.[5] If,
in the experiment I have described, we were to substitute two pieces of
fat for the fragments of phosphorus, the results would be precisely
similar. The fat burned in oxygen gas would emit intense light and heat;
but the total amount of these forces evolved would be neither greater
nor less than that developed during the slower and therefore less
brilliant combustion of the fat in ordinary atmospheric air. Now, as we
can demonstrate that an ounce of fat will emit a certain amount of heat,
if burned within a minute of time, and that neither a larger nor a
smaller amount will be developed if the combustion of the fat extend
over a period of five minutes, I think we may fairly assume that the
amount of heat evolved by the complete oxidation of a specific quantity
of fat is constant under all conditions, except, as I have already
explained, at high temperatures, when a portion of the heat is converted
into light.

In the animal organism fat is burned. The process of combustion no
doubt is a very slow one, but still the total amount of heat evolved
is just the same as if the fat were consumed in a furnace. When the
fat constituting a candle is burned, what becomes of it? Its elements,
carbon and hydrogen (we may disregard its small amount of oxygen)
combine with the oxygen of the air, and form carbonic acid gas and
water. What becomes of the fat consumed within the animal body? It also
is converted into carbonic acid gas and water. It is not difficult
to prove these statements to be facts. A candle will not burn in
atmospheric air which has been deprived of its oxygen, because there is
no substance present with which the elements of the taper can combine,
consequently the process of combustion cannot go on. Now, a man may in
one respect be compared with this taper. He is partly made up of fat;
that fat is consumed by the oxygen of the air, and the heat developed
thereby keeps the body warm. In the process of respiration oxygen is
introduced into the lungs, and from thence, by means of the blood
vessels, is conveyed throughout every part of the body. In some way, at
present not thoroughly understood, the elements of the fat combine with
the oxygen, and are converted into carbonic acid gas and water, which
are exhaled from the lungs and from the surface of the body.

Fat is a constituent of both animals and plants. The animal derives a
portion of its fat directly from the vegetable; but it possesses the
power of for