of
respiration.

560. To understand the process by which heat is generated in the human
system and in animals, it will be necessary to state: 1st. That the
apparent heat of a body, as perceived by the touch, or as indicated by
a thermometer, is not the measurement of heat contained in the body,
or its capacity for heat.

_Illustration._ If we mix one pound of water, at the temperature of
60 deg., with another pound at 91 deg., the resulting temperature will be
exactly the medium, or 75 1/2 deg. But, if we mix a pound of water at 60
deg. with a pound of quicksilver at 91 deg., the resulting temperature
will be only 61 deg., because the capacity of water for heat is so much
greater than that of quicksilver, that the heat which raised the
quicksilver 31 deg. will raise the water only 1 deg.

561. 2d. When the density and the arrangement of the atoms of a body
are changed, its capacity to hold heat in a latent state is altered.
If it will retain more, heat will be absorbed from contiguous and
surrounding substances; but, if its capacity for caloric is lessened,
heat will be set free and