ATMOSPHERIC DUST IN THE ARCTIC REGIONS. 149 



The relation of the suspended dust-particles to various at- 

 mospheric phenomena has been the subject of much recent 

 study, and each year adds to its interest. It is scarcely more 

 than twenty years since Tyndall showed its biological impor- 

 tance as being the active cause of ordinary fermentation and 

 putrefaction, and also as causing the blue color of the sky. 

 It is not yet fifteen years since Aitken showed the part it 

 played in the formation of clouds, fog, and rain. Since then 

 numerous observers have confirmed their conclusions and 

 studied its relation to the colors of the sky, the nature of haze, 

 and other atmospheric phenomena. 



The systematic investigations have, however, been mostly 

 restricted to work in laboratories or observations on the free 

 air of a few regions in Europe and the United States, sup- 

 plemented by casual observations on certain atmospheric phe- 

 nomena observed elsewhere in the temperate and tropical 

 zones. So far as I know, the recorded observations made in 

 the polar regions are confined to the fact that dust has been 

 found in the snow by several Arctic explorers. Captain 

 Nares found it in the stratified snow over the " palaeocrystic 

 sea" in the far north, and others have noted it in the snows of 

 Greenland and Lapland. 



For a better understanding of the relations of atmospheric 

 dust to certain phenomena in the Arctic regions it may be 

 well to state a few elementary facts in physics and the con- 

 ditions that exist in the other climates. 



Watery vapor is a colorless gas and as invisible as the other 

 gases are that constitute most of the atmosphere. It only 

 becomes visible when condensed as cloud, fog, snow, or rain. 

 Ordinary air will hold only a certain amount of this invisible 

 vapor, the quantity depending on the temperature. When it 

 contains all of the vapor it will hold at any temperature it is 

 said to be saturated. If the temperature is raised, the amount 

 of vapor required to saturate air increases, and on the other 



