CHAP. ix. THE IMPORTANCE OF DUST. 77 



also to the presence of suspended matter, which Pro- 

 fessor Tyndall thought might be so fine that it would re- 

 quire ages of quiet subsidence to reach the bottom. All 

 the evidence goes to show, therefore, that the exquisite 

 blue tints of sky and ocean, as well as all the sunset hues 

 of sky and cloud, of mountain peak and alpine snows, 

 are due to the finer particles of that very dust which, in 

 its coarser forms, we find so annoying and even dan- 

 gerous. 



But if this production of color and beauty were the 

 only useful function of dust, some persons might be dis- 

 posed to dispense with it in order to escape its less agree- 

 able effects. It has, however, been recently discovered 

 that dust has another part to play in nature; a part so 

 important that it is doubtful whether we could even live 

 without it. To the presence of dust in the higher atmos- 

 phere we owe the formation of mists, clouds, and gentle 

 beneficial rains, instead of waterspouts and destructive 

 torrents. 



It is barely twenty years ago since the discovery was 

 made, first in France by Coulier and Mascart, but more 

 thoroughly worked out by Mr. John Aitken in 1880. 

 He found that if a jet of steam is admitted into two 

 large glass receivers, one filled with ordinary air, the 

 other with air which has been filtered through cotton 

 wool so as to keep back all particles of solid matter, the 

 first will be instantly filled with condensed vapor in the 

 usual cloudy form, while the other vessel will remain 

 quite transparent. Another experiment was made, more 

 nearly reproducing what occurs in nature. Some water 

 was placed in the two vessels prepared as before. When 



