twenty-four hours, a milch cow 70 3/4
ounces; so that the horse requires 13 pounds 3 1/2 ounces, and the
cow 11 pounds 10 3/4 ounces of oxygen. [2]

As no part of the oxygen taken into the system of an animal is given
off in any other form than combined with carbon or hydrogen, and as
in a normal condition, or state of health, the carbon and hydrogen
so given off are replaced by those elements in the food, it is
evident that the amount of nourishment required by an animal for its
support must be in a direct ratio with the quantity of oxygen taken
in to its system. Two animals which in equal times take up by means
of the lungs and skin unequal quantities of oxygen, consume an
amount of food unequal in the same ratio. The consumption of oxygen
in a given time may be expressed by the number of respirations; it
is, therefore, obvious that in the same animal the quantity of
nourishment required must vary with the force and number of
respirations. A child breathes quicker than an adult, and,
consequently, requires food more frequently and proportionably in
larger quantity, and bears hunger less easily. A bird deprived of
food dies on the third day, while a serpent, confined under a bell,
respires so slowly that the quantity of carbonic acid generated in
an hour can scarcely be observed, and it will live three months, or
longer, without food. The number of respirations is fewer in a state
of rest than during labour or exercise: the quantity of food
necessary in both cases must be in the same ratio. An excess of
food, a want of a due amount of respired oxygen, or of exercise, as
also great exercise (which obliges us to take an increased supply of
food), together with weak organs of digestion, are incompatible with
health.

But the quantity of oxygen received by an animal through the lungs
not only depends upon the number of respirations, but also upon the
temperature of the respired air. The size of the thorax of an animal
is unchangeable; we may therefore regard the volume of air which
enters at every inspiration as uniform. But its weight, and
consequently the amount of oxygen it contains, is not constant. Air
is expanded by heat, and contracted by cold--an equal volume of hot
and cold air contains, therefore, an unequal amount of oxygen. In
summer atmospheric air contains water in the form of vapour, it is
nearly deprived of it in winter; the volume of oxygen in the same
volume of air is smaller in summer than in winter.