volcanic ashes. 3. That it is necessarily a dust carried up to a great
height by a strong current of air or whirlwind from a dried-up
swamp-region. 4. That the dust neither demonstrably nor necessarily
comes from Africa, notwithstanding that the wind may blow from thence
as the nearest land when the dust falls, because there are in it no
forms whatsoever exclusively native to Africa.' These were remarkable
facts, but warranted by the evidence: one, if not more, of the
animalcules was proved to be peculiar to America, and that country was
naturally inferred to be the quarter from which they had been derived.

The inquiry once begun was followed up; other specimens of dust were
submitted to the same critical test, and found generally to contain a
much greater number and variety of infusoria than the first--mostly
fresh-water forms, but with a few of marine origin; whence the
conclusion, that they had been brought from a coast-region; and
especially remarkable was the fact, that among all the forms there was
not one peculiar to the African continent. One example was known to
belong to the Isle of France, the others were chiefly South American.
After an examination of six specimens, obtained at different
intervals, Ehrenberg discovered that they contained four organisms in
common. 'I now consider myself,' he observes, 'justified in the
conclusion, that all the Atlantic dust may come only from one and the
same source, notwithstanding its extent and annual amount. The
constant yellow and reddish colour of the dust, produced by
ferruginous matter, its falling with the trade-winds and not with the
harmattan, increase the interest of the phenomena.'

It had always been supposed, that the dust which traversed the
Mediterranean was borne from the Great Sahara; but in a quantity
collected on board the ship _Revenge_, at Malta, an infusoria peculiar