These discoveries were made about the same time,
Dalton having the credit of originating them. Various modifications
of the principle have been from time to time presented to public
attention.
Whether the constituents of the atmosphere are chemically or
mechanically combined,--one of the things about which the learned are
not fully agreed,--it is found to be chemically the same in its
constituents, all over the world, whether collected on mountains or on
plains, on the sea or on the land, whether obtained by aeronauts miles
above the earth or by miners in their deepest excavations. On the
theory of its mechanical combination, however, as by volume, and that
each constituent acts freely for itself and according to its own laws,
important speculations (conclusions, indeed) have arisen, both as
regards temperature and climatic differences. It should be observed,
that volume, as we have used the word, is the apparent space occupied,
and differs from mass, which is the _effective_ space occupied, or the
real bulk of matter, while density is the relation of mass to volume,
or the quotient resulting from the division of the one by the other.
Those empty spaces which render the volume larger than the mass are
technically called its pores.
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