had been numbered among
the elements, though considered liable to vitiation or foulness. The
great discovery of oxygen gas placed its chemical relations in their
proper position. One after another, other gases, both simple and
compound, were discovered. Then it was recognized that the atmosphere is
the common receptacle for all gases and vapours, and the problem
whether, in the course of ages, it has ever undergone change in its
constitution arose for solution.

[Sidenote: The antagonism of animals and plants.] The negative
determination of that problem, so far as a few thousand years are
concerned, was necessarily followed by a recognition of the antagonism
of animals and plants, and their mutually balancing each other, the
latter accomplishing their duty under the influence of the sun, though
he is a hundred millions of miles distant. From this it appeared that it
is not by incessant interventions that the sum total of animal life is
adjusted to that of vegetable, but that, in this respect, the system of
government of the world is by the operation of natural causes and law, a
conclusion the more imposing since it contemplates all living things,
and includes even man himself. The detail of these investigations proved
that the organic substance of plants is condensed from the inorganic air
to which that of all animals returns, the particles running in
ever-repeating cycles, now in the air, now in plants, now in animals,
now in the air again, the impulse of movement being in the sun, from
whom has come the force incorporated in plant tissues, and eventually
disengaged in our fires, shining in our flames, oppressing us in fevers,
and surprising us in blushes.

[Sidenote: The winds; their origin and nature.] Organic disturbances by
respiration and the growth of plants being in the lowest stratum of the
air, its uniformity of composition would be impossible were it not for
the agency of the winds and the diffusion of gases, which it was found
would take place under any pressure. The winds were at length properly
referred to the influence of the sun, whose heat warms the air, causing
it to ascend, while other portions flow in below. The explanation of
land and sea breezes was given, and in the trade-wind was found a proof
of the rotation of the earth. At a later period followed the explanation
of monsoons in the alternate heating and cooling of Asia and Africa on
opposite sides of the line, and of tornadoes, which are disks