round the
globe.

It is well known, that when two gases of different specific gravity are
brought into contact, even though the heavier be the lowermost, they
soon become uniformly diffused by mutual absorption through the whole
space which they occupy. By virtue of this law, the heavy carbonic acid
finds its way upwards through the lighter air of the atmosphere, and
conveys nourishment to the lichen which covers the mountain top.

If the quantity of food consumed by terrestrial animals, and the
elements imbibed by the roots and leaves of plants, were derived
entirely from that supply of hydrogen, carbon, oxygen, nitrogen, and
other elements, given out into the atmosphere and the waters by the
putrescence of organic substances, then we might imagine that the
vegetable mould would, after a series of years, neither gain nor lose a
single particle by the action of organic beings; and this conclusion is
not far from the truth; but the operation which renovates the vegetable
and animal mould is by no means so simple as that here supposed.
Thousands of carcases of terrestrial animals are floated down, every
century, into the sea; and, together with forests of drift-timber, are
imbedded in subaqueous deposits, where their elements are imprisoned in
solid strata, and may there remain locked up throughout whole geological
epochs before they again become subservient to the purposes of life.

On the other hand, fresh supplies are derived by the atmosphere and by
running water, as before stated, from the disintegration of rocks and
their organic contents, and through the agency of mineral springs from
the interior of the earth, from whence all the elements before
mentioned, which enter principally into the composition of animals and
vegetables, are continually evolved. Even nitrogen is found, by
chemists, to be contained very generally in the waters of mineral
springs.

_Vegetation not an antagonist power counterbalancing the action of
running water._--If we suppose that the copious supply from the nether
regions, by springs and volcanic vents, of carbonic acid and other
gases, together with the decomposition of rocks, may be just sufficient
to counterbalance that loss of matter which, having already served for
the nourishment of animals and plants, is annually carried down in
organized forms, and buried in subaqueous strata, we concede the utmost
that is consistent with probability. An opinion, however, has been
expressed, th