which float in the air. Some observers find
many, and some few, seeds and eggs, according
as the observers look for proofs of generation
from germs and sperms, or for proofs of
development. M. Pouchet, the zealous opponent of
those he calls the panspermists (the most
eminent and numerous of the physiologists) gives
the following account of the things he has
found: At sea, and on hill-tops, these flotsam
and jetsam of the air are rare, while they are
marvellously plentiful in old and crowded towns.
Food, clothes, furniture, houses, everything, in
fact, furnishes particles to become the motes
dancing on the sunbeams. Flour is most
common. Particles of the corn for which Joseph
and his brethren went to Egypt may be still
hidden in cracks and crannies, or floating about
— adhering to an insect's wing, or caught in a
snow-flake. There are scales of moths, skeletons
of infusoria, and living animalcules in
the air. Dust and smoke contribute to the
mote-dances. But the floating things do not
remain in the air; they go into the insides
of animals. Not merely flour, but living
microscopical crustaceans have been found alive in
the lungs of a dead man. Filiments of wool
and silk, richly dyed, were once drawn from the
bones of a peacock, the pet of a château. Flour
has been discovered stuffing the bones of a
baker's poultry; while the bones of the fowls
of a dealer in charcoal contained particles of
charcoal dust. Dust of leaves and bark
penetrates inside the woodpecker. Vegetable dust,
and filaments of cotton and wool, are found
inside the bones of crows and magpies.

What is the law in such cases? Heat
has much to do with locomotion. Heat affects
gravity, the law which makes stars and tears
round. The earth is continually rolling out
of cold air into warm. Glaciers likewise
glide towards the warmth; icebergs float from
icy towards tepid seas; and the aerial and
aqueous oceans obey the solar and lunar rays.
Temperature has much to do with the pullings
and pushings of particles, or electrical and
magnetical phenomena, to which are ascribed the
rotations in vegetable cells. The antheridiæ,
or pollenaria, of mosses and ferns rotate
according to the law of the Archimedean screw.
No less distinguished a physiologist than
Professor J. Mueller, of Berlin, has emitted the
opinion that, whilst the bones of birds are
undoubtedly made empty that they might be lighter
than they would be if they held marrow, the air
in the aerial bags does not much lessen the
weight of the birds, because it is nearly as
dense as common air. He does not, however,
say who the observer was who weighed the
gases in the aerial bags and bones of birds when
flying; and how he ascertained their density
when they were inside the pelican or the
albatross, apparently asleep in the storm. Birds
are six or seven degrees warmer than men,
because they contain, proportionally, more
carbon and hydrogen to combine with oxygen,
and produce warmth. The difference between
summer heat and blood heat is twenty degrees;
and, probably, six or seven degrees more in
birds than in men. The temperature of a
young sparrow eight days old fell sixteen or
seventeen degrees in an hour of separation
from its nest and its mother. Fledging is
warming; feathering is lightening. The heart
of a hybernating bat beats some fifty times a
minute, and of a summer bat two hundred
times. Fishes are called cold-blooded animals,
yet they are half a degree to a degree and a
half warmer than the water they swim in. The
fish that can live in ice, keep the water around
them from freezing. The arterial blood of a
bat is less crimson in winter than in summer,
and arterial blood in general is a degree or two
warmer than veinous blood. Animal heat, the
chemists tell us, is due to the combination in
the lungs of the oxygen of the air with the
carbon and hydrogen of the blood. The
carbonic acid which is exhaled is developed in the
whole vascular system. If the observation and
calculations of Magnus are to be credited,
arterial blood is more gaseous than veinous
blood. Carbonic acid gas and oxygen gas
both existing in the blood, numerous experiments
seem to prove that the carbonic gas is
expelled, not by the atmospheric air, but by
the other gases. Blood contains oxygen, carbon,
and azotic gases; veinous blood more carbonic
gas than arterial, and arterial more oxygen
than veinous blood. The miracle of the vivification
of the blood, the change from death to
life, is ascribed to this predominance of oxygen
gas. Carbonic acid gas is disengaged in respiration
when lungs are distended; in fermentation
when liquids are swelled; in combustion,
which turns solid wood and coal into smoke
and flame; in putrefaction, which brings dead
bodies above the surface of water; and this
disengagement of gas, wherever it occurs, makes
its subjects lighter.

A summary of the facts I have collected
will make it more and more evident still that
buoyancy in air, as in water, is due to the
presence of gases. Flying animals are built to
hold gases everywhere— in their bones, their
bodies, their skins; and, as their blood is
several degrees warmer than the blood of
walking or running animals, their gases are,
probably, several degrees lighter. Azote, or
hydrogen, or whatever the gas held in the
gaseous structures may be, it is proportionally
warmer, and therefore proportionally lighter,
than air. But the bat, it is said, has not the
structure of birds, and yet it flies well.

A word on bats. I have just mentioned that
the heart of the flying bat beats four beats for
one beat of the hybernating bat; and I have
been proving that greater warmth implies greater
lightness. Digestion having gone on during
hybernation, and all the stores of fat having
been absorbed, the bat awakes from torpor
extremely light and thin, a resuscitated mummy,
and, from hanging to projections, takes every
evening to a few hours of flying and feeding.
The bat is, like the bird, provided with aerial
oars, although they have membranes instead of