152 LIVING LIGHTS. 



of the remarkable resemblance which unmistakably exists 

 between plutonic rocks and meteoric stones." 



Such enormous masses of material could well explain the 

 rosy and other curious lights that are from- time to time 

 observed. But cosmic dust is not the only matter in the 

 air that could occasion the phenomena ; the atmosphere is 

 constantly filled with innumerable forms caught up by cur- 

 rents and carried to inconceivable heights, and thus to great 

 distances, to be precipitated to the earth in hail, snow, or 

 rain. 



Near St. Domingo, Darwin tells us, the atmosphere became 

 thick and hazy from the impalpable fine dust that actually 

 injured their astronomical instruments. " The morning 

 before we anchored at Porto Praya," he says, "I collected 

 a little packet of this brown-colored 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 fell on a vessel a few 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 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 



