t remains being a mixture of azotic and carbonic acid
gases.

When a candle is enclosed in a given quantity of pure oxygen gas, it
will burn four times as long as in the same quantity of atmospheric
air.

In the same manner it has been proved, that an animal will be four
times as long in consuming a given quantity of pure oxygen gas, as in
rendering unfit for respiration the same quantity of atmospheric air.

Here then we observe a striking similarity between combustion and
animal respiration. The ancients seem to have had a more accurate
idea of respiration than most of the philosophers who followed them.
They supposed that the air contained a principle proper for the
support and nourishment of life, which they called pabulum vitae.
This idea, which was unconnected with any hypothesis, was followed by
systems destitute of foundation. Sometimes it was thought that the
air in the lungs incessantly acted as a stimulus or spur to drive on
the circulation; sometimes the lungs were considered in the light of
a pair of bellows, or fan, to cool the body, which was supposed to be
heated by a thousand imaginary causes: and when philosophers were
convinced, by experiments, that the bulk of the air was diminished by
respiration, they explained it by saying, that the air had lost its
spring.

Modern chemistry however enables us to explain the phenomena of
respiration in a satisfactory manner.

In order to see this, we shall proceed to examine the changes
produced by respiration; firstly, on the air, and secondly, on the
blood.

The air which has served for respiration, is found to contain a
mixture of azotic and carbonic acid gas, with a small quantity of
oxygen gas; and a considerable quantity of water is thrown off from
the lungs, in the form of vapour, during respiration.

From a variety of facts, it appears that oxygen gas is decomposed in
the lungs during respiration; a part of it unites, as we shall
afterwards see, with the iron contained in the blood, and converts it
into an oxid; another and greater portion unites with the carbon,
brought by the venous blood from all parts of the body to the lungs,
and thus forms carbonic acid gas; while another portion of the oxygen
unites with the hydrogen, brought in the same manner by the blood,
and forms water. Thus then we are able to account for the different
products of respiration.

Hence we see, that the explanation of animal heat follows as a simple
and beautiful coro