The true cause of death in these cases is the respiratory process,
that is, the action of the atmosphere.

A deficiency of food, and a want of power to convert the food into a
part of the organism, are both, equally, a want of resistance; and
this is the negative cause of the cessation of the vital process.
The flame is extinguished, because the oil is consumed; and it is
the oxygen of the air which has consumed it.

In many diseases substances are produced which are incapable of
assimilation. By the mere deprivation of food, these substances are
removed from the body without leaving a trace behind; their elements
have entered into combination with the oxygen of the air.

From the first moment that the function of the lungs or of the skin
is interrupted or disturbed, compounds, rich in carbon, appear in
the urine, which acquires a brown colour. Over the whole surface of
the body oxygen is absorbed, and combines with all the substances
which offer no resistance to it. In those parts of the body where
the access of oxygen is impeded; for example, in the arm-pits, or in
the soles of the feet, peculiar compounds are given out,
recognisable by their appearance, or by their odour. These compounds
contain much carbon.

Respiration is the falling weight--the bent spring, which keeps the
clock in motion; the inspirations and expirations are the strokes of
the pendulum which regulate it. In our ordinary time-pieces, we know
with mathematical accuracy the effect produced on their rate of
going, by changes in the length of the pendulum, or in the external
temperature. Few, however, have a clear conception of the influence
of air and temperature on the health of the human body; and yet the
research into the conditions necessary to keep it in the normal
state is not more difficult than in the case of a clock.

LETTER VIII

My dear Sir,

Having attempted in my last letter to explain to you the simple and
admirable office subserved by the oxygen of the atmosphere in its
combination with carbon in the animal body, I will now proceed to
present you with some remarks upon those materials which sustain its
mechanisms in motion, and keep up their various functions,--namely,
the Aliments.

If the increase in mass in an animal body, the development and
reproduction of its organs depend upon the blood, then those
substances only which are capable of being converted into blood can
be properly regarded as nourishment. In ord