sur la Composition Chimique des Vegetaux, p. 36, 42; Liebig, 'Org.
Chemie', s. 229-345; Boussingault, 'Econ. Rurale', t. i., p. 142-153.
Fogs which have a peculiar smell at some seasons of the year, remind us of
these accidental admixtures in the lower strata of the atmosphere. Winds
and currents of air caused by the heating of the ground even carry up to a
considerable elevation solid substances reduced to a fine powder. The dust
which darkens the air for an extended area, and falls on the Cape Verd
Islands, to which Darwin has drawn attention, contains, according to
Ehrenberg's discovery, a host of silicious-shelled infusoria.
As principal features of a general descriptive picture of the atmosphere, we
may enumerate:
1. 'Variations of atmospheric pressure': to which belong the horary
oscillations, occurring with such regularity in the tropics, where they
produce a kind of ebb and flow in the atmosphere, which can not be ascribed
to the attraction of the moon,* and which differs so considerably according
to geographical latitude, the seasons of the year, and the elevation above
the level of the sea.
[footnote] *Bouvard, by the application of the formulae, in 1827, which
Laplace had deposited with the Board of Longitude shortly before his death,
found that the portion of the horary oscillations of the pressure of the
atmosphere, which depends on the attraction of the moon, can not raise the
mercury in the barometer at Paris more than the 0.
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