I have sent (June 1845) a full account of the falling of this dust to the Geological Society.) In five little packets which I sent him, he has ascertained no less than sixty-seven different organic forms! The infusoria, with the exception of two marine species, are all inhabitants of fresh-water.

I have found no less than fifteen different accounts of dust having fallen on vessels when far out in the Atlantic.

From the direction of the wind whenever it has fallen, and from its having always fallen during those months when the harmattan is known to raise clouds of dust high into the atmosphere, we may feel sure that it all comes from Africa.

It is, however, a very singular fact, that, although Professor Ehrenberg knows many species of infusoria peculiar to Africa, he finds none of these in the dust which I sent him.

On the other hand, he finds in it two species which hitherto he knows as living only in South America.