proportion as they occupy a higher grade in
the zoological scale. During life, therefore,
from man to the last of the zoophytes, every
creature generates heat. It is true there is
an enormous interval between the fox and
the grouse of Captains Parry and Back, and
the frog, who is only just able to turn the
balance of warmth on the positive side;
nevertheless, the wonderful phenomenon of
the production of vital heat exists equally
for the reptile as for the mammal and the
bird. The inferior animals, it must be
remembered, are so completely at the mercy of
the physical condition of the medium in
which they live, that their mode of existence
entirely depends on external circumstances.
During summer they are lively, active, and
in the full enjoyment of their vital powers;
at the approach of winter, they begin to
languish; and, if the cold around them
increases, they fall into a condition so utterly
benumbed, that the functions of life seem to
be suspended. Their heat-producing power
becomes excessively weak; and their
temperature, without falling below that of
surrounding bodies, gradually approximates to
it in proportion as their torpor masters them.
It follows that their proper temperature
should be studied at the time when their
vital energy is in the plenitude of its exercise.

The real source of vital heat has been
known only within the last eighty years, to
speak in round numbers. Various
hypotheses, of greater or less plausibility, had
been maintained previously. Many of these
may be permitted to slumber in the calm
obscurity in which their obsoleteness now
envelops them. Our own Brodie has
maintained that the nervous system in warm-
blooded animals exercises a powerful influence
on their temperature, and inquired what was
the nature of the relation between this cause
and its effect. Was the brain directly or
indirectly a necessary instrument in the
production of heat? French physiologists, with
less reserve and cautiousness, have made
experiments by cutting off the heads of
animals (to suppress the action of the brain,)
and have then concluded that the said animals
died of cold, in spite of the artificial
respiration which they took care to establish
through a hole in the wind-pipe! Such
mutilations, of course, prove nothing to the
minds of any except the partisans of a
theory. Delarive's electrical hypothesis,
given to the world in eighteen hundred and
twenty, is merely an instance of the favour
at that date enjoyed by electro-dynamics as
important agents in the phenomena of life.

Meanwhile, one grand fact reigns
paramount through the whole history of animal
warmth. All creatures are provided with
various apparatus which enables the air to
penetrate into the interior of their bodies,
and to mingle with their fluids. Boyle's
experiments proved that no animal can live
in a vacuum ; and that, consequently, the
action of the air is necessary for the maintenance
of life. Other contemporary
physiologists proved that a candle is extinguished,
and that an animal dies, if shut up for too
long a time in the same mass of confined air;
and that, in both cases alike, a certain
portion of the air disappears. Long discussions
took place to ascertain whether the air
became combined with the blood entirely and
bodily, or whether it merely yielded certain
principles; endeavours were likewise made
to ascertain the real cause of the death of
animals in air confined in close vessels. Each
philosopher interpreted the fact after his own
way of thinking. Haller prudently advised
them to look out for some yet unknown
cause of sudden death. Cigna, of Turin,
demonstrated by clever experiments, that
the red colour of arterial blood is due to the
action of the atmospheric air.

Priestly made a masterly stride in
advance; he proved that common air, when
vitiated by the combustion of a candle, by
fermentation, by putrefaction, by the production
of rust on metals, and finally by respiration,
are alike fatal to animal life; that all
those modifications of air contain fixed air, or
carbonic acid; and that, to remove its
deleterious properties, to make it again
respirable, he had only to place it for several days
in contact with a plant vegetating vigorously.
Comprehending the value of his
experiments, he enunciated very just ideas
respecting the reciprocal office of the plants
and the vegetables scattered over the surface
of the globe, as means of maintaining the
atmosphere in an invariable state of composition.
He afterwards discovered oxygen,
which he called dephlogisticated air; he
showed that this gas is not injurious to
animals, but that, on the contrary, it serves for
their respiration a little longer time than
common air. Subsequently, he read before the
Royal Society conclusive proofs that common
air and dephlogisticated air alone possess the
property of restoring to venous blood the
brilliant colour of arterial blood, and that
this action takes effect even through a
moistened organic membrane; while bright
arterial blood assumes the dark hue of venous
blood when placed in contact with phlogisticated
air (azote), inflammable air (hydrogen),
and fixed air (carbonic acid). After this
great physiological progress, we expect to see
Priestly give the finishing stroke to his work
by clearly and decidedly propounding the true
theory of respiration; but the false notions
of his time led him astray, and he just missed
giving the true explanation of the grand vital
function, all whose detailed phenomena he
held so firmly within his grasp. Phlogiston
was the ignis fatuus which beguiled his steps
from the direct path, to wander fruitlessly in
a morass of error. The failure is as if
Columbus had died on board ship the day
before the New World arose on the horizon.
It is difficult to find in the whole range of