72 DISCKEPANCIES. 



The very dust of the air is found to be peopled with 

 living plants and animals, and that where we should least 

 have expected to find it so stocked ; nay, where we should 

 scarcely have looked for clouds of dust at all, — far out on 

 the lone ocean, hundreds of miles from land. In Mr 

 Darwin's voyage, he noticed, as he approached the Cape 

 Verd Islands, this curious phenomenon : — " Generally the 

 atmosphere is hazy ; and this is caused by the falling of 

 impalpably fine dust, which was found to have slightly 

 injured the astronomical instruments. The mornmg be- 

 fore we anchored at Porto Praya, I collected a little 

 packet of this brown-coloured fine dust, which appeared 

 to have been filtered from the wind by the gauze of the 

 vane at the masthead. Mr Lyell has also given me four 

 packets of dust which feU on a vessel a fe^ hundred miles 

 northward of these islands. Professor Ehrenberg finds 

 that this dust consists, in great part, of infusoria* with 

 siliceous shields, and of the siliceous tissue of plants. In 

 five little packets which I sent him, he has ascertained no 

 less than sixty-seven different organic forms ! The infu- 

 soria, with the exception of two marine species, are all 

 inhabitants of fresh water. I have found no less thani 

 fifteen difi'erent accounts of dust having fallen on vessel 

 when far out in the Atlantic. Prom the direction of the 

 wind whenever it has fallen, and from its having alwaya 

 fallen during those months when the harmattan is kno^ 

 to raise clouds of dust high into the atmosphere, we may ] 

 feel sure that it all comes from Africa. It is, however, a j 



* Constituting the Diatomacece of modern science. • 



