ill, therefore, be a fit
period to conclude our present conversation. What further remarks we
have to make on the animal economy shall be reserved for our next
interview.

CONVERSATION XXVI.

ON ANIMAL HEAT; AND ON VARIOUS ANIMAL PRODUCTS.

EMILY.

Since our last interview, I have been thinking much of the theory of
respiration; and I cannot help being struck with the resemblance which
it appears to bear to the process of combustion. For in respiration, as
in most cases of combustion, the air suffers a change, and a portion of
its oxygen combines with carbon, producing carbonic acid gas.

MRS. B.

I am much pleased that this idea has occurred to you: these two
processes appear so very analogous, that it has been supposed that a
kind of combustion actually takes place in the lungs; not of the blood,
but of the superfluous carbon which the oxygen attracts from it.

CAROLINE.

A combustion in our lungs! that is a curious idea indeed! But, Mrs. B.,
how can you call the action of the air on the blood in the lungs
combustion, when neither light nor heat are produced by it?

EMILY.

I was going to make the same objection. --Yet I do not conceive how the
oxygen can combine with the carbon, and produce carbonic acid, without
disengaging heat?

MRS. B.

The fact is, that heat is disengaged.* Whether any light be evolved,
I cannot pretend to determine; but that heat is produced in considerable
and very sensible quantities is certain, and this is the principal, if
not the only source of ANIMAL HEAT.

[Footnote *: It has been calculated that the heat produced by
respiration in 12 hours, in the lungs of a healthy person, is such
as would melt about 100 pounds of ice.]

EMILY.

How wonderful! that the very process which purifies and elaborates the
blood, should afford an inexhaustible supply of internal heat?

MRS. B.

This is the theory of animal heat in its original simplicity, such
nearly as it was first proposed by Black and Lavoisier. It was equally
clear and ingenious; and was at first generally adopted. But it was
objected, on second consideration, that if the whole of the animal heat
was evolved in the lungs, it would necessarily be much less in the
extremities of the body than immediately at its source; which is not
found to be the case. This objection, however, which was by no means
frivolous, is now satisfactorily removed by the following
consideration:-- Venous blood has been fo