ATMOSPHERIC DUST IX THE ARCTIC REGIONS. 151 



the normal amount required for saturation. If the smallest 

 amount of common air (dust-laden as it always is) be admitted 

 into such supersaturated air, cloud immediately forms. As 

 Aitken expresses it, " dust is the germ of which clouds and fog 

 are the developed phenomena." 



Atmospheric dust is of very varied origin and fineness. A 

 part of it is organic, coming from the vegetable and animal 

 world, either as part of original growth or organic material 

 partially decayed. Some of it consists of the solid particles in 

 smoke ; a part consists of pulverized soil raised by the winds ; 

 some doubtless comes from dust in space or the combustion of 

 shooting-stars when they reach the atmosphere. These various 

 ingredients are mingled in every conceivable proportion and 

 are of every degree of fineness, from coarse drifting sands 

 down to particles of molecular fineness. Illuminated by the 

 sun or other light, the air becomes visible because of this dust. 

 The rays are scattered, and diffused daylight and twilight are 

 among the results. The particles may be so small as to break 

 up the light-waves and by diffraction and polarization pro- 

 duce colors reds, blues, greens, yellows, and their various 

 combinations, according to the size and character of the 

 particles. 



Moisture begins to accumulate on these particles long be- 

 fore cloud begins to form, changing the color and visibility of 

 the atmosphere, so the aspect of haze changes with the dry- 

 ness of the air as well as with the size of the particles. 



The sirocco of Egypt is a hot southerly wind from the 

 desert, so laden with fine dust that it is an Arabian saying 

 that it will penetrate the shell of an egg. Travellers describe 

 the color of the sun seen through this air as red or dull 

 purple. The harmattan is a scorching easterly wind blowing 

 from the interior of the Sahara toward the Atlantic between 

 Capes Lopez and Verde. It is so laden with fine dust that it 

 is often described as a " dry fog," through which the sun ap- 



