Mr Charles Darwin, in the narrative of his voyage in the Beagle, states that while he was at St Jago, one of the Cape de Verd islands, in January 1832:  ’The atmosphere was generally very hazy; this appears chiefly due to an impalpable dust, which is constantly falling, even on vessels far out at sea.  The dust,’ he goes on to say, ’is of a brown colour, and under the blow-pipe, easily fuses into a black enamel.  It is produced, as I believe, from the wear and tear of volcanic rocks, and must come from the coast of Africa.’  The same opinion was held by scientific men generally, as well of the dust met with in the North Atlantic, as of that which sometimes falls on the islands and shores of the Mediterranean:  Africa was supposed to be the original source of the air-borne particles.  Some of the dust, however, having been sent to Ehrenberg of Berlin, that celebrated savant, after a microscopical examination, laid an account of his inquiry before the Akademie der Wissenschaften, in May 1844, in which he shewed that the dust, so far from being inorganic, contained numerous specimens of a species of flint-shelled animalcules, or infusoria, known as polygastrica, and minute portions of terrestrial plants.  The investigation led him to certain conclusions:  ’1.  That meteoric dust-rain is of terrestrial origin. 2.  That the same is not a rain of 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.’