own temperature; and that this is so
wisely adapted to the circumstances of its economy, or so dependent
upon them, that, whatever be the temperature of the atmosphere, it
will have very little influence either in diminishing or increasing
that of the animal.

In order that we may see how this effect is produced, we must examine
the chemical properties of the air. Previously to this, however, it
will be necessary to point out briefly how bodies are affected, with
respect to heat, when they change their form.

When a body passes from a state of solidity to that of fluidity, it
absorbs a quantity of heat, which becomes chemically combined with
it, and insensible to the touch or the thermometer; in the same
manner, when it passes from a fluid state to that of vapour or gas,
it combines with a still larger quantity of heat, which remains
latent in it, so long as it continues in the state of gas, but when
it returns to the liquid or solid state, it gives out the heat which
was combined with it, which, being set at liberty, flows into the
surrounding bodies, and augments their temperature.

This is evinced by the conversion of ice into water, and of water
into steam; and by the return of steam into water. It is evinced
likewise by the evaporation of ether, and by numberless other
experiments.

Modern chemistry has shown that the atmosphere is not a homogeneous
fluid, but consists of two elastic fluids, endowed with opposite and
different properties.

If a combustible body, for instance a candle, be confined in a given
quantity of atmospheric air, it will burn only for a certain time;
after it is extinguished, if another combustible body be lighted and
immersed in the same air, it will not burn, but will immediately be
extinguished.

It has been proved by chemical experiments, that in this instance,
the combustible body absorbs that portion of the air which is fitted
for combustion, but produces no change on that which is unfit: so
that, according to this, the air of the atmosphere consists of two
elastic fluids, one of which is capable of supporting combustion, and
the other not; and that they exist in the proportion of one part of
the former to three of the latter nearly.

These two parts may be separated from each other, and experiments
made with them.

Many metals, and particularly manganese, when exposed to the
atmosphere, attract the combustible air from it, without touching the
other; and it may be procured