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 to Chili was met with, which, with other characteristics, proved the dust to be the same as that observed on the Atlantic. Their colour, too, was identical; while the Sahara is a 'dazzling white sand:' hence the dust brought across the Mediterranean by the sirocco was not peculiar to Africa. The conclusion here arrived at was still further verified by another sirocco-storm in May 1846, which extended to Genoa, and bore with it a dust that 'covered the roofs of the city in great abundance.' This, as was clearly ascertained, contained formations identical with those which had been collected off the Cape de Verd; and it was shewn that the dust-showers of the Atlantic, and those of Malta and Genoa, were 'always of a yellow ochre-like colour--not gray, like those of the kamsin, in North Africa.' The peculiar colour of the dust was found to be caused by iron-oxide; and from one-sixth to one-third of the whole proved to consist 'of determinable organic parts.' In the following year, 1847, Ehrenberg had another opportunity of testing his conclusions, in specimens of dust which had fallen in Italy and Sicily in 1802 and 1813; the same result came out on examination; 'several species peculiar to South America, and none peculiar to Africa.'