emistry, the science of dead matter, we possess many
facts and a few principles or laws; but whenever the functions of life
are considered, though the facts are numerous, yet there is, as yet,
scarcely any approach to general laws, and we must usually end where we
begin by confessing our entire ignorance.

_Eub_.--I will not allow this ignorance to be entire. Something,
undoubtedly, has been gained by the knowledge of the circulation of the
blood and its aeration in the lungs--these, if not laws, are at least
fundamental principles.

_The Unknown_.--I speak only of the functions in their connection with
life. We are still ignorant of the source of animal heat, though half a
century ago the chemists thought they had proved it was owing to a sort
of combustion of the carbon of the blood.

_Phil_.--As we return to our inn I hope you will both be so good as give
me your views of the nature of this function, so important to all living
things; tell me what you _know_, or what you _believe_, or what others
_imagine they know_.

_The Unknown_.--The powers of the organic system depend upon a continued
state of change. The waste of the body produced in muscular action,
perspiration, and various secretions, is made up for by the constant
supply of nutritive matter to the blood by the absorbents, and by the
action of the heart the blood is preserved in perpetual motion through
every part of the body. In the lungs, or bronchia, the venous blood is
exposed to the influence of air and undergoes a remarkable change, being
converted into arterial blood. The obvious chemical alteration of the
air is sufficiently simple in this process: a certain quantity of carbon
only is added to it, and it receives an addition of heat or vapour; the
volumes of elastic fluid inspired and expired (making allowance for
change of temperature) are the same, and if ponderable agents only were
to be regarded it would appear as if the only use of respiration were to
free the blood from a certain quantity of carbonaceous matter. But it is
probable that this is only a secondary object, and that the change
produced by respiration upon the blood is of a much more important kind.
Oxygen, in its elastic state, has properties which are very
characteristic: it gives out light by compression, which is not certainly
known to be the case with any other elastic fluid except those with which
oxygen has entered without undergoing combustion; and from the fire it
p